Device Power Modes

Introduction

This document provides details on the different WF200 device power modes which can be used by the host to optimize the power efficiency of its application. To optimize even further the power consumption, the WF(M)200 can make use of its different Wi-Fi power modes (i.e., active, Fast-PS, PS-Poll and U-APSD). The Wi-Fi power modes are documented here.

Device Power Saving

First, a good RF design is needed to achieve a good link with other Wi-Fi devices, leading to less retries and thus reduced power consumption during active phases (Tx and Rx) using the 38 MHz clock.

See UG382 (WF200) and UG395 (WFM200) for details on RF-related hardware design.

WF(M)200 Device Power Save Modes

• Device Power Save Modes reduce power consumption during the inactive phases, i.e. when there is no Tx or Rx activity.
• Entering a device power save mode is only possible when GPIO/WUP is LOW and functional.

The best power saving mode an application can reach depends on its hardware. We consider four device power modes:

'-' means that the presence or absence of the hardware does not matter for the power mode.

Low-Speed Clock

When in power save mode, using a slower clock instead of the 38 MHz clock is more energy efficient, since silicon power consumption is a direct relation between the number of clock cycles per second and the number of active gates.

A Low Power Clock (LP_CLK) running at 32 KHz can be provided to WF(M)200. This clock is used in Sleep with XO and Sleep modes instead of the 38 MHz clock. The LP_CLK frequency drift within 1 second should be lower than +/-100 ppm. The sleep current reaches 22 µA with LP_CLK, as opposed to 1.2 mA with the normal clock (38.4 MHz), as detailed in the datasheet.

To enable the use of a low power clock, the hardware design has to provide a 32.768 KHz on LP_CLK. The LP_CLK is detected automatically after reset, so no configuration is required.

38 MHz Clock

As described above, the WF(M)200 can save power using a 32 KHz clock while in Sleep mode. In most cases, the crystal oscillator driving the 38 MHz clock is switched off, thus saving some power.

Depending on the hardware design it may be necessary to keep the 38 MHz clock active if it's fed to another device. In this situation, the WF(M)200 will still run on the 32 KHz clock, but the crystal oscillator will be kept on to maintain the 38 MHz clock. Power savings won't be optimum. This case is the Sleep with XO mode. To share the 38 MHz clock:

• Set HF_CLK/XTAL_SHARED: off; in your PDS file is you're not sharing the 38 MHz clock (best case)

The WFM200 has no 38 MHz sharing capability, so HF_CLK/XTAL_SHARED: off; is mandatory with WFM200.

• Set HF_CLK/XTAL_SHARED: on; in your PDS file if the Xtal oscillator (i.e the 38 MHz clock) needs to be permanently available for another device.

GPIO/WUP

GPIO/WUP is an output for the host and an input for the WF(M)200.

The GPIO/WUP (wake-up) pin allows the Host to enable or disable device power save modes in the WF(M)200.

• Hardware-related power saving only occurs when GPIO/WUP is LOW.
• When GPIO/WUP is HIGH, the WF(M)200 does not enter device power save.

Although the Host indicates to WF(M)200 via GPIO/WUP that it can enter device power save, the WF(M)200 may not be able to do so, depending on Wi-Fi activity. To enable the WUP GPIO, set GPIO_WUP/PIN_MODE: func; in your PDS file.

GPIO/WIRQ

GPIO/WIRQ is an output for WF(M)200 and an input for the Host. WF(M)200 raises the WIRQ signal when a Wi-Fi message intended for the host is received, indicating that the host needs to retrieve it. If the host is in suspend state it goes back to an active state.

Messages solely intended to maintain the connection to the AP (which can be processed by the firmware) are not sent over the SPI or SDIO bus, so GPIO/WIRQ is not raised in this situation.

To enable the WIRQ GPIO, set GPIO_WIRQ/PIN_MODE: func; in your PDS file.

Enabling Device Power Save

FMAC API

Refer to the FMAC Device Power Save documentation.

LMAC API

With the Low-MAC driver (used with Linux platforms), two conditions must be met to enable Device Power Save:

• The GPIO pin used as GPIO/WUP needs to be declared in the device tree
• Wi-Fi Power Save needs to be enabled (For more information, refer to LMAC Wi-Fi Power Save Enablement)

Additional Power Saving Considerations

SPI/SDIO Clock

• Use the highest possible SPI or SDIO clock to reduce transfer times
• If possible on the host side, stop the SPI or SDIO clock when enabling sleep (when GPIO/WUP is pulled LOW by the host)

Pull-ups

• Maximize all pull-ups resistors

Power Supplies

• Use 1.8 V for VDD_RF, VDD_D, VDD_IO
• VDD_PA needs to be 3.3 V

See AN1219: Power Consumption Measurement Setup and Results on WF(M)200 for additional details on power consumption.