From 6495ceb1c745c188872c99658edf3644a9457989 Mon Sep 17 00:00:00 2001
From: deadprogram <ron@hybridgroup.com>
Date: Tue, 29 Sep 2026 01:05:31 +0200
Subject: [PATCH] machine/esp32xx: clear the I2C bus with a GPIO STOP when
 configuring

Without a bus clear, an MPU-6050 on a XIAO ESP32-C3 NACKs the first
transaction after a chip reset. This does the same as the software
i2c_master_clear_bus in ESP-IDF v4.4.8. SCL is clocked while SDA is
held low, then a STOP is sent with the pins as GPIOs.
---
 src/machine/machine_esp32xx_i2c.go | 33 ++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/machine/machine_esp32xx_i2c.go b/src/machine/machine_esp32xx_i2c.go
index ab62cd063b..aeda5e0c74 100644
--- a/src/machine/machine_esp32xx_i2c.go
+++ b/src/machine/machine_esp32xx_i2c.go
@@ -44,6 +44,7 @@ func (i2c *I2C) Configure(config I2CConfig) error {
 
 	i2c.initClock(config)
 	i2c.initNoiseFilter()
+	i2c.clearBus(config)
 	i2c.initPins(config)
 	i2c.initFrequency(config)
 	i2c.startMaster()
@@ -72,6 +73,36 @@ func (i2c *I2C) initNoiseFilter() {
 	i2c.Bus.FILTER_CFG.Set(0x377)
 }
 
+// clearBus clocks SCL until SDA is released and then sends a STOP, both as GPIOs.
+// Same as ESP-IDF https://github.com/espressif/esp-idf/blob/v4.4.8/components/driver/i2c.c#L579
+func (i2c *I2C) clearBus(config I2CConfig) {
+	const halfPeriodNS = 5000
+	wait := func() {
+		end := nanotime() + halfPeriodNS
+		for nanotime() < end {
+		}
+	}
+	scl, sda := config.SCL, config.SDA
+	scl.Configure(PinConfig{Mode: PinOutput})
+	scl.pinReg().SetBits(esp.GPIO_PIN_PAD_DRIVER)
+	sda.Configure(PinConfig{Mode: PinOutput})
+	sda.pinReg().SetBits(esp.GPIO_PIN_PAD_DRIVER)
+	scl.Low()
+	sda.High()
+	wait()
+	for i := 0; i < 9 && !sda.Get(); i++ {
+		scl.High()
+		wait()
+		scl.Low()
+		wait()
+	}
+	sda.Low()
+	scl.High()
+	wait()
+	sda.High()
+	wait()
+}
+
 //go:inline
 func (i2c *I2C) initPins(config I2CConfig) {
 	config.SDA.configure(PinConfig{Mode: PinOutput}, i2c.funcSDA)
@@ -139,8 +170,6 @@ func (i2c *I2C) startMaster() {
 func (i2c *I2C) resetMaster() {
 	// reset FSM
 	i2c.Bus.SetCTR_FSM_RST(1)
-	// No bus clear (SCL_RST_SLV), a GT911 did not ACK the next transaction after it.
-	// ESP-IDF does not clear the bus on init either https://github.com/espressif/esp-idf/blob/v4.4.8/components/driver/i2c.c#L673
 	i2c.Bus.SetSCL_STRETCH_CONF_SLAVE_SCL_STRETCH_EN(1)
 	i2c.Bus.SetCTR_CONF_UPGATE(1)
 	i2c.Bus.FILTER_CFG.Set(0x377)