Power Save Modes#

The Power Save Modes for SiWx917 in NCP mode are broadly classified into the following:

  • Connected Power Save Modes

  • Unconnected Power Save Modes

When the SiWx917 is set into Power Save Mode after a wireless connection (connect to Access Point via Wi-Fi or a BLE central device), it is said to be in Connected Power Save Mode. The Connected Power Save modes are classified into two types:

  • Associated Power Save

  • Associated Power Save with Low Latency

When the SiWx917 is set into Power Save Mode before establishing a wireless connection, it is said to be in Unconnected Power Save Mode. The Unconnected Power Save modes are classified into two types:

  • Standby Power Save with RAM Retention

  • Standby Power Save without RAM Retention

Before going through each of the Power Save Modes, the prerequisite is to understand the Power Save Handshake Mechanism between host and SiWx917 in its Power Save Mode.

Note:

  1. For Connected Power Save Modes, ULP mode with RAM Retention configuration alone is supported, while for Unconnected Power Save Modes, ULP mode with and without RAM Retention configurations are supported.

  2. The sleep/wake state switching in Power Save Modes can be via any of the selected handshake mechanisms mentioned in the Power Save Handshake Mechanism section.

Power Save Mode Handshake Mechanism#

During its Power Save Mode, when the SiWx917 is in Sleep state, if the host application wants to send data/command to it, the host must wake the SiWx917 up. As the SiWx917's host Interface is powered off in Sleep state, a handshake mechanism between the host and SiWx917 is used for sleep/wake state switching. The handshake mechanism can be one of the following:

  • General Purpose Input/Output (GPIO)-based

  • Message-based

Note: While setting SiWx917 into Power Save Mode, no Handshake Mechanism is used. The Power Save Handshake Mechanism comes into the picture only when SiWx917 is in Power Save Mode.

GPIO-Based Handshake#

The GPIO-based Handshake, as the name specifies uses GPIOs for handshake. During its Power Save Mode, the SiWx917 uses the following GPIOs for receiving sleep/wake requests from the host, and for sending its sleep/wake indications to the host.

  • UULP_VBAT_GPIO_2 (Input to SiWx917): The host uses this GPIO for the following scenarios:

    • If the host application wants to send data/command to SiWx917, it makes a wake request by asserting this GPIO.

    • After communicating with SiWx917, the host lets the SiWx917 sleep by de-asserting this GPIO.

  • UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3 (Input to host): The SiWx917 uses either of these GPIOs to send sleep/wake indications to the host.

Software Configuration:

  • For making Ultra Low Power configuration with GPIO-based handshake, enable the SL_SI91X_FEAT_ULP_GPIO_BASED_HANDSHAKE feature in the Feature bit map (sl_wifi_device_configuration_t.boot_config.feature_bit_map).

  • By default, the UULP_VBAT_GPIO_3 should be used by the host to receive sleep/wake indications from SiWx917. Should you want to use UULP_VBAT_GPIO_0 for receiving sleep/wake indications from SiWx917, enable SL_SI91X_FEAT_SLEEP_GPIO_SEL_BITMAP feature in config feature bit map (sl_wifi_device_configuration_t.boot_config.config_feature_bit_map).

Note: If UULP_VBAT_GPIO_3 is used for sleep/wake indications, do not choose 32 KHz bypass clock on UULP_VBAT_GPIO_3 (BIT(23)) for ULP operation. In this case, choose the internal 32 KHz RC clock or 32 KHz external crystal clock for ULP mode operation.

Message-based Handshake#

If the host has a limited number of GPIOs available, the Message-based handshake can be used. The Message-based handshake, as the name specifies exchanges wake/sleep information through messages over the Host Interface for handshake.

  • If the host application wants to send data/command to SiWx917, it needs to wait until it receives a "WKP" message from the host.

  • Whenever the SiWx917 wakes up, it sends a "WKP" message to the host.

  • Whenever the SiWx917 wants to enter sleep state, it requests the host by sending an "SLP" message.

  • After receiving the "SLP" message from the SiWx917, the host, if done sending data/commands to SiWx917, gives sleep permission by sending an "ACK" message. The SiWx917 does not enter a sleep state until it receives an "ACK" message from the host.

Note: In Rev. 1.0 version of the document, the Message-based handshake is not supported. Refer to the Release Notes or Revision History of this document to check about Message-based handshake support.

