EZSP-UART Physical Interfaces#

The following figure represents the EZSP-UART physical interfaces.

Serial Data: Tx and Rx#

The EZSP protocol sends data in both directions, utilizing a standard Universal Asynchronous Receiver-Transmitter (UART). Both the Tx and Rx pins are utilized on the NCP and Host. When connecting the NCP and Host, the connections should be from NCP Tx to Host Rx and NCP Rx to Host Tx. This interface is required for NCP-to-Host communications.

Flow Control: RTS and CTS#

EZSP requires flow control for operation. This can be in the form of either XON/XOFF software flow control or RTS/CTS hardware flow control. RTS/CTS hardware flow control is part of the standard UART interface. When connecting the NCP and Host, the connections should be from NCP CTS to Host RTS and NCP RTS to Host CTS. With this interface, RTS enables transmission from the Host to the NCP and CTS enables NCP transmissions to the Host.

If needed, the Host can restrain the NCP from sending to it by deasserting the NCP’s CTS input. The NCP will stop sending immediately, except to finish sending a byte already in progress. The NCP will do the same to the Host when it requires the Host to stop sending data.

While hardware or software flow control can be utilized, EZSP-UART will have better performance and speed using hardware flow control. Hardware flow control allows operation up to 115,200 bps (other baud rates may be selected by a configuration value programmed into flash during manufacturing).

Reset Control: nRESET#

nRESET is a hardware input that can reset the NCP. For normal operation, the nRESET line should be held high. While pulling the line low, the NCP will hold in a reset state. Upon returning the line high, on the rising edge of the signal, the part will restart operation. Silicon Labs NCP chips include an internal pull-up resistor on this input so adding one is not necessary.

The Host must be able to reset the NCP to run the EZSP protocol. This can normally be accomplished utilizing software reset frames sent via EZSP from the Host to the NCP. However, in cases where the NCP fails to respond to EZSP frames, this hardware backup can be utilized.

While this interface is optional for NCP-to-Host communications, Silicon Labs recommends including nRESET in all Host-NCP hardware designs as a backup for situations where continued NCP operation is impossible or where unforeseen NCP operation blocks the Host.

For secure designs (utilizing secure EZSP) or security products, these connections should be considered mandatory.

Bootloader Recovery Pin#

The Bootload line allows the Host to force an NCP into bootloader mode during a reset or power cycle. For normal operation, the Boot- load line should be held high. During a reset (while the nRESET line is held low) or a power cycle, the Bootload line should be pulled low. Upon release of the nRESET line or powering up the NCP, the low input on the Bootload pin will signal the bootload software (Gecko or legacy) to override normal NCP operation, allowing the Host to update the firmware of the NCP. Because the Bootload pin can be configured on any input, a pull-up resistor should be placed on this input or set internally.

In Simplicity Studio, the GPIO associated with this Bootloader Pin can be found in the bootloader project: GPIO Activation component.

Bootloading is normally accomplished utilizing software bootload frames sent via EZSP from the Host to the NCP. However, in cases where the NCP fails to respond to EZSP frames, this hardware backup, in conjunction with nRESET, can be utilized.

While this interface is optional for NCP-to-Host communications Silicon Labs recommends including the Bootload pin in all Host-NCP hardware designs as a backup for situations where unforeseen operation or corruption block the NCP from responding to any software commands from the Host.

For secure designs (utilizing secure EZSP) or security products, these connections should be considered mandatory.

Note that when utilizing EZSP over CPC, the security can be implemented by configuring CPC to run with Security Enabled. This will encrypt all packets between the NCP and the Host.

Pin Connections#

The following table lists the pin connections.