From cd628bcde6d86cac40be31993bac4338120d75ff Mon Sep 17 00:00:00 2001
From: Ayke van Laethem <aykevanlaethem@gmail.com>
Date: Fri, 18 Jun 2021 09:29:09 +0200
Subject: [PATCH] nrf52840: add support for flashing with the BOSSA tool

This only works with a custom bossac build from Arduino, not with the
upstream version. It avoids needing the manual "double tap" to enter
bootloader mode before flashing firmware.
---
 .../machine_nrf52840_usb_reset_bossa.go       | 26 +++++++++++++++++++
 .../machine_nrf52840_usb_reset_none.go        |  2 +-
 targets/nano-33-ble.json                      |  2 +-
 3 files changed, 28 insertions(+), 2 deletions(-)
 create mode 100644 src/machine/machine_nrf52840_usb_reset_bossa.go

diff --git a/src/machine/machine_nrf52840_usb_reset_bossa.go b/src/machine/machine_nrf52840_usb_reset_bossa.go
new file mode 100644
index 00000000..42c6ad00
--- /dev/null
+++ b/src/machine/machine_nrf52840_usb_reset_bossa.go
@@ -0,0 +1,26 @@
+// +build nrf52840,nrf52840_reset_bossa
+
+package machine
+
+import (
+	"device/arm"
+	"device/nrf"
+)
+
+const DFU_MAGIC_SERIAL_ONLY_RESET = 0xb0
+
+// checkShouldReset is called by the USB-CDC implementation to check whether to
+// reset into the bootloader/OTA and if so, resets the chip appropriately.
+func checkShouldReset() {
+	if usbLineInfo.dwDTERate == 1200 && usbLineInfo.lineState&usb_CDC_LINESTATE_DTR == 0 {
+		EnterSerialBootloader()
+	}
+}
+
+// EnterSerialBootloader resets the chip into the serial bootloader. After
+// reset, it can be flashed using serial/nrfutil.
+func EnterSerialBootloader() {
+	arm.DisableInterrupts()
+	nrf.POWER.GPREGRET.Set(DFU_MAGIC_SERIAL_ONLY_RESET)
+	arm.SystemReset()
+}
diff --git a/src/machine/machine_nrf52840_usb_reset_none.go b/src/machine/machine_nrf52840_usb_reset_none.go
index 08c615eb..796a021e 100644
--- a/src/machine/machine_nrf52840_usb_reset_none.go
+++ b/src/machine/machine_nrf52840_usb_reset_none.go
@@ -1,4 +1,4 @@
-// +build nrf52840,!nrf52840_reset_uf2
+// +build nrf52840,!nrf52840_reset_uf2,!nrf52840_reset_bossa
 
 package machine
 
diff --git a/targets/nano-33-ble.json b/targets/nano-33-ble.json
index 53b4fcac..df22221f 100644
--- a/targets/nano-33-ble.json
+++ b/targets/nano-33-ble.json
@@ -1,6 +1,6 @@
 {
 	"inherits": ["nrf52840"],
-	"build-tags": ["nano_33_ble"],
+	"build-tags": ["nano_33_ble", "nrf52840_reset_bossa"],
 	"flash-command": "bossac_arduino2 -d -i -e -w -v -R --port={port} {bin}",
 	"flash-1200-bps-reset": "true",
 	"linkerscript": "targets/nano-33-ble.ld"