From ea003da13f37357a248a1f9aa4300957936a7762 Mon Sep 17 00:00:00 2001
From: Konstantin Sharlaimov <konstantin.sharlaimov@gmail.com>
Date: Fri, 17 Jul 2026 10:45:50 +0200
Subject: [PATCH] Add UART line inversion support (#5522)

* feat(machine): add UART line inversion support.

Add InvertTX and InvertRX to UARTConfig to allow enabling hardware
line inversion on supported targets. Added hardware implementation for
RP2 (RP2040, RP2350), STM32 (newer families), SAM (SAMD51, SAME5x),
and ESP (ESP32, ESP32-C3, ESP32-C6).

* refactor(machine): Refactor UART inversion with pin setter helpers.

RP2: Extract setOutOver/setInOver methods on Pin, removing inline
IO control register manipulation from UART configure.

SAM: Switch to SetCTRLA_TXINV/SetCTRLA_RXINV methods, dropping
the unused device/sam import and raw SetBits/ClearBits calls.

---------

Co-authored-by: Konstantin Sharlaimov <ksharlaimov@inavflight.com>
---
 src/machine/machine_atsamd21.go            |  3 +++
 src/machine/machine_atsamd51.go            |  3 +++
 src/machine/machine_esp32.go               | 10 ++++++++++
 src/machine/machine_esp32c3.go             | 12 ++++++++++--
 src/machine/machine_esp32c6.go             | 12 ++++++++++--
 src/machine/machine_rp2_gpio.go            | 10 ++++++++++
 src/machine/machine_rp2_uart.go            | 10 ++++++++++
 src/machine/machine_sam_uart_invert.go     | 17 +++++++++++++++++
 src/machine/machine_sam_uart_noinvert.go   |  7 +++++++
 src/machine/machine_stm32_uart.go          |  3 +++
 src/machine/machine_stm32_uart_invert.go   | 17 +++++++++++++++++
 src/machine/machine_stm32_uart_noinvert.go |  7 +++++++
 src/machine/serial.go                      |  2 ++
 13 files changed, 109 insertions(+), 4 deletions(-)
 create mode 100644 src/machine/machine_sam_uart_invert.go
 create mode 100644 src/machine/machine_sam_uart_noinvert.go
 create mode 100644 src/machine/machine_stm32_uart_invert.go
 create mode 100644 src/machine/machine_stm32_uart_noinvert.go

diff --git a/src/machine/machine_atsamd21.go b/src/machine/machine_atsamd21.go
index b46dcef94..5768d8f04 100644
--- a/src/machine/machine_atsamd21.go
+++ b/src/machine/machine_atsamd21.go
@@ -621,6 +621,9 @@ func (uart *UART) Configure(config UARTConfig) error {
 	//sercom->USART.CTRLB.reg |= SERCOM_USART_CTRLB_TXEN | SERCOM_USART_CTRLB_RXEN ;
 	uart.Bus.CTRLB.SetBits(sam.SERCOM_USART_CTRLB_TXEN | sam.SERCOM_USART_CTRLB_RXEN)
 
+	// Configure RX/TX inversion
+	uart.setInversion(config)
+
 	// Enable USART1 port.
 	// sercom->USART.CTRLA.bit.ENABLE = 0x1u;
 	uart.Bus.CTRLA.SetBits(sam.SERCOM_USART_CTRLA_ENABLE)
diff --git a/src/machine/machine_atsamd51.go b/src/machine/machine_atsamd51.go
index 158be4911..1c7ea0411 100644
--- a/src/machine/machine_atsamd51.go
+++ b/src/machine/machine_atsamd51.go
@@ -1102,6 +1102,9 @@ func (uart *UART) Configure(config UARTConfig) error {
 	//sercom->USART.CTRLB.reg |= SERCOM_USART_CTRLB_TXEN | SERCOM_USART_CTRLB_RXEN ;
 	uart.Bus.CTRLB.SetBits(sam.SERCOM_USART_INT_CTRLB_TXEN | sam.SERCOM_USART_INT_CTRLB_RXEN)
 
+	// Configure RX/TX inversion
+	uart.setInversion(config)
+
 	// Enable USART1 port.
 	// sercom->USART.CTRLA.bit.ENABLE = 0x1u;
 	uart.Bus.CTRLA.SetBits(sam.SERCOM_USART_INT_CTRLA_ENABLE)
diff --git a/src/machine/machine_esp32.go b/src/machine/machine_esp32.go
index 3f715e6de..060a3b0ba 100644
--- a/src/machine/machine_esp32.go
+++ b/src/machine/machine_esp32.go
@@ -332,10 +332,20 @@ func (uart *UART) Configure(config UARTConfig) {
 
 	if config.RX != NoPin {
 		config.RX.configure(PinConfig{Mode: PinInputPullup}, uart.TXRXSignal)
+		if config.InvertRX {
+			inFunc(uart.TXRXSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX)<<esp.GPIO_FUNC_IN_SEL_CFG_IN_SEL_Pos | esp.GPIO_FUNC_IN_SEL_CFG_IN_INV_SEL)
+		} else {
+			inFunc(uart.TXRXSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX)<<esp.GPIO_FUNC_IN_SEL_CFG_IN_SEL_Pos)
+		}
 	}
 