Connected Power Save Modes#

In Connected Power Save Modes, the SiWx917 enters Sleep state when idle and switches to active state for transmitting/receiving data to/from AP/host MCU. Before going through Connected Power Save Modes, it is important to understand the sleep/wake state switching mechanism and wake interval concepts of SiWx917 during Connected Power Save Mode.

Sleep/Wake State Switching Mechanism#

  1. The connecting station (here SiWx917) makes an Association request to the AP. In the Association frame, it sends the Listen Interval which indicates how often the station wakes to listen to the AP beacon frames. Based on the Listen Interval, the AP holds the data frames destined for the station.

  2. The AP shall respond with an Association Response frame in which it specifies the Association Identifier (AID) to its connecting station.

  3. The SiWx917 can be configured to wake every Delivery Traffic Indication Message (DTIM) Interval or Beacon Interval or Listen interval or Target Wake Time (TWT) Wake Interval during its Connected Power Save Mode.

    • Beacon Interval-based wakeup: Beacon Interval is the period between two subsequent beacon frames transmitted by AP. The station wakes every beacon interval.

    • DTIM Interval-based wakeup: The DTIM period specifies how often an AP beacon includes Buffered Traffic Indication to its connected clients via the TIM element in the beacon frame. When the AP includes TIM information in a beacon frame, the beacon is called a DTIM beacon. DTIM interval is the time between two subsequent DTIM beacons transmit-ted by AP. DTIM Interval = Beacon Interval*DTIM Period.

    • Listen Interval-based wakeup: Based on the listen interval configured in the application, the station wakes up at the nearest integral multiples of DTIM Interval/Beacon Interval broadcasted by the connected AP which is just less than or equal to the Listen Interval. This is explained in detail in the Listen Interval section.

      Note: Configuring larger listen intervals greater than 1000 ms might lead to AP disconnecting the SiWx917. It is highly recommended to use 1000 ms as a listen interval for low-power consumption. APs supporting Wi-Fi 6 offer longer sleep intervals beyond 1000 ms.

    • TWT Wake Interval-based wakeup: The station wakes every TWT Wake Interval configured in the application. The APs confirming Wi-Fi 6 support TWT. TWT is explained in detail in the Target Wake Time section.

  4. When the station is set in Connected Power Save Mode by the host, it sends a Null data frame to the AP with the Power Management (PWR MGT) bit set in the Frame Control field, indicating that it is entering the Sleep State.

  5. The AP acknowledges the Station's Null data frame.

  6. When its connected stations are asleep,

    • if the AP has any broadcast/multicast frames to transmit, it buffers them. It indicates this information to its connected stations in the immediate DTIM beacon with the Multicast field in the Traffic Indication Map (TIM) set to "True" and transmits the broadcast/multicast frames to all its connected stations.

    • if the AP has unicast data frames for a station, it buffers the frames. The AP informs the station about the buffered data via the Partial Virtual Bitmap (PVB) field in the TIM element in the immediate DTIM beacon. The PVB determines the list of AIDs set. For example, if a station is assigned with AID "3" during its association, whenever there are data packets buffered for this station, the AP sets bit "3" in the PVB.

    Note: If the broadcast/multicast frames are not important for your application, call sl_wifi_filter_broadcast() API to ignore these frames before calling sl_wifi_set_performance_profile() API.

  7. When the station wakes up to receive an AP beacon, it checks whether its AID is set in the Partial Virtual Bitmap (PVB) and if set retrieves the buffered frames from the AP.

  8. The process of retrieving the unicast data from AP is different for different Connected Power Save Modes. The Associated Power Save Mode uses the Maximum Power Save Polling (Max PSP) Mechanism and Associated Power Save with Low Latency uses the Fast Power Save Polling (Fast PSP) mechanism. The Max PSP and Fast PSP mechanisms shall be explained in detail in the upcoming sections of the document.

  9. After receiving the data frames, the station goes back to sleep state.

Beacon Interval#

  • The SiWx917 wakes every Beacon Interval (BI) configured in the AP. The more the Beacon Interval, the longer the SiWx917 stays asleep, thereby reducing the current consumption.

  • The following figure illustrates the BI-based wakeup of SiWx917 for AP’s BI = 100 ms and DTIM period =3. In this case, the SiWx917’s wake interval = 100 ms.

    BI-based WakeupBI-based Wakeup

  • Configuration: Set the structure variable sl_wifi_performance_profile_t.dtim_aligned_type to SL_SI91X_ALIGN_WITH_BEACON while calling sl_wifi_set_performance_profile() API.

