Add setInverted() options to SerialPIO (#1451)

Call `SerialPIO::setInverted(txinv, rxinv)` before `SerialPIO::begin()` to enable.

---------
Co-authored-by: Mykle Hansen <mykle@mykle.com>
Co-authored-by: Earle F. Philhower, III <earlephilhower@yahoo.com>

cores/rp2040/SerialPIO.cpp

@@ -46,22 +46,24 @@ static pio_program_t *pio_make_uart_prog(int repl, const pio_program_t *pg) {
     return p;
 }
 
-static PIOProgram *_getTxProgram(int bits) {
-    auto f = _txMap.find(bits);
+static PIOProgram *_getTxProgram(int bits, bool inverted) {
+    int key = inverted ? -bits : bits;
+    auto f = _txMap.find(key);
     if (f == _txMap.end()) {
-        pio_program_t * p = pio_make_uart_prog(bits, &pio_tx_program);
-        _txMap.insert({bits, new PIOProgram(p)});
-        f = _txMap.find(bits);
+        pio_program_t * p = pio_make_uart_prog(bits, inverted ? &pio_tx_inv_program : &pio_tx_program);
+        _txMap.insert({key, new PIOProgram(p)});
+        f = _txMap.find(key);
     }
     return f->second;
 }
 
-static PIOProgram *_getRxProgram(int bits) {
-    auto f = _rxMap.find(bits);
+static PIOProgram *_getRxProgram(int bits, bool inverted) {
+    int key = inverted ? -bits : bits;
+    auto f = _rxMap.find(key);
     if (f == _rxMap.end()) {
-        pio_program_t * p = pio_make_uart_prog(bits, &pio_rx_program);
-        _rxMap.insert({bits, new PIOProgram(p)});
-        f = _rxMap.find(bits);
+        pio_program_t * p = pio_make_uart_prog(bits, inverted ? &pio_rx_inv_program : &pio_rx_program);
+        _rxMap.insert({key, new PIOProgram(p)});
+        f = _rxMap.find(key);
     }
     return f->second;
 }
@@ -96,7 +98,7 @@ void __not_in_flash_func(SerialPIO::_handleIRQ)() {
         return;
     }
     while (!pio_sm_is_rx_fifo_empty(_rxPIO, _rxSM)) {
-        uint32_t decode = _rxPIO->rxf[_rxSM];
+        uint32_t decode = _rxPIO->rxf[_rxSM] ^ (_rxInverted ? 0xffffffff : 0);
         decode >>= 33 - _rxBits;
         uint32_t val = 0;
         for (int b = 0; b < _bits + 1; b++) {
@@ -189,7 +191,7 @@ void SerialPIO::begin(unsigned long baud, uint16_t config) {
 
     if (_tx != NOPIN) {
         _txBits = _bits + _stop + (_parity != UART_PARITY_NONE ? 1 : 0) + 1/*start bit*/;
-        _txPgm = _getTxProgram(_txBits);
+        _txPgm = _getTxProgram(_txBits, _txInverted);
         int off;
         if (!_txPgm->prepare(&_txPIO, &_txSM, &off)) {
             DEBUGCORE("ERROR: Unable to allocate PIO TX UART, out of PIO resources\n");
@@ -216,7 +218,7 @@ void SerialPIO::begin(unsigned long baud, uint16_t config) {
         _reader = 0;
 
         _rxBits = 2 * (_bits + _stop + (_parity != UART_PARITY_NONE ? 1 : 0) + 1) - 1;
-        _rxPgm = _getRxProgram(_rxBits);
+        _rxPgm = _getRxProgram(_rxBits, _rxInverted);
         int off;
         if (!_rxPgm->prepare(&_rxPIO, &_rxSM, &off)) {
             DEBUGCORE("ERROR: Unable to allocate PIO RX UART, out of PIO resources\n");
@@ -346,6 +348,11 @@ void SerialPIO::flush() {
     delay((1000 * (_txBits + 1)) / _baud);
 }
 
+void SerialPIO::setInverted(bool invTx, bool invRx) {
+    _txInverted = invTx;
+    _rxInverted = invRx;
+}
+
 size_t SerialPIO::write(uint8_t c) {
     CoreMutex m(&_mutex);
     if (!_running || !m || (_tx == NOPIN)) {
@@ -364,7 +371,7 @@ size_t SerialPIO::write(uint8_t c) {
     }
     val <<= 1;  // Start bit = low
 
-    pio_sm_put_blocking(_txPIO, _txSM, val);
+    pio_sm_put_blocking(_txPIO, _txSM, _txInverted ? ~val : val);
 
     return 1;
 }

cores/rp2040/SerialPIO.h

@@ -41,6 +41,8 @@ public:
     void begin(unsigned long baud, uint16_t config) override;
     void end() override;
 
+    void setInverted(bool invTx = true, bool invRx = true);
+
     virtual int peek() override;
     virtual int read() override;
     virtual int available() override;
@@ -63,6 +65,8 @@ protected:
     int _stop;
     bool _overflow;
     mutex_t _mutex;
+    bool _txInverted = false;
+    bool _rxInverted = false;
 
     PIOProgram *_txPgm;
     PIO _txPIO;

cores/rp2040/pio_uart.pio

@@ -29,7 +29,7 @@
 ; We shift out the start and stop bit as part of the FIFO
     set x, 9
 
-    pull               side 1 ; Force stop bit
+    pull               side 1 ; Force stop bit high
 