 	if config.TX != NoPin {
 		config.TX.configure(PinConfig{Mode: PinOutput}, uart.TXRXSignal)
+		if config.InvertTX {
+			config.TX.outFunc().Set(uart.TXRXSignal | esp.GPIO_FUNC_OUT_SEL_CFG_INV_SEL)
+		} else {
+			config.TX.outFunc().Set(uart.TXRXSignal)
+		}
 	}
 
 	if config.RTS != NoPin {
diff --git a/src/machine/machine_esp32c3.go b/src/machine/machine_esp32c3.go
index 2fd1746b3..1fa90e774 100644
--- a/src/machine/machine_esp32c3.go
+++ b/src/machine/machine_esp32c3.go
@@ -447,9 +447,17 @@ func (uart *UART) setupPins(config UARTConfig, regs registerSet) {
 	config.TX.Configure(PinConfig{Mode: PinInputPullup})
 
 	// link TX with GPIO signal X (technical reference manual 5.10) (this is not interrupt signal!)
-	config.TX.outFunc().Set(regs.gpioMatrixSignal)
+	if config.InvertTX {
+		config.TX.outFunc().Set(regs.gpioMatrixSignal | esp.GPIO_FUNC_OUT_SEL_CFG_INV_SEL)
+	} else {
+		config.TX.outFunc().Set(regs.gpioMatrixSignal)
+	}
 	// link RX with GPIO signal X and route signals via GPIO matrix (GPIO_SIGn_IN_SEL 0x40)
-	inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX))
+	if config.InvertRX {
+		inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX) | esp.GPIO_FUNC_IN_SEL_CFG_IN_INV_SEL)
+	} else {
+		inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX))
+	}
 }
 
 func (uart *UART) configureInterrupt(intrMapReg *volatile.Register32) { // Disable all UART interrupts
diff --git a/src/machine/machine_esp32c6.go b/src/machine/machine_esp32c6.go
index 50996dd72..683de1f7c 100644
--- a/src/machine/machine_esp32c6.go
+++ b/src/machine/machine_esp32c6.go
@@ -460,9 +460,17 @@ func (uart *UART) setupPins(config UARTConfig, regs registerSet) {
 	config.TX.Configure(PinConfig{Mode: PinInputPullup})
 
 	// link TX with GPIO signal X (technical reference manual, GPIO matrix)
-	config.TX.outFunc().Set(regs.gpioMatrixSignal)
+	if config.InvertTX {
+		config.TX.outFunc().Set(regs.gpioMatrixSignal | esp.GPIO_FUNC_OUT_SEL_CFG_INV_SEL)
+	} else {
+		config.TX.outFunc().Set(regs.gpioMatrixSignal)
+	}
 	// link RX with GPIO signal X and route signals via GPIO matrix
-	inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX))
+	if config.InvertRX {
+		inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX) | esp.GPIO_FUNC_IN_SEL_CFG_IN_INV_SEL)
+	} else {
+		inFunc(regs.gpioMatrixSignal).Set(esp.GPIO_FUNC_IN_SEL_CFG_SEL | uint32(config.RX))
+	}
 }
 
 func (uart *UART) configureInterrupt(intrMapReg *volatile.Register32) {
diff --git a/src/machine/machine_rp2_gpio.go b/src/machine/machine_rp2_gpio.go
index 25d76261f..94226ad6f 100644
--- a/src/machine/machine_rp2_gpio.go
+++ b/src/machine/machine_rp2_gpio.go
@@ -141,6 +141,16 @@ func (p Pin) setSchmitt(trigger bool) {
 	p.padCtrl().ReplaceBits(boolToBit(trigger)<<rp.PADS_BANK0_GPIO0_SCHMITT_Pos, rp.PADS_BANK0_GPIO0_SCHMITT_Msk, 0)
 }
 
