From 9bbad6700b3b17ad4fe9709f1d4138cd17111b48 Mon Sep 17 00:00:00 2001
From: Dima Jolkin <dimajolkin@gmail.com>
Date: Sun, 15 Feb 2026 21:11:41 +0200
Subject: [PATCH] esp32s3-usbserial: added usbserial printing

---
 src/machine/machine_esp32s3.go | 88 +++++++++++++++++++++++++++++++++-
 src/runtime/runtime_esp32s3.go |  4 ++
 targets/esp32s3.json           |  2 +-
 3 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/src/machine/machine_esp32s3.go b/src/machine/machine_esp32s3.go
index 65261bd3c..9c2bd9001 100644
--- a/src/machine/machine_esp32s3.go
+++ b/src/machine/machine_esp32s3.go
@@ -309,4 +309,90 @@ func (uart *UART) writeByte(b byte) error {
 
 func (uart *UART) flush() {}
 
-// TODO: SPI
+// USB Serial/JTAG Controller for ESP32-S3
+type USB_DEVICE struct {
+	Bus *esp.USB_DEVICE_Type
+}
+
+var (
+	_USBCDC = &USB_DEVICE{
+		Bus: esp.USB_DEVICE,
+	}
+
+	USBCDC Serialer = _USBCDC
+)
+
+func initUSB() {
+	USBCDC.Configure(UARTConfig{BaudRate: 115200})
+}
+
+type Serialer interface {
+	WriteByte(c byte) error
+	Write(data []byte) (n int, err error)
+	Configure(config UARTConfig) error
+	Buffered() int
+	ReadByte() (byte, error)
+	DTR() bool
+	RTS() bool
+}
+
+func (usbdev *USB_DEVICE) Configure(config UARTConfig) error {
+	return nil
+}
+
+func (usbdev *USB_DEVICE) WriteByte(c byte) error {
+	// Host is connected - wait for TX FIFO space with reasonable timeout
+	timeout := 10000 // Generous timeout for connected host
+	for usbdev.Bus.GetEP1_CONF_SERIAL_IN_EP_DATA_FREE() == 0 && timeout > 0 {
+		timeout--
+	}
+
+	// Even with host connected, don't hang forever
+	if timeout == 0 {
+		return nil
+	}
+
+	// Write byte to USB Serial/JTAG endpoint
+	usbdev.Bus.SetEP1_RDWR_BYTE(uint32(c))
+
+	// Trigger transmission
+	usbdev.Bus.SetEP1_CONF_WR_DONE(1)
+
+	return nil
+}
+
+func (usbdev *USB_DEVICE) Write(data []byte) (n int, err error) {
+	for _, c := range data {
+		err = usbdev.WriteByte(c)
+		if err != nil {
+			return n, err
+		}
+		n++
+	}
+	return n, nil
+}
+
+func (usbdev *USB_DEVICE) ReadByte() (byte, error) {
+	// TODO: Implement USB Serial/JTAG input reading
+	return 0, errors.New("ReadByte not implemented")
+}
+
+func (usbdev *USB_DEVICE) Buffered() int {
+	// Return number of bytes available to read
+	return int(usbdev.Bus.GetEP1_CONF_SERIAL_OUT_EP_DATA_AVAIL())
+}
+
+func (usbdev *USB_DEVICE) DTR() bool {
+	// Data Terminal Ready - not applicable for USB Serial/JTAG
+	return false
+}
+
+func (usbdev *USB_DEVICE) RTS() bool {
+	// Request To Send - not applicable for USB Serial/JTAG
+	return false
+}
+
+func (usbdev *USB_DEVICE) flush() {
+	// Force transmission of any buffered data
+	usbdev.Bus.SetEP1_CONF_WR_DONE(1)
+}
diff --git a/src/runtime/runtime_esp32s3.go b/src/runtime/runtime_esp32s3.go
index 5a612d6df..ab0d85d73 100644
--- a/src/runtime/runtime_esp32s3.go
+++ b/src/runtime/runtime_esp32s3.go
@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"device/esp"
+	"machine"
 )
 
 // This is the function called on startup after the flash (IROM/DROM) is
@@ -71,6 +72,9 @@ func main() {
 		clearbss()
 	}
 
+	// Initialize UART.
+	machine.InitSerial()
+
 	// Initialize main system timer used for time.Now.
 	initTimer()
 
diff --git a/targets/esp32s3.json b/targets/esp32s3.json
index f245b82ab..051cc8542 100644
--- a/targets/esp32s3.json
+++ b/targets/esp32s3.json
@@ -4,7 +4,7 @@
 	"features": "+atomctl,+bool,+clamps,+coprocessor,+debug,+density,+div32,+esp32s3,+exception,+fp,+highpriinterrupts,+interrupt,+loop,+mac16,+memctl,+minmax,+miscsr,+mul32,+mul32high,+nsa,+prid,+regprotect,+rvector,+s32c1i,+sext,+threadptr,+timerint,+windowed",
 	"build-tags": ["esp32s3", "esp"],
 	"scheduler": "tasks",
-	"serial": "uart",
+	"serial": "usb",
 	"linker": "ld.lld",
 	"default-stack-size": 2048,
 	"rtlib": "compiler-rt",