DTIM Interval#

  • The SiWx917 wakes every DTIM Interval, as per the DTIM period configured in the AP. The less the DTIM Interval, the less the Rx latency to retrieve the buffered frames from AP.

  • The following figure illustrates the DTIM Interval-based wakeup of SiWx917 for AP’s BI = 100 ms and DTIM period = 3. In this case, the SiWx917’s wake interval = 300 ms.

    DTIM Interval-based WakeupDTIM Interval-based Wakeup

  • Configuration: Set the structure variable sl_wifi_performance_profile_t.dtim_aligned_type to SL_SI91X_ALIGN_WITH_DTIM_BEACON while calling sl_wifi_set_performance_profile() API.

Listen Interval#

  • The Listen Interval indicates how often the SiWx917 in Connected Power Save Mode wakes to listen to AP beacons.

  • By default, the Listen Interval is set to 1000 ms in the WiSeConnect SDK v3.x.

  • The wake interval of SiWx917 can be configured to be aligned with DTIM Interval or Beacon Interval of AP.

  • For example, if the Listen Interval is 1000 ms, the DTIM period is 3 and beacon interval is 100 ms at AP (it means every 3rd beacon is a DTIM beacon),

    • the SiWx917 wakes every 900 ms (<= Listen Interval) if configured to be aligned with DTIM period of AP (DTIM Interval-based wakeup).

    • the SiWx917 wakes every 1000 ms (<= Listen Interval) if configured to be aligned with beacon interval of AP (Beacon Interval-based wakeup). In this case, the SiWx917 does not take DTIM period of AP into consideration.

  • Configuration: The Listen Interval can be configured in two methods:

    • During association with an AP.

      • Set the listen_interval to a desired value in multiples of DTIM period of AP and call the following API. void sl_si91x_set_listen_interval(uint32_t listen_interval)

      • Set the join_feature_bitmap to SI91X_JOIN_FEAT_LISTEN_INTERVAL_VALID and call this API before calling Wi-Fi connection APIs: sl_status_t sl_si91x_set_join_configuration(sl_wifi_interface_t interface, uint8_t join_feature_bitmap)

      • Call sl_net_up() API.

    • After establishing a Wi-Fi connection with the AP, pass the listen interval as an argument for power save API.

      • Set the join_feature_bitmap to SI91X_JOIN_FEAT_PS_CMD_LISTEN_INTERVAL_VALID.

      • Call the sl_net_up() for associating with an AP.

      • Call the following API with the profile structure variable's member (sl_wifi_performance_profile_t.listen_interval) set to a desired listen interval. This listen interval should be less than listen interval set during association request: sl_status_t sl_wifi_set_performance_profile(const sl_wifi_performance_profile_t *profile)

      Listen Interval-based WakeupListen Interval-based Wakeup

  • The following figure illustrates the LI-based wakeup of SiWx917 for AP’s BI = 100 ms and DTIM period = 3, LI = 800 ms. In this case, the SiWx917’s wake interval = 600 ms with DTIM-aligned configuration and 800 ms with Beacon-aligned configuration.

Note: For Listen Interval based wake up, the broadcast and multicast frames transmitted by the AP when the device is asleep may be lost.

Associated Power Save Mode#

The Associated Power Save Mode is a combination of Max PSP Mode with GPIO-based handshake between the host and SiWx917.

Associated Power Save ModeAssociated Power Save Mode

Communication between AP and SiWx917:

The SiWx917, when in Associated Power Save Mode, can send data to AP at any instance. For retrieving unicast data buffered at the AP, the Max PSP mechanism is used.

Maximum Power Save Polling (Max PSP):

  • Whenever the AP receives data frames that are destined for a station (here SiWx917), it buffers the frames.

  • The AP informs this to the station by setting the corresponding station's AID in the TIM element of the next immediate DTIM beacon.

  • On the next wake-up (based on DTIM Interval of BI or listen interval), the Station receives the beacon and checks for the TIM.

  • If the AID of the station is set, it sends a Power Save Polling (PS-Poll) frame with its AID to the AP to retrieve a data frame.

  • The AP acknowledges the PS-Poll frame and transmits a data frame with the "More Data" field set to 1 in case there are more data frames buffered for the station.

  • After receiving a data frame, the SiWx917 station sends it to the host.

