How do I connect more than 3 SPI devices (IDFGH-1225)

[amoghskulkarni]

spi_master.c driver is implemented in such a way that there is a hard limit on the maximum number of SPI devices that can be communicated with (as shown here). I have two questions regarding this issue -

1. Why this hard limit?
2. Is there a work around for connecting more than 3 SPI devices?

P.S. - I tried to manually change the macro value to more than 3, but it didn't change anything (besides not giving the error). I found a reddit thread that discusses this issue and a possible workaround, but I didn't understand how to use the workaround in my code.

[nopnop2002]

Hello.
This is my idea, and I have not tried.

spi_device_interface_config_t devcfg;
memset( &devcfg, 0, sizeof( spi_device_interface_config_t ) );
devcfg.clock_speed_hz = SPI_Frequency;
devcfg.spics_io_num = -1; // CS GPIO pin is not use.
devcfg.queue_size = 1;
spi_device_handle_t handle;
ret = spi_bus_add_device( HSPI_HOST, &devcfg, &handle);

CS of SPI device #1 is GPIO_XX
CS of SPI device #2 is GPIO_YY
CS of SPI device #3 is GPIO_ZZ

You set GPIO_XX GPIO_YY GPIO_ZZ manualy.

[ginkgm]

Hi all,
The hardware only has three CS signals, so that we set the limit to 3 in the early version.
If you want to support more chips, you can set NO_CS definition to greater value. And use software cs (controlled by gpio_set_level functions) in the pretrans and posttrans callbacks to activate corresponding slave.

Please take care of all cases where 3 or NO_CS are used in spi_master.c yourself. For example, you have to forbid the cs initialization when the device is 4 or larger:
spi_master.c:433

    //Set CS pin, CS options
    if (dev_config->spics_io_num >= 0) {
        spicommon_cs_initialize(host, dev_config->spics_io_num, freecs, !(spihost[host]->flags&SPICOMMON_BUSFLAG_NATIVE_PINS));
    }

BTW, kindly notice: more slaves connected to the same SPI bus will cause the timing to be worse. The delay of all lines will be greater, which will limit your working frequency.

[amoghskulkarni]

@ginkgm Thank you for the explanation. I am currently trying to get the software CS working for more than 3 devices, will be dropping any new development update/question on this issue. Please feel free to close the issue.

[amoghskulkarni]

If you want to support more chips, you can set NO_CS definition to greater value. And use software cs (controlled by gpio_set_level functions) in the pretrans and posttrans callbacks to activate corresponding slave.

@ginkgm I tried this and my 4th SPI device still wouldn't respond. My setup looks something like this -

void swcs_pre_cb (spi_transaction_t *trans){
    // reset the pin
    gpio_set_level(CS_PIN, 0);
    return;
}

void swcs_post_cb (spi_transaction_t *trans) {
    // set the pin
    gpio_set_level(CS_PIN, 1);
    return;
}

...
extern "C" void app_main(void)
{
...
    spi_device_interface_config_t dev_config;

    dev_config.command_bits = 0;
    dev_config.address_bits = 8;
    dev_config.dummy_bits = 0;
    dev_config.mode = 0;
    dev_config.duty_cycle_pos = 128;  // default 128 = 50%/50% duty
    dev_config.cs_ena_pretrans = 0;  // 0 not used
    dev_config.cs_ena_posttrans = 0;  // 0 not used
    dev_config.clock_speed_hz = 1000000;
    dev_config.spics_io_num = -1; // SW cs 
    dev_config.flags = 0;  // 0 not used
    dev_config.queue_size = 1;
    dev_config.pre_cb = swcs_pre_cb;
    dev_config.post_cb = swcs_post_cb;

    hspi.addDevice(&dev_config, &spi_handle);

   gpio_set_direction(CS_PIN, GPIO_MODE_INPUT_OUTPUT);
   PIN_FUNC_SELECT(CS_PIN, PIN_FUNC_GPIO);
...
}

The exact same configuration works for the first 3 devices with NULL callbacks and different spics_io_num values, which brings me to the following comment of yours -

Please take care of all cases where 3 or NO_CS are used in spi_master.c yourself. For example, you have to forbid the cs initialization when the device is 4 or larger:
spi_master.c:433

I could not determine which parts in spi_master.c need changes, because wherever NO_CS is being used, it is used in the bus acquiring logic which my 4th device needs anyway.

P.S. For my 4th device, I am taking care that the CS pins don't get initialized within spi_master.c driver functions by providing -1 to spics_io_num variable of the config object.

[o-marshmallow]

Closing this issue, if you are still countering this issue, please open a new ticket.