+// setOutOver sets the output override (OUTOVER field) for the pin.
+func (p Pin) setOutOver(over uint32) {
+	p.ioCtrl().ReplaceBits(over<<rp.IO_BANK0_GPIO0_CTRL_OUTOVER_Pos, rp.IO_BANK0_GPIO0_CTRL_OUTOVER_Msk, 0)
+}
+
+// setInOver sets the input override (INOVER field) for the pin.
+func (p Pin) setInOver(over uint32) {
+	p.ioCtrl().ReplaceBits(over<<rp.IO_BANK0_GPIO0_CTRL_INOVER_Pos, rp.IO_BANK0_GPIO0_CTRL_INOVER_Msk, 0)
+}
+
 // setFunc will set pin function to fn.
 func (p Pin) setFunc(fn pinFunc) {
 	// Set input enable, Clear output disable
diff --git a/src/machine/machine_rp2_uart.go b/src/machine/machine_rp2_uart.go
index 37e2ca9c2..8405ebe60 100644
--- a/src/machine/machine_rp2_uart.go
+++ b/src/machine/machine_rp2_uart.go
@@ -52,9 +52,19 @@ func (uart *UART) Configure(config UARTConfig) error {
 	// set GPIO mux to UART for the pins
 	if config.TX != NoPin {
 		config.TX.Configure(PinConfig{Mode: PinUART})
+		if config.InvertTX {
+			config.TX.setOutOver(rp.IO_BANK0_GPIO0_CTRL_OUTOVER_INVERT)
+		} else {
+			config.TX.setOutOver(rp.IO_BANK0_GPIO0_CTRL_OUTOVER_NORMAL)
+		}
 	}
 	if config.RX != NoPin {
 		config.RX.Configure(PinConfig{Mode: PinUART})
+		if config.InvertRX {
+			config.RX.setInOver(rp.IO_BANK0_GPIO0_CTRL_INOVER_INVERT)
+		} else {
+			config.RX.setInOver(rp.IO_BANK0_GPIO0_CTRL_INOVER_NORMAL)
+		}
 	}
 	if config.RTS != 0 {
 		config.RTS.Configure(PinConfig{Mode: PinOutput})
diff --git a/src/machine/machine_sam_uart_invert.go b/src/machine/machine_sam_uart_invert.go
new file mode 100644
index 000000000..2c7b2d0ca
--- /dev/null
+++ b/src/machine/machine_sam_uart_invert.go
@@ -0,0 +1,17 @@
+//go:build sam && (atsamd51 || atsame5x)
+
+package machine
+
+// Configure UART TX/RX line inversion using SVD-generated APIs.
+func (uart *UART) setInversion(config UARTConfig) {
+	if config.InvertTX {
+		uart.Bus.SetCTRLA_TXINV(1)
+	} else {
+		uart.Bus.SetCTRLA_TXINV(0)
+	}
+	if config.InvertRX {
+		uart.Bus.SetCTRLA_RXINV(1)
+	} else {
+		uart.Bus.SetCTRLA_RXINV(0)
+	}
+}
diff --git a/src/machine/machine_sam_uart_noinvert.go b/src/machine/machine_sam_uart_noinvert.go
new file mode 100644
index 000000000..5d4f9a0f9
--- /dev/null
+++ b/src/machine/machine_sam_uart_noinvert.go
@@ -0,0 +1,7 @@
+//go:build sam && !atsamd51 && !atsame5x
+
+package machine
+
+// Hardware inversion is not supported on SAMD21.
+func (uart *UART) setInversion(config UARTConfig) {
+}
diff --git a/src/machine/machine_stm32_uart.go b/src/machine/machine_stm32_uart.go
index 4c6a54658..f6758742d 100644
--- a/src/machine/machine_stm32_uart.go
+++ b/src/machine/machine_stm32_uart.go
@@ -56,6 +56,9 @@ func (uart *UART) Configure(config UARTConfig) {
 	// Set baud rate
 	uart.SetBaudRate(config.BaudRate)
 
+	// Configure RX/TX inversion
+	uart.setInversion(config)
+
 	// Enable USART port, tx, rx and rx interrupts
 	uart.Bus.CR1.Set(stm32.USART_CR1_TE | stm32.USART_CR1_RE | stm32.USART_CR1_RXNEIE | stm32.USART_CR1_UE)
 
diff --git a/src/machine/machine_stm32_uart_invert.go b/src/machine/machine_stm32_uart_invert.go
new file mode 100644
index 000000000..42c049d77
--- /dev/null
+++ b/src/machine/machine_stm32_uart_invert.go
@@ -0,0 +1,17 @@
+//go:build stm32 && !(stm32f1 || stm32f4)
+
+package machine
+
+// Configure UART TX/RX line inversion using SVD-generated APIs.
+func (uart *UART) setInversion(config UARTConfig) {
+	if config.InvertTX {
+		uart.Bus.SetCR2_TXINV(1)
+	} else {
+		uart.Bus.SetCR2_TXINV(0)
+	}
+	if config.InvertRX {
+		uart.Bus.SetCR2_RXINV(1)
+	} else {
+		uart.Bus.SetCR2_RXINV(0)
+	}
+}
diff --git a/src/machine/machine_stm32_uart_noinvert.go b/src/machine/machine_stm32_uart_noinvert.go
new file mode 100644
index 000000000..f0dc9f860
--- /dev/null
+++ b/src/machine/machine_stm32_uart_noinvert.go
@@ -0,0 +1,7 @@
+//go:build stm32 && (stm32f1 || stm32f4)
+
+package machine
+
+// Hardware inversion is not supported on F1/F4.
+func (uart *UART) setInversion(config UARTConfig) {
+}
diff --git a/src/machine/serial.go b/src/machine/serial.go
index 4aacc502e..3149bd6a0 100644
--- a/src/machine/serial.go
+++ b/src/machine/serial.go
@@ -13,6 +13,8 @@ type UARTConfig struct {
 	RX       Pin
 	RTS      Pin
 	CTS      Pin
+	InvertTX bool // Invert TX line (active low becomes active high, etc.)
+	InvertRX bool // Invert RX line (active low becomes active high, etc.)
 }
 
 // NullSerial is a serial version of /dev/null (or null router): it drops