  • The station sends a PS-Poll frame to retrieve each data frame from the AP.

  • While sending the last data frame to the station, the AP shall set the "More data" field to "0".

  • After receiving the last data frame, the station goes back to sleep state.

The Max PSP saves more power but produces lower throughputs. In case of receiving bulk amount of data, sending a PS-Poll frame to retrieve each frame affects the throughput and induces a considerable amount of delay.

Communication from SiWx917 to the host:

  • Whenever the SiWx917 wants to send data to the host, no handshake is required.

  • During Connected Power Save Mode, the SiWx917 acknowledges its sleep/wake states to the host. Whenever the SiWx917 wakes up for every DTIM/Beacon/Listen Interval, it asserts the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3. After receiving the AP beacon, if the TIM element of the beacon indicates no data buffered to it, the SiWx917 goes back to sleep by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3.

Communication from the host to SiWx917:

  • Whenever the host wants to send data/commands to SiWx917, a GPIO-based handshake is required. The host makes a wake re-quest to SiWx917 by asserting the UULP_VBAT_GPIO_2. The host must wait until it receives a wake indication from SiWx917.

  • After SiWx917 wakes up from Sleep state, it gives a wake indication to the host by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3.

  • The host, after sending the data/commands to SiWx917, gives sleep permission to SiWx917 by de-asserting the UULP_VBAT_GPIO_2.

  • The SiWx917 acknowledges the host's sleep permission by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3 and goes back to sleep state.

The following flowchart illustrates the Associated Power Save Mode:

Associated Power Save Mode FlowchartAssociated Power Save Mode Flowchart

Configuration:

  1. After a wireless connection, the SiWx917 can be set into Associated Power Save by calling the below API with the profile structure variable's member (sl_wifi_performance_profile_t.sl_performance_profile_t) set to ASSOCIATED_POWER_SAVE.

    sl_status_t sl_wifi_set_performance_profile(const sl_wifi_performance_profile_t *profile)

  2. The listen interval can be configured as described in the Sleep/Wake States Switching Mechanism section.

Associated Power Save with Low Latency Mode#

The Associated Power Save with Low Latency Mode is a combination of Fast PSP Mode with GPIO-based handshake between the host and SiWx917.

Associated Power Save with Low Latency ModeAssociated Power Save with Low Latency Mode

Communication between AP and SiWx917:

The SiWx917, when in the associated Power Save Mode with Low Latency, can send data to AP at any instance. For retrieving unicast data buffered at the AP, the Fast PSP mechanism is used.

Fast Power Save Polling (Fast PSP):

  • Whenever the AP receives data frames that are destined for a station (here SiWx917), it buffers the frames.

  • The AP informs this to the station by setting the corresponding station's AID in the TIM element of the next immediate DTIM beacon.

  • On the next wake-up (based on DTIM or listen interval), the station receives the beacon and checks for the TIM.

  • If the AID of the Station is set, the station exits power save mode, switches to active mode, and sends a Null data frame with PWR MGT bit set to "0" to the AP to retrieve all the data packets from AP.

  • The AP acknowledges the Null data frame and it AP transmits a data frame with the "More Data" field set to 1 in case there are more data frames buffered for the Station.

  • After receiving a data frame, the SiWx917 station sends it to the host.

  • After receiving the last data frame, the station waits for the monitor interval time configured and checks for data frames from AP.

  • If no data is to be received, the station goes back to sleep state by sending a Null data frame with the PWR MGT bit set to "1" to the AP.

The Fast PSP saves less power and produces better throughput when compared to the Max PSP. If both throughput and power save are important for your application, use Fast PSP.

Communication from SiWx917 to the host:

The procedure is the same as mentioned in the Communication from SiWx917 to host subsection of Associated Power Save Mode section.

Communication from the host to SiWx917:

The procedure is the same as mentioned in the Communication from SiWx917 to host subsection of Associated Power Save Mode section.

The following flowchart illustrates the Associated Power Save with Low Latency Mode:

Associated Power Save with Low Latency Mode FlowchartAssociated Power Save with Low Latency Mode Flowchart

Configuration:

  1. After a wireless connection, the SiWx917 can be set into Associated Power Save with Low Latency by calling the below API with the profile structure variable's member (sl_wifi_performance_profile_t.sl_performance_profile_t) set to ASSOCIATED_POWER_SAVE_LOW_LATENCY. The Monitor Interval can be configured by defining the profile structure variable's member (sl_wifi_performance_profile_t.monitor_interval).

    sl_status_t sl_wifi_set_performance_profile(const sl_wifi_performance_profile_t *profile)

  2. The Listen Interval can be configured as described Sleep/Wake States Switching Mechanism subsection.

Enhanced Max PSP Feature#

In Associated Power Save Mode, during unicast data retrieval, some Access Points do not deliver buffered data destined for the station for a PS-Poll frame. This interoperability issue can be avoided with the Enhanced Max PSP feature.

