Allow SPI to disable RX or TX pins

Pass in `NOPIN` to `setTX` or `setRX` to make a unidirectional SPI
interface.

Fixes #3115

Author: earlephilhower

cores/rp2040/Arduino.h

@@ -170,6 +170,9 @@ constexpr uint64_t __bitset(const int (&a)[N], size_t i = 0U) {
 #define __FIRSTANALOGGPIO 26
 #endif
 
+// For peripherals where a pin may be not used
+#define NOPIN ((pin_size_t) 0xff)
+
 #ifdef __cplusplus
 using namespace arduino;
 #endif

cores/rp2040/SerialPIO.h

@@ -29,7 +29,6 @@ extern "C" typedef struct uart_inst uart_inst_t;
 
 class SerialPIO : public arduino::HardwareSerial {
 public:
-    static const pin_size_t NOPIN = 0xff; // Use in constructor to disable RX or TX unit
     SerialPIO(pin_size_t tx, pin_size_t rx, size_t fifoSize = 32);
     ~SerialPIO();
 

docs/piouart.rst

@@ -12,13 +12,13 @@ are supported, as well as data sizes from 5- to 8-bits.  Fifosize, if not
 specified, defaults to 32 bytes.
 
 To instantiate only a serial transmit or receive unit, pass in
-``SerialPIO::NOPIN`` as the ``txpin`` or ``rxpin``.
+``NOPIN`` as the ``txpin`` or ``rxpin``.
 
 For example, to make a transmit-only port on GP16
 
 .. code:: cpp
 
-        SerialPIO transmitter( 16, SerialPIO::NOPIN );
+        SerialPIO transmitter( 16, NOPIN );
 
 For detailed information about the Serial ports, see the
 Arduino `Serial Reference <https://www.arduino.cc/reference/en/language/functions/communication/serial/>`_ .

docs/spi.rst

@@ -16,6 +16,9 @@ SPI pinouts can be set **before SPI.begin()** using the following calls:
 Note that the ``CS`` pin can be hardware or software controlled by the sketch.
 When software controlled, the ``setCS()`` call is ignored.
 
+Passing in ``NOPIN`` to either ``setRX`` or ``setTX`` will disable SPI on that
+interface, making it behave as a unidirectional (output- or input-only SPI interface.
+
 The Arduino `SPI documentation <https://docs.arduino.cc/language-reference/en/functions/communication/SPI/>`_ gives
 a detailed overview of the library, except for the following RP2040-specific
 changes:

libraries/SPI/src/SPI.cpp

@@ -281,6 +281,11 @@ bool SPIClassRP2040::setRX(pin_size_t pin) {
         return true;
     }
 
+    if (!_running && (pin == NOPIN)) {
+        _RX = pin;
+        return true;
+    }
+
     if (_running) {
         panic("FATAL: Attempting to set SPI%s.RX while running", spi_get_index(_spi) ? "1" : "");
     } else {
@@ -362,6 +367,11 @@ bool SPIClassRP2040::setTX(pin_size_t pin) {
         return true;
     }
 
+    if (!_running && (pin == NOPIN)) {
+        _TX = pin;
+        return true;
+    }
+
     if (_running) {
         panic("FATAL: Attempting to set SPI%s.TX while running", spi_get_index(_spi) ? "1" : "");
     } else {
@@ -372,13 +382,22 @@ bool SPIClassRP2040::setTX(pin_size_t pin) {
 
 void SPIClassRP2040::begin(bool hwCS) {
     DEBUGSPI("SPI::begin(%d), rx=%d, cs=%d, sck=%d, tx=%d\n", hwCS, _RX, _CS, _SCK, _TX);
-    gpio_set_function(_RX, GPIO_FUNC_SPI);
+
+    if ((_RX == NOPIN) && (_TX == NOPIN)) {
+        panic("SPI%s: TX and RX assigned to NOPIN", spi_get_index(_spi) ? "1" : "");
+    }
+
+    if (_RX != NOPIN) {
+        gpio_set_function(_RX, GPIO_FUNC_SPI);
+    }
     _hwCS = hwCS;
     if (hwCS) {
         gpio_set_function(_CS, GPIO_FUNC_SPI);
     }
     gpio_set_function(_SCK, GPIO_FUNC_SPI);
-    gpio_set_function(_TX, GPIO_FUNC_SPI);
+    if (_TX != NOPIN) {
+        gpio_set_function(_TX, GPIO_FUNC_SPI);
+    }
     // Give a default config in case user doesn't use beginTransaction
     beginTransaction(_spis);
     endTransaction();
@@ -391,12 +410,16 @@ void SPIClassRP2040::end() {
         _initted = false;
         spi_deinit(_spi);
     }
-    gpio_set_function(_RX, GPIO_FUNC_SIO);
+    if (_RX != NOPIN) {
+        gpio_set_function(_RX, GPIO_FUNC_SIO);
+    }
     if (_hwCS) {
         gpio_set_function(_CS, GPIO_FUNC_SIO);
     }
     gpio_set_function(_SCK, GPIO_FUNC_SIO);
-    gpio_set_function(_TX, GPIO_FUNC_SIO);
+    if (_TX != NOPIN) {
+        gpio_set_function(_TX, GPIO_FUNC_SIO);
+    }
     _spis = SPISettings(0, LSBFIRST, SPI_MODE0);
 }
 

