teensy40: move txBuffer allocation to UART declaration

src/machine/board_teensy40.go

@@ -137,8 +137,9 @@ const (
 
 var (
 	UART1 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART6,
+		Bus:      nxp.LPUART6,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D0 (PA3 [AD_B0_03])
 			mux: nxp.IOMUXC_LPUART6_RX_SELECT_INPUT_DAISY_GPIO_AD_B0_03_ALT2,
 			sel: &nxp.IOMUXC.LPUART6_RX_SELECT_INPUT,
@@ -149,8 +150,9 @@ var (
 		},
 	}
 	UART2 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART4,
+		Bus:      nxp.LPUART4,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D7 (PB17 [B1_01])
 			mux: nxp.IOMUXC_LPUART4_RX_SELECT_INPUT_DAISY_GPIO_B1_01_ALT2,
 			sel: &nxp.IOMUXC.LPUART4_RX_SELECT_INPUT,
@@ -161,8 +163,9 @@ var (
 		},
 	}
 	UART3 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART2,
+		Bus:      nxp.LPUART2,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D15 (PA19 [AD_B1_03])
 			mux: nxp.IOMUXC_LPUART2_RX_SELECT_INPUT_DAISY_GPIO_AD_B1_03_ALT2,
 			sel: &nxp.IOMUXC.LPUART2_RX_SELECT_INPUT,
@@ -173,8 +176,9 @@ var (
 		},
 	}
 	UART4 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART3,
+		Bus:      nxp.LPUART3,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D16 (PA23 [AD_B1_07])
 			mux: nxp.IOMUXC_LPUART3_RX_SELECT_INPUT_DAISY_GPIO_AD_B1_07_ALT2,
 			sel: &nxp.IOMUXC.LPUART3_RX_SELECT_INPUT,
@@ -185,8 +189,9 @@ var (
 		},
 	}
 	UART5 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART8,
+		Bus:      nxp.LPUART8,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D21 (PA27 [AD_B1_11])
 			mux: nxp.IOMUXC_LPUART8_RX_SELECT_INPUT_DAISY_GPIO_AD_B1_11_ALT2,
 			sel: &nxp.IOMUXC.LPUART8_RX_SELECT_INPUT,
@@ -197,15 +202,17 @@ var (
 		},
 	}
 	UART6 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART1,
+		Bus:      nxp.LPUART1,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		// LPUART1 not connected via IOMUXC
 		//   RX: D24 (PA12 [AD_B0_12])
 		//   TX: D25 (PA13 [AD_B0_13])
 	}
 	UART7 = UART{
-		Buffer: NewRingBuffer(),
-		Bus:    nxp.LPUART7,
+		Bus:      nxp.LPUART7,
+		Buffer:   NewRingBuffer(),
+		txBuffer: NewRingBuffer(),
 		muxRX: muxSelect{ // D28 (PC18 [EMC_32])
 			mux: nxp.IOMUXC_LPUART7_RX_SELECT_INPUT_DAISY_GPIO_EMC_32_ALT2,
 			sel: &nxp.IOMUXC.LPUART7_RX_SELECT_INPUT,

src/machine/machine_mimxrt1062_uart.go

@@ -15,6 +15,10 @@ type UART struct {
 	Buffer    *RingBuffer
 	Interrupt interrupt.Interrupt
 
+	// txBuffer should be allocated globally (such as when UART is created) to
+	// prevent it being reclaimed or cleaned up prematurely.
+	txBuffer *RingBuffer
+
 	// these hold the input selector ("daisy chain") values that select which pins
 	// are connected to the LPUART device, and should be defined where the UART
 	// instance is declared. see the godoc comments on type muxSelect for more
@@ -31,7 +35,6 @@ type UART struct {
 	configured   bool
 	msbFirst     bool
 	transmitting volatile.Register32
-	txBuffer     *RingBuffer
 }
 
 func (uart *UART) isTransmitting() bool { return uart.transmitting.Get() != 0 }
@@ -197,9 +200,6 @@ func (uart *UART) Sync() error {
 
 // WriteByte writes a single byte of data to the UART interface.
 func (uart *UART) WriteByte(c byte) error {
-	if nil == uart.txBuffer {
-		uart.txBuffer = NewRingBuffer()
-	}
 	uart.startTransmitting()
 	for !uart.txBuffer.Put(c) {
 	}