Functionality and Usage:

  • Initially, the SiWx917 is in Associated Power Save Mode.

  • After sending a PS-Poll frame to AP, if the AP delivers the buffered data within 20ms, the SiWx917 continues to be in Associated Power Save Mode.

  • When the AP does acknowledge PS-Poll and does not deliver the buffered data within 20ms, the SiWx917 switches to Associated Power Save with Low Latency Mode.

  • To use this feature, do the following configuration:

  • Enable the SL_SI91X_ENABLE_ENHANCED_MAX_PSP in config_feature_bit_map (sl_wifi_device_configuration_t.boot_config.config_feature_bit_map).

  • Set the Performance Profile to ASSOCIATED_POWER_SAVE_LOW_LATENCY.

Note: To switch the SiWx917 from connected power save to active mode, call the sl_wifi_set_performance_profile() API with the profile structure variable's member (sl_wifi_performance_profile_t.sl_performance_profile_t) set to HIGH_PERFORMANCE.

Target Wake Time (TWT)#

TWT is a beneficial Wi-Fi 6 feature which allows connected stations to manage power efficiently with the reduced network contention.

  • Reduces network contention: In the Legacy Power Save Modes, the Wi-Fi stations go to sleep and wake up at random times to perform data transfer without knowing the wake-up timings of other stations. With TWT, the AP schedules wake timings for its connected stations, ensuring no two Wi-Fi stations wake at the same time. This method helps avoid packet collisions, thus reducing retransmissions and in turn reducing the station's current consumption.

There are two types of TWT:

  • Individual TWT: The connected stations negotiate the TWT session parameters (TWT Wake interval and Wake duration) independently with the AP.

  • Broadcast TWT: AP broadcasts predefined TWT parameters (TWT Wake Interval and Wake duration) in its beacon and schedules different wake times for different connected stations.

The Wake duration, Wake Interval, and Sleep duration are derived from the TWT parameters.

  • Wake duration: The duration for which the station is awake to transmit/receive data frames to/from the AP. This duration is also called as TWT Service Period (SP).

  • Wake Interval: The time interval between two successive SPs' start times.

  • Sleep duration: The time between the end of current SP and the start of next/future SP.

Configuration:

  • Configure the TWT parameters using the sl_wifi_performance_profile_t. sl_wifi_twt_request_t structure.

    typedef struct {
  uint8_t wake_duration; 
  uint8_t wake_duration_tol; 
  uint8_t wake_int_exp; 
  uint8_t wake_int_exp_tol; 
  uint16_t wake_int_mantissa; 
  uint16_t wake_int_mantissa_tol; 
  uint8_t implicit_twt; 
  uint8_t un_announced_twt; 
  uint8_t triggered_twt; 
  uint8_t negotiation_type; 
  uint8_t twt_channel
  uint8_t twt_protection; 
  uint8_t twt_flow_id
  uint8_t restrict_tx_outside_tsp;  
  uint8_t twt_retry_limit
  uint8_t twt_retry_interval; 
  uint8_t req_type
  uint8_t twt_enable
  uint8_t wake_duration_unit; 
} sl_wifi_twt_request_t;

    Note: To know more about the above structure and its configuration, refer to TWT TCP Client example in the WiSeConnect SDK v3.x.

  • As per above structure parameters, below are the derivations:

    • The Wake duration or TWT Service Period is calculated as wake_duration*wake_duration_unit.

    • The TWT Wake Interval is calculated as (wake_int_mantissa)*(2^wake_int_exp).

    • The TWT Sleep duration is calculated as TWT Wake Interval - Wake duration.

  • Define a sl_wifi_performance_profile_t. sl_wifi_twt_request_t structure variable and pass it as an argument to the below API.

    sl_status_t sl_wifi_enable_target_wake_time(sl_wifi_twt_request_t *twt_req)

TWT-based Power Save Mechanism#

This section explains the TWT Power Saving Mechanism of SiWx917 with following TWT parameters set:

  • Individual TWT - the connected station (SiWx917) initiates a negotiation with its individual TWT Wake interval and Wake duration.