libraries/SPI/src/SPI.h

@@ -31,21 +31,21 @@
 class SPIClassRP2040 : public arduino::HardwareSPI {
 public:
     /**
-            @brief Create a PIO-based SPI instance, pins can be changed before begin() call
+        @brief Create a PIO-based SPI instance, pins can be changed before begin() call
 
-            @param [in] spi SPI hardware instance (spi0/spi1)
-            @param [in] rx MISO GPIO
-            @param [in] cs CS GPIO
-            @param [in] sck SCK GPIO
-            @param [in] tx MOSI GPIO
+        @param [in] spi SPI hardware instance (spi0/spi1)
+        @param [in] rx MISO GPIO
+        @param [in] cs CS GPIO
+        @param [in] sck SCK GPIO
+        @param [in] tx MOSI GPIO
     */
     SPIClassRP2040(spi_inst_t *spi, pin_size_t rx, pin_size_t cs, pin_size_t sck, pin_size_t tx);
 
     /**
-           @brief Send an 8-bit byte of data and return read-back 8-bit value
+        @brief Send an 8-bit byte of data and return read-back 8-bit value
 
-           @param [in] data Data to send
-           @returns Read back byte from SPI interface
+        @param [in] data Data to send
+        @returns Read back byte from SPI interface
     */
     byte transfer(uint8_t data) override;
 
@@ -67,11 +67,11 @@ class SPIClassRP2040 : public arduino::HardwareSPI {
     void transfer(void *buf, size_t count) override;
 
     /**
-            @brief Sends one buffer and receives into another under a single CS. Can set rx or txbuf to nullptr
+        @brief Sends one buffer and receives into another under a single CS. Can set rx or txbuf to nullptr
 
-            @param [in] txbuf Buffer to transmit or nullptr to send 0s
-            @param [out] rxbuf Buffer to read back into or nullptr to ignore returned data
-            @param [in] count Numbner of bytes to transmit/receive
+        @param [in] txbuf Buffer to transmit or nullptr to send 0s
+        @param [out] rxbuf Buffer to read back into or nullptr to ignore returned data
+        @param [in] count Numbner of bytes to transmit/receive
     */
     void transfer(const void *txbuf, void *rxbuf, size_t count) override;
 
@@ -108,134 +108,134 @@ class SPIClassRP2040 : public arduino::HardwareSPI {
 
 
     /**
-           @brief Begin an SPI transaction, sets SPI speed and masks necessary interrupts
+        @brief Begin an SPI transaction, sets SPI speed and masks necessary interrupts
 
-           @param [in] SPISettings SPI configuration parameters, including the clock speed
+        @param [in] SPISettings SPI configuration parameters, including the clock speed
     */
     void beginTransaction(SPISettings settings) override;
 
     /**
-            @brief Ends an SPI transaction, unmasks and masked GPIO interrupts
+        @brief Ends an SPI transaction, unmasks and masked GPIO interrupts
     */
     void endTransaction(void) override;
 
     /**
-           @brief Sets the MISO(RX) pin. Call before begin()
+        @brief Sets the MISO(RX) pin. Call before begin()
 
-           @param [in] pin The GPIO number to assign to
-           @returns True on success
+        @param [in] pin The GPIO number to assign to or NOPIN
+        @returns True on success
     */
     bool setRX(pin_size_t pin);
 
     /**
-            @brief Sets the MISO(RX) pin. Call before begin()
+        @brief Sets the MISO(RX) pin. Call before begin()
 
