Allow using software controlled Chip Select (CS) for MCP23S17.

  * This is useful when multiple devices, with a different address, are sharing the same CS line on the SPI bus.

components/mcp23x17/mcp23x17.c

@@ -149,6 +149,15 @@ static esp_err_t write_reg_bit_8(mcp23x17_t *dev, uint8_t reg, bool val, uint8_t
 
 #else
 
+static esp_err_t spi_transmit(mcp23x17_t *dev, spi_transaction_t *t)
+{
+    esp_err_t err;
+    if (dev->use_software_cs) gpio_set_level(dev->cs_pin, /*level=*/0);
+    err = spi_device_transmit(dev->spi_dev, t);
+    if (dev->use_software_cs) gpio_set_level(dev->cs_pin, /*level=*/1);
+    return err;
+}
+
 static esp_err_t read_reg_16(mcp23x17_t *dev, uint8_t reg, uint16_t *val)
 {
     CHECK_ARG(dev && val);
@@ -162,7 +171,7 @@ static esp_err_t read_reg_16(mcp23x17_t *dev, uint8_t reg, uint16_t *val)
     t.tx_buffer = tx;
     t.length = 32;   // 32 bits
 
-    CHECK(spi_device_transmit(dev->spi_dev, &t));
+    CHECK(spi_transmit(dev, &t));
 
     *val = (rx[3] << 8) | rx[2];
 
@@ -180,7 +189,7 @@ static esp_err_t write_reg_16(mcp23x17_t *dev, uint8_t reg, uint16_t val)
     t.tx_buffer = tx;
     t.length = 32;   // 32 bits
 
-    CHECK(spi_device_transmit(dev->spi_dev, &t));
+    CHECK(spi_transmit(dev, &t));
 
     return ESP_OK;
 }
@@ -208,7 +217,7 @@ static esp_err_t read_reg_8(mcp23x17_t *dev, uint8_t reg, uint8_t *val)
     t.tx_buffer = tx;
     t.length = 24;   // 24 bits
 
-    CHECK(spi_device_transmit(dev->spi_dev, &t));
+    CHECK(spi_transmit(dev, &t));
 
     *val = rx[2];
 
@@ -226,7 +235,7 @@ static esp_err_t write_reg_8(mcp23x17_t *dev, uint8_t reg, uint8_t val)
     t.tx_buffer = tx;
     t.length = 24;   // 24 bits
 
-    CHECK(spi_device_transmit(dev->spi_dev, &t));
+    CHECK(spi_transmit(dev, &t));
 
     return ESP_OK;
 }
@@ -310,9 +319,18 @@ esp_err_t mcp23x17_init_desc_spi(mcp23x17_t *dev, spi_host_device_t host, uint32
     }
 
     dev->addr = addr;
+    dev->cs_pin = cs_pin;
 
     memset(&dev->spi_cfg, 0, sizeof(dev->spi_cfg));
-    dev->spi_cfg.spics_io_num = cs_pin;
+    if (dev->use_software_cs) {
+        // Configure software Chip Select (CS) and pull the pin high.
+        dev->spi_cfg.spics_io_num = -1;
+        CHECK(gpio_set_direction(dev->cs_pin, GPIO_MODE_OUTPUT));
+        CHECK(gpio_set_level(dev->cs_pin, /*level=*/1));
+    } else {
+        // Configure hardware CS.
+        dev->spi_cfg.spics_io_num = dev->cs_pin;
+    }
     dev->spi_cfg.clock_speed_hz = clock_speed_hz;
     dev->spi_cfg.mode = 0;
     dev->spi_cfg.queue_size = 1;

components/mcp23x17/mcp23x17.h

@@ -65,6 +65,12 @@ typedef struct
     spi_device_interface_config_t spi_cfg;
     spi_device_handle_t spi_dev;
     uint8_t addr;
+    // Whether to use a software Chip Select (CS) line instead of the hardware
+    // one. This is useful when multiple MCP23S17 chips are sharing the same CS
+    // line on the SPI bus.
+    bool use_software_cs; //!< Use software CS control instead of hardware.
+    // CS GPIO pin number, populated by the `mcp23x17_init_desc_spi` function.
+    gpio_port_t cs_pin;   //!< GPIO pin number for CS.
 } mcp23x17_t;
 
 #endif