  • Implicit TWT - the TWT requesting STA (SiWx917) calculates the next TWT by adding a fixed value to the current TWT value.

  • Unannounced TWT - the TWT requesting STA (SiWx917) does not announce its wake-up to AP through PS-POLLs or UAPSD Trigger frames.

  • Non-Triggered TWT - The TWT SP does not contain any triggered frames.

Communication between AP and SiWx917:

  • The SiWx917 sends an Association frame with TWT Requester Support bit set in the High Efficiency (HE) MAC capabilities field.

  • After connecting to an Access Point, the SiWx917 transmits a TWT Setup frame with the TWT parameters depending on the request type (Request TWT/Suggest TWT/Demand TWT).

  • If the AP agrees on the TWT parameters, it transmits a TWT Accept frame.

  • The SiWx917 sends a Null data frame and goes to sleep.

  • Whenever the AP receives destined packets for a station (SiWx917), it buffers them.

  • The AP indicates this to the station by setting the corresponding AID in the TIM element of the next beacon.

  • The SiWx917 wakes just before its TWT to read the incoming beacon from the AP for Time Synchronization.

  • As soon as it reads the beacon, the SiWx917 goes back to sleep and wakes at start of TWT SP.

  • During its Wake duration/TWT SP, if the AID of SiWx917 is set in the TIM element, the SiWx917 exits power save mode, switches to active mode, and retrieves all the data packets from AP during TWT SP.

  • The AP transmits a data frame with the "More Data" field set to 1 in case there are more data packets buffered for the Station.

  • After receiving a data packet, the SiWx917 sends it to the host.

  • In case there are no data frames to be received, the station will be active for the rest of wake duration (if remains) and goes back to Sleep state.

  • After TWT SP, if there are more data frames to be transmitted or received from the AP, the device still goes to sleep and shall carry out these data transfers at its next TWT SP.

TWT Mechanism can be used when your host application has predictable and deterministic data traffic. The host application should be confident about the times at which data traffic is expected should be known prior.

TWT Power Saving MechanismTWT Power Saving Mechanism

Communication from SiWx917 to the host:

  • Whenever the SiWx917 wants to send data to the host, no handshake is required.

  • The SiWx917 acknowledges its sleep/wake states to the host. Whenever the SiWx917 wakes every TWT Wake Interval, it as-serts the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3. After TWT SP, the SiWx917 goes back to sleep by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3.

Communication from the host to SiWx917:

  • Whenever the host wants to send data/commands to SiWx917, a GPIO-based handshake is required. The host makes a wake request to SiWx917 by asserting the UULP_VBAT_GPIO_2. The host must wait until it receives a wake indication from SiWx917.

  • The SiWx917 shall give a wake indication only when it wakes at next TWT SP by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3.

  • The host can communicate with SiWx917 only when SiWx917 is awake for serving TWT SP. The host, after sending the data/command to SiWx917, gives sleep permission to SiWx917 by de-asserting the UULP_VBAT_GPIO_2.

  • If the host has more data/commands to send beyond TWT SP/SiWx917’s Wake duration, it must wait until next TWT SP.

Configuration:

  • After calling sl_net_up() API, register a callback function for TWT negotiation response events using the below API:

    static inline sl_status_t sl_wifi_set_twt_config_callback(sl_wifi_twt_config_callback_t function, void *optional_arg)

  • Trigger the TWT parameters negotiation by calling the below API with the profile structure variable member sl_wifi_performance_profile_t.sl_wifi_twt_request_t set with TWT setup configuration.

    sl_status_t sl_wifi_enable_target_wake_time(sl_wifi_twt_request_t *twt_req)

  • Once the AP sends a TWT response frame, the registered callback function gets triggered. The callback function gives the agreed TWT parameters during TWT parameters negotiation.