 ; Send the bits
 bitloop:
@@ -41,6 +41,26 @@ wait_bit:
 
     
 
+; inverted-logic version (inverts the stop bit)
+
+.program pio_tx_inv
+.side_set 1 opt
+
+
+; We shift out the start and stop bit as part of the FIFO
+    set x, 9
+
+    pull               side 0 ; Force stop bit low
+
+; Send the bits
+bitloop:
+    out pins, 1
+    mov y, isr                ; ISR is loaded by the setup routine with the period-1 count
+wait_bit:
+    jmp y-- wait_bit
+    jmp x-- bitloop
+
+
 % c-sdk {
 
 static inline void pio_tx_program_init(PIO pio, uint sm, uint offset, uint pin_tx) {
@@ -68,7 +88,7 @@ static inline void pio_tx_program_init(PIO pio, uint sm, uint offset, uint pin_t
 }
 
 %}
-
+    
 
 .program pio_rx
 
@@ -91,6 +111,27 @@ wait_half:
     push                ; Stuff it and wait for next start
 
 
+.program pio_rx_inv
+
+; IN pin 0 and JMP pin are both mapped to the GPIO used as UART RX.
+
+start:
+    set x, 18           ; Preload bit counter...we'll shift in the start bit and stop bit, and each bit will be double-recorded (to be fixed by RP2040 code)
+    wait 1 pin 0        ; Stall until start bit is asserted
+
+bitloop:
+   ; Delay until 1/2 way into the bit time
+    mov y, osr
+wait_half:
+    jmp y-- wait_half
+
+    ; Read in the bit
+    in pins, 1          ; Shift data bit into ISR
+    jmp x-- bitloop     ; Loop all bits
+    
+    push                ; Stuff it and wait for next start
+
+
 % c-sdk {
 static inline void pio_rx_program_init(PIO pio, uint sm, uint offset, uint pin) {
     pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, false);

cores/rp2040/pio_uart.pio.h

@@ -2,6 +2,8 @@
 // This file is autogenerated by pioasm; do not edit! //
 // -------------------------------------------------- //
 
+#pragma once
+
 #if !PICO_NO_HARDWARE
 #include "hardware/pio.h"
 #endif
@@ -37,6 +39,39 @@ static inline pio_sm_config pio_tx_program_get_default_config(uint offset) {
     sm_config_set_sideset(&c, 2, true, false);
     return c;
 }
+#endif
+
+// ---------- //
+// pio_tx_inv //
+// ---------- //
+
+#define pio_tx_inv_wrap_target 0
+#define pio_tx_inv_wrap 5
+
+static const uint16_t pio_tx_inv_program_instructions[] = {
+    //     .wrap_target
+    0xe029, //  0: set    x, 9
+    0x90a0, //  1: pull   block           side 0
+    0x6001, //  2: out    pins, 1
+    0xa046, //  3: mov    y, isr
+    0x0084, //  4: jmp    y--, 4
+    0x0042, //  5: jmp    x--, 2
+    //     .wrap
+};
+
+#if !PICO_NO_HARDWARE
+static const struct pio_program pio_tx_inv_program = {
+    .instructions = pio_tx_inv_program_instructions,
+    .length = 6,
+    .origin = -1,
+};
+
+static inline pio_sm_config pio_tx_inv_program_get_default_config(uint offset) {
+    pio_sm_config c = pio_get_default_sm_config();
+    sm_config_set_wrap(&c, offset + pio_tx_inv_wrap_target, offset + pio_tx_inv_wrap);
+    sm_config_set_sideset(&c, 2, true, false);
+    return c;
+}
 
 static inline void pio_tx_program_init(PIO pio, uint sm, uint offset, uint pin_tx) {
     // Tell PIO to initially drive output-high on the selected pin, then map PIO
@@ -90,6 +125,39 @@ static inline pio_sm_config pio_rx_program_get_default_config(uint offset) {
     sm_config_set_wrap(&c, offset + pio_rx_wrap_target, offset + pio_rx_wrap);
     return c;
 }
+#endif
+
+// ---------- //
+// pio_rx_inv //
+// ---------- //
+
+#define pio_rx_inv_wrap_target 0
+#define pio_rx_inv_wrap 6
+
+static const uint16_t pio_rx_inv_program_instructions[] = {
+    //     .wrap_target
+    0xe032, //  0: set    x, 18
+    0x20a0, //  1: wait   1 pin, 0
+    0xa047, //  2: mov    y, osr
+    0x0083, //  3: jmp    y--, 3
+    0x4001, //  4: in     pins, 1
+    0x0042, //  5: jmp    x--, 2
+    0x8020, //  6: push   block
+    //     .wrap
+};
+
+#if !PICO_NO_HARDWARE
+static const struct pio_program pio_rx_inv_program = {
+    .instructions = pio_rx_inv_program_instructions,
+    .length = 7,
+    .origin = -1,
+};
+
+static inline pio_sm_config pio_rx_inv_program_get_default_config(uint offset) {
+    pio_sm_config c = pio_get_default_sm_config();
+    sm_config_set_wrap(&c, offset + pio_rx_inv_wrap_target, offset + pio_rx_inv_wrap);
+    return c;
+}
 
 static inline void pio_rx_program_init(PIO pio, uint sm, uint offset, uint pin) {
     pio_sm_set_consecutive_pindirs(pio, sm, pin, 1, false);