Add RP2040 single-wire UART write path for TMC2209 stepper drivers

.gitignore

@@ -1 +1,2 @@
 .DS_Store
+*.sublime-workspace

src/source/TMC2208Stepper.cpp

@@ -7,6 +7,10 @@
 #include "TMC_MACROS.h"
 #include "SERIAL_SWITCH.h"
 
+#if defined(ARDUINO_ARCH_RP2040) && defined(RP2040_SINGLE_WIRE_UART_PIN)
+  #include <hardware/gpio.h>
+#endif
+
 // Protected
 // addr needed for TMC2209
 TMC2208Stepper::TMC2208Stepper(Stream * SerialPort, float RS, uint8_t addr) :
@@ -225,16 +229,50 @@ void TMC2208Stepper::write(uint8_t addr, uint32_t regVal) {
 
 	preWriteCommunication();
 
-	for(uint8_t i=0; i<=len; i++) {
+	for (uint8_t i = 0;  i<= len; i++) {
 		bytesWritten += serial_write(datagram[i]);
 	}
 	postWriteCommunication();
 
 	delay(replyDelay);
 }
 
+#if defined(ARDUINO_ARCH_RP2040) && defined(RP2040_SINGLE_WIRE_UART_PIN)
+
+	#include <hardware/gpio.h>
+
+	// Bit-bang one 8N1 UART byte on the single-wire PDN_UART pin.
+	// Temporarily switches GPIO to SIO output for TX, then restores UART RX mode.
+	// 9 µs/bit ≈ 111 kbaud (-3.7% vs 115200) — within the ±6.25% 8N1 tolerance.
+	static void _rp2040_sw_uart_write_byte(uint8_t byte) {
+	    constexpr uint32_t T = 9; // µs per bit
+	    gpio_set_function(RP2040_SINGLE_WIRE_UART_PIN, GPIO_FUNC_SIO);
+	    gpio_set_dir(RP2040_SINGLE_WIRE_UART_PIN, GPIO_OUT);
+	    gpio_put(RP2040_SINGLE_WIRE_UART_PIN, 1); // idle HIGH before start bit
+	    // Start bit
+	    gpio_put(RP2040_SINGLE_WIRE_UART_PIN, 0);
+	    delayMicroseconds(T);
+	    // 8 data bits, LSB first
+	    for (int i = 0; i < 8; i++) {
+	        gpio_put(RP2040_SINGLE_WIRE_UART_PIN, (byte >> i) & 1);
+	        delayMicroseconds(T);
+	    }
+	    // Stop bit
+	    gpio_put(RP2040_SINGLE_WIRE_UART_PIN, 1);
+	    delayMicroseconds(T);
+	    // Restore UART1 RX function on GPIO9
+	    gpio_set_function(RP2040_SINGLE_WIRE_UART_PIN, GPIO_FUNC_UART);
+	}
+
+#endif // ARDUINO_ARCH_RP2040 && RP2040_SINGLE_WIRE_UART_PIN
+
 uint64_t TMC2208Stepper::_sendDatagram(uint8_t datagram[], const uint8_t len, uint16_t timeout) {
-	while (available() > 0) serial_read(); // Flush
+	// === STEP 0: flush stale RX bytes ===
+	const uint32_t _flush_t0 = micros();
+	while (available() > 0) {
+		serial_read();
+		if ((micros() - _flush_t0) > (uint32_t)timeout * 1000UL) break;
+	}
 
 	#if HAS_HALF_DUPLEX_MODE
 		if (RXTX_pin > 0) {
@@ -243,29 +281,33 @@ uint64_t TMC2208Stepper::_sendDatagram(uint8_t datagram[], const uint8_t len, ui
 		}
 	#endif
 
-	for(int i=0; i<=len; i++) serial_write(datagram[i]);
+	// === STEP 1: write request ===
+	#if defined(ARDUINO_ARCH_RP2040) && defined(RP2040_SINGLE_WIRE_UART_PIN)
+		noInterrupts();
+		for (int i = 0; i <= len; i++) _rp2040_sw_uart_write_byte(datagram[i]);
+		interrupts();
+	#else
+		for (int i = 0; i <= len; i++) serial_write(datagram[i]);
+	#endif
 
 	#if HAS_HALF_DUPLEX_MODE
 		if (RXTX_pin > 0) {
 			pinMode(RXTX_pin, INPUT_PULLUP);
 		}
 	#endif
 
+	// === STEP 2: reply delay ===
 	delay(this->replyDelay);
 
-	// scan for the rx frame and read it
-	uint32_t ms = millis();
+	// === STEP 3: sync scan with micros() timeout ===
+	uint32_t _sync_t0 = micros();
 	uint32_t sync_target = (static_cast<uint32_t>(datagram[0])<<16) | 0xFF00 | datagram[2];
 	uint32_t sync = 0;
 
 	do {
-		uint32_t ms2 = millis();
-		if (ms2 != ms) {
-			// 1ms tick
-			ms = ms2;
-			timeout--;
+		if ((micros() - _sync_t0) > (uint32_t)timeout * 1000UL) {
+			return 0;
 		}
-		if (!timeout) return 0;
 
 		int16_t res = serial_read();
 		if (res < 0) continue;
@@ -276,26 +318,21 @@ uint64_t TMC2208Stepper::_sendDatagram(uint8_t datagram[], const uint8_t len, ui
 
 	} while (sync != sync_target);
 
+	// === STEP 4: body read ===
 	uint64_t out = sync;
-	ms = millis();
+	uint32_t _body_t0 = micros();
 	timeout = this->abort_window;
 
-	for(uint8_t i=0; i<5;) {
-		uint32_t ms2 = millis();
-		if (ms2 != ms) {
-			// 1ms tick
-			ms = ms2;
-			timeout--;
+	for (uint8_t i = 0; i < 5; i++) {
+		if ((micros() - _body_t0) > (uint32_t)timeout * 1000UL) {
+			return 0;
 		}
-		if (!timeout) return 0;
 
 		int16_t res = serial_read();
 		if (res < 0) continue;
 
 		out <<= 8;
 		out |= res & 0xFF;
-
-		i++;
 	}
 
 	#if HAS_HALF_DUPLEX_MODE