  • Next, set the SiWx917 into Associated Power Save (or with Low Latency) by calling the below API with the profile structure variable's member (sl_wifi_performance_profile_t.sl_performance_profile_t) set to ASSOCIATED_POWER_SAVE (or ASSOCIATED_POWER_SAVE_LOW_LATENCY).

    sl_status_t sl_wifi_set_performance_profile(const sl_wifi_performance_profile_t *profile)

  • To tear down the TWT session, set thesl_wifi_twt_request_t. twt_enable to “0” and call the below API.

    sl_status_t sl_wifi_disable_target_wake_time(sl_wifi_twt_request_t *twt_req)

Automatic TWT#

Automatic TWT (Auto TWT) is a Silicon Labs’s implementation that configures TWT parameters automatically based on the user application requirements. The user can provide the application requirements in terms of average Throughput and Rx latency. Based on these inputs, the SiWx917 automatically configures the TWT parameters and negotiates them with the AP.

Working and Configuration:

  • After calling sl_net_up() API, register a callback function for TWT negotiation response events using the below API:

    static inline sl_status_t sl_wifi_set_twt_config_callback(sl_wifi_twt_config_callback_t function, void *optional_arg)

  • The Auto TWT selection parameters are configured using the sl_wifi_performance_profile_t. sl_wifi_twt_selection_t structure.

    typedef struct {
  uint8_t twt_enable; 
  uint16_t average_tx_throughput; 
  uint32_t tx_latency; 
  uint32_t rx_latency; 
  uint16_t device_average_throughput; 
  uint8_t estimated_extra_wake_duration_percent; 
  uint8_t twt_tolerable_deviation; 
  uint32_t default_wake_interval_ms
  uint32_t default_minimum_wake_duration_ms; 
  uint8_t beacon_wake_up_count_after_sp; 
} sl_wifi_twt_selection_t;

  • From the above structure, the configurable values are:

    • twt_enable : 1- Setup ; 0 - teardown

    • rx_latency : The maximum allowed receive latency, in milliseconds, when an Rx packet is buffered at the AP. If rx_latency is less than <= 2 sec, session creation is not possible. If configured as 0, then by default 2sec is considered.

    • avg_tx_throughput : The expected average Tx throughput in Kbps should be between 0 and half of device average throughput.

  • Define a sl_wifi_performance_profile_t. sl_wifi_twt_selection_t structure variable and pass it as an argument to the below API.

    sl_status_t sl_wifi_target_wake_time_auto_selection(sl_wifi_twt_selection_t *twt_auto_request)

  • When the above API is called, the SiWx917 automatically configures the TWT parameters and triggers a TWT negotiation with the AP.

  • Once the AP sends a TWT response frame, the registered callback function gets triggered. The callback function gives the agreed TWT parameters during TWT parameters negotiation.

  • Next, set the SiWx917 into Associated Power Save (or with Low Latency) by calling the below API with the profile structure varia-ble's member (sl_wifi_performance_profile_t.sl_performance_profile_t) set to ASSOCIATED_POWER_SAVE (or ASSOCIATED_POWER_SAVE_LOW_LATENCY).

    sl_status_t sl_wifi_set_performance_profile(const sl_wifi_performance_profile_t *profile)

  • The SiWx917 performs Tx/Rx based on the user configured parameters ensuring low latency for transmitting and receiving data.

    Auto TWT Power Save MechanismAuto TWT Power Save Mechanism

Benefits of Automatic TWT Over Standard TWT#

  • Power Optimization when compared to standard TWT.

  • Low Latency for transmitting and receiving application data.

Unconnected Power Save Modes#

In Unconnected Power Save Mode, the SiWx917 enters:

  • Sleep state when idle and switches to active state for communicating with host MCU.

  • Deep Sleep state when idle and exits Power Save Mode for communicating with host MCU.

Standby Power Save with RAM Retention Mode#

  • The Standby Power Save with RAM Retention Mode uses a GPIO-based handshake between the host and SiWx917.

  • The SiWx917's RAM contents and current state are retained.

  • The host should not send any wireless commands (scan for Wi-Fi networks, connect to a Wi-Fi network) to SiWx917 when configured in this mode.

  • To perform scan or connect to a Wireless network, the SiWx917 should exit Power Save Mode. To achieve this, the host should call sl_wifi_set_performance_profile() with sl_wifi_performance_profile_t.sl_performance_profile_t set to HIGH_PERFORMANCE to make SiWx917 exit power save mode and switch to active mode and then send the wireless commands.

Standby Power Save with RAM Retention ModeStandby Power Save with RAM Retention Mode

Communication from SiWx917 to host:

When the SiWx917 wants to send a command response to the host, no handshake is required. The SiWx917 shall not send any asynchronous data to the host.

Communication from host to SiWx917:

  • Whenever the host wants to send a command to SiWx917, a GPIO-based handshake is required.

