Slave Slack(SS) Function Enable in SPI

Mbed OS
1

Hi,
I have a question about ssel pin in SPI.
According to the Mbed OS 6 Document, It shows that only CS is config to DigitalOut.
It means that CS is controlled by GPIO, not by SPI right?
I think it may cost more latency during SPI transactions.
Can you help me how to config CS without DigitalOut API?

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2

Hello MinHsuan_Wang

According to the Mbed API documentation for SPI you can use for that purpose the constructor with four arguments:

SPI ( PinName mosi ,
PinName miso ,
PinName sclk ,
PinName ssel = NC
)

This constructor passes the SSEL pin selection to the target HAL. Not all targets support SSEL, so this cannot be relied on in portable code.

Parameters

mosi SPI Master Out, Slave In pin.
miso SPI Master In, Slave Out pin.
sclk SPI Clock pin.
ssel SPI Chip Select pin.

To find out which pin can be used as Slave (Chip) Select on STM targets open the corresponding PeripheralPins.c file and search for PinMap_SPI_SSEL[]. For example, in case of the NUCLEO-F446RE you can find the following array in the mbed-os/targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F446xE/TARGET_NUCLEO_F446RE/PeripheralPins.c:

...
MBED_WEAK const PinMap PinMap_SPI_SSEL[] = {
    {PA_4,       SPI_1, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF5_SPI1)},
    {PA_4_ALT0,  SPI_3, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF6_SPI3)},
    {PA_15,      SPI_1, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF5_SPI1)},
    {PA_15_ALT0, SPI_3, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF6_SPI3)},
    {PB_4,       SPI_2, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF7_SPI2)},
    {PB_9,       SPI_2, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF5_SPI2)},
    {PB_12,      SPI_2, STM_PIN_DATA(STM_MODE_AF_PP, GPIO_NOPULL, GPIO_AF5_SPI2)},
    {NC, NC, 0}
};
...

So if you’d like to use the SPI_1 peripheral then you can configure/initialize it as follows:

SPI spi(D11, D12, D13, PA_4);  // mosi, miso, sclk, ssel

NOTE:
SSEL (Slave Select) = SS (Slave Select) = CS (Chip Select)

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