-            @param [in] pin The GPIO number to assign to
-            @returns True on success
+        @param [in] pin The GPIO number to assign to
+        @returns True on success
     */
     inline bool setMISO(pin_size_t pin) {
         return setRX(pin);
     }
 
     /**
-            @brief Sets the CS pin. Call before begin()
+        @brief Sets the CS pin. Call before begin()
 
-            @param [in] pin The GPIO number to assign to
-            @returns True on success
+        @param [in] pin The GPIO number to assign to
+        @returns True on success
     */
     bool setCS(pin_size_t pin);
 
     /**
-            @brief Sets the SCK pin. Call before begin()
+        @brief Sets the SCK pin. Call before begin()
 
-            @param [in] pin The GPIO number to assign to
-            @returns True on success
+        @param [in] pin The GPIO number to assign to
+        @returns True on success
     */
     bool setSCK(pin_size_t pin);
 
     /**
-          @brief Sets the MOSI(TX) pin. Call before begin()
+        @brief Sets the MOSI(TX) pin. Call before begin()
 
-          @param [in] pin The GPIO number to assign to
-          @returns True on success
+        @param [in] pin The GPIO number to assign to
+        @returns True on success
     */
     bool setTX(pin_size_t pin);
 
     /**
-          @brief Sets the MOSI(TX) pin. Call before begin()
+        @brief Sets the MOSI(TX) pin. Call before begin()
 
-          @param [in] pin The GPIO number to assign to
-          @returns True on success
+        @param [in] pin The GPIO number to assign to or NOPIN
+        @returns True on success
     */
     inline bool setMOSI(pin_size_t pin) {
         return setTX(pin);
     }
 
     /**
-            @brief Call once to init/deinit SPI class, select pins, etc.
+        @brief Call once to init/deinit SPI class, select pins, etc.
     */
     virtual void begin() override {
         begin(false);
     }
 
     /**
-           @brief Call once to init/deinit SPI class, select pins, etc.
+        @brief Call once to init/deinit SPI class, select pins, etc.
 
-           @param [in] hwCS Pass in true to enable HW-controlled CS.  Otherwise application needs to assert/deassert CS.
+        @param [in] hwCS Pass in true to enable HW-controlled CS.  Otherwise application needs to assert/deassert CS.
     */
     void begin(bool hwCS);
 
     /**
-           @brief Call to deinit and disable the SPI interface.
+        @brief Call to deinit and disable the SPI interface.
     */
     void end() override;
 
     /**
-            @brief Deprecated, do not use
+        @brief Deprecated, do not use
 
-            @param [in] order Deprecated
+        @param [in] order Deprecated
     */
     void setBitOrder(BitOrder order) __attribute__((deprecated));
 
     /**
-            @brief Deprecated, do not use
+        @brief Deprecated, do not use
 
-            @param [in] order Deprecated
+        @param [in] order Deprecated
     */
     void setDataMode(uint8_t uc_mode) __attribute__((deprecated));
 
     /**
-            @brief Deprecated, do not use
+        @brief Deprecated, do not use
 
-            @param [in] order Deprecated
+        @param [in] order Deprecated
     */
     void setClockDivider(uint8_t uc_div) __attribute__((deprecated));
 
     /**
-            @brief Ensure specific GPIO interrupt is disabled during and SPI transaction to protect against re-entrancy.  Multiple GPIOs supported by multiple calls.
+        @brief Ensure specific GPIO interrupt is disabled during and SPI transaction to protect against re-entrancy.  Multiple GPIOs supported by multiple calls.
 
-            @param [in] interruptNumber GPIO pin to mask
+        @param [in] interruptNumber GPIO pin to mask
     */
     virtual void usingInterrupt(int interruptNumber) override {
         _helper.usingInterrupt(interruptNumber);
     }
 
     /**
-            @brief Remove a GPIO from the masked-during-transaction list.
+        @brief Remove a GPIO from the masked-during-transaction list.
 
-            @param [in] interruptNumber GPIO pin to unmask
+        @param [in] interruptNumber GPIO pin to unmask
     */
     virtual void notUsingInterrupt(int interruptNumber) override {
         _helper.notUsingInterrupt(interruptNumber);
     }
 
     /**
-           @brief Deprecated, do not use
+        @brief Deprecated, do not use
     */
     virtual void attachInterrupt() override  __attribute__((deprecated)) { /* noop */ }
 
     /**
-           @brief Deprecated, do not use
+        @brief Deprecated, do not use
     */
     virtual void detachInterrupt() override  __attribute__((deprecated)) { /* noop */ }