  • The host makes a wakeup request to SiWx917 by asserting the ULP_GPIO_5 (in Low Power Mode)/UULP_VBAT_GPIO_2 (in Ultra Low Power Mode). The host must wait until it receives a wake indication from SiWx917.

  • The host, after sending the command to SiWx917, gives sleep permission to SiWx917 by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3.

  • The SiWx917 processes the received command, responds to the host, acknowledges the host's sleep permission by de-asserting the UULP_VBAT_GPIO_0/UULP_VBAT_GPIO_3 and goes back to sleep state.

The following flowchart illustrates the Standby Power Save with RAM Retention Mode:

Standby Power Save with RAM Retention Mode FlowchartStandby Power Save with RAM Retention Mode Flowchart

Standby Power Save Mode#

  • The Standby Power Save Mode uses a GPIO-based handshake between the host and SiWx917.

  • The SiWx917's RAM contents and current state are not retained.

  • The host should not send any commands to SiWx917, when in Standby Power Save Mode.

Standby Power Save without RAM Retention ModeStandby Power Save without RAM Retention Mode

Communication from SiWx917 to host:

The SiWx917 shall not send any asynchronous data to the host when in Standby Power Save Mode.

Communication from host to SiWx917:

To send data/commands to SiWx917, the host should bring the SiWx917 back to active mode. For this, the host needs to re-initialize the SiWx917 as the previous state of the device is not retained.

Configuration:

  • After the device initialization and before establishing a wireless connection, the SiWx917 can be set into Standby Power Save Mode by calling the below API with the profile structure variable's member (sl_wifi_performance_profile_t.sl_performance_profile_t) to STANDBY_POWER_SAVE.

  • To switch the SiWx917 from Standby Power Save mode to active mode, the host needs to call si91x_req_wakeup() API and re-initialize the SiWx917 by calling the sl_net_init() API to initialize the device and bring it back to active mode.

How to Choose Power Save Modes?#

This section describes how to choose a right Power Save Mode depending on the application and use case. Provided below are some typical use cases and Power Save Mode to be used in those use cases.

  1. Let us say for an application, Rx latency is critical, and the data must be retrieved within one second. In such cases, choose Associated Power Save Mode with a listen interval of 1 second. If data retrieval should happen in less than a second, choose DTIM Interval or Beacon Interval based wake up without listen interval configuration.

  2. Consider an application which,

    • performs periodic sensor data publishes to the cloud every 30 seconds.

    • upon a user request, performs instant download of heavy content updates/files of say > 500 KB size from a Cloud Server 15-20 times a day with low latency (up to 1 second to initiate a download). In this case, as the download speed/receive throughput should be higher and should be completed without any latency, choose Associated Power Save with Low Latency with a listen interval of 1 second. Here, Fast PSP brings high Rx throughputs.

  3. Consider a use case with predictable and deterministic application traffic. The application expects the SiWx917 to sleep for longer duration and perform data transmission or reception for 100-200 ms duration every 2 minutes. Beyond this 100-200 ms duration, the application shall not transmit any data. In this case, the TWT-based Power Save Mechanism can be used with a TWT Wake Interval of 2 minutes and TWT Wake Duration of 200 ms.

  4. Let us consider an application with the following requirements:

    • The time at which the Rx data arrives is unpredictable. The Rx data reception is not periodic.

    • Expects an average Tx throughput of 10 Mbps with a maximum Receive Latency of 5 seconds.

    • Performs data transmissions less frequently, say 20-30 times a day. In this case, the Auto TWT-based Power Save Mechanism can be used which ensures longer sleep duration providing reduced current consumption as well as low latency while receiving the data that arrives at random times.

  5. Consider an application that scans for Wi-Fi Networks at regular intervals to get the Wi-Fi Networks/APs statistics in the vicinity. The scan needs to be performed once a minute. In this case, the SiWx917 can be set in Standby Power Save with RAM Retention Mode when idle (for 1 minute), set to Active mode to perform scan, and can be set back to Standby Power Save with RAM Retention Mode. This process reduces SiWx917’s average current consumption.

  6. Consider applications that publish data to the cloud 2-3 times a day. In such cases, the device can be set to Standby Power Save Mode. Whenever data must be published, the SiWx917 can be set into Active Mode, connect to a Wi-Fi network, perform data publish, disconnect from Wi-Fi, and set back into Standby Power Save Mode.