Zephyr port: open the serial port at 1200 baud to enter the bootloader by mikeysklar · Pull Request #11570 · adafruit/circuitpython

mikeysklar · 2026-10-12T19:09:32Z

What

Opening the USB serial port at 1200 baud now resets the board into its bootloader.

Why

The README said this worked, but the Zephyr port never handled it.

Raspberry Pi boards no longer need BOOTSEL to reflash.

The RP2 settings live in adaboot (adafruit/adaboot#9), so Zephyr Arduino can share them.

Board	Test	Before move	After move
nRF54LM20 DK	1200 baud touch	36/36	20/20
Feather nRF52840	1200 baud touch, then UF2 copy	23/23	build only
Pico W	1200 baud touch, then UF2 copy	25/25	10/10
	`on_next_reset(UF2 / BOOTLOADER)`, then UF2 copy	3/3, 5/5	3/3, 3/3
	Power off, cold boot stays in CircuitPython	30/30	10/10
	Open at 115200 or held at 1200, no reset	18/18	5/5
Pico 2 W	the four Pico W tests	not tested	10/10, 3/3 + 3/3, 10/10, 5/5

Not tested: nRF54L15 DK (UART console). Pico, Pico 2 and Feather RP2040 are build-tested only.

Written with Claude. The "after move" tests were run on the farm by Claude; I reviewed them.

🤖 Generated with Claude Code

The README said the touch worked, but nothing handled it: the TinyUSB handler is not built for this port. Watch line coding and DTR on the first CDC. At 1200 baud with DTR low, recheck 100 ms later, detach USB and call reset_to_bootloader(), which now asks for UF2 mode so the Adafruit nRF52 bootloader shows its drive. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

On the RP2 boards the touch and on_next_reset(UF2) only rebooted into CircuitPython, so every reflash needed BOOTSEL. Add Zephyr's boot-mode retention region to the five RP2 overlays and enable the retained-memory chain. Zephyr's rom_bootloader.c then enters the ROM bootloader. A "CP" prefix and CRC8 keep cold-boot SRAM from being read as a request. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

dhalbert

This looks OK to me. @tannewt checking with you as well in case you have an adaboot consideration.

mikeysklar · 2026-10-13T01:36:29Z

Checked on a Feather nRF52840: adafruit-nrfutil --touch 1200 serial DFU still works 3/3 with this PR, and the bootloader now also shows the UF2 drive. Adaboot itself ignores the UF2 flag, so it needs no change.

tannewt

Thanks for fixing this. I think some of it should be moved to adaboot.

…touch

RP2 boards have no MCUboot, so adaboot_request_update_mode() had nothing to signal on them. Wire Zephyr's boot-mode retention into the five RP2 layout dtsis (4-byte region with an "AB" prefix and CRC8, so cold-boot SRAM is not read as a request) and add a module Kconfig that enables RETAINED_MEM, RETENTION and RETENTION_BOOT_MODE on RP2 when the region is chosen. Zephyr's RPI_PICO_ROM_BOOTLOADER then follows and enters the ROM UF2 bootloader on the next boot. This moves the settings out of CircuitPython (adafruit/circuitpython#11570) so any application using adaboot gets the same behavior. Signed-off-by: Mikey Sklar <mikeysklar@gmail.com> Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

The RP2 boot-mode retention region and its Kconfig defaults now live in adaboot, next to the RP2 flash layouts, so Zephyr Arduino can share them. Drop the CP copies and bump adaboot. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tannewt

One Q about the delay.

tannewt · 2026-10-16T17:03:50Z

+         msg->type == USBD_MSG_CDC_ACM_CONTROL_LINE_STATE) &&
+        msg->dev == touch_cdc) {
+        if (touch_requested()) {
+            k_work_schedule(&touch_work, K_MSEC(100));


Why do you need a delay? TinyUSB doesn't:

circuitpython/supervisor/shared/usb/usb_device.c

Lines 81 to 89 in 8253c03

if (!dtr) {

cdc_line_coding_t coding;

// Use whichever CDC is itf 0.

tud_cdc_get_line_coding(&coding);

if (coding.bit_rate == 1200) {

reset_to_bootloader();

}

} else {

It isn't needed. I took it from ArduinoCore-samd, which waits 250 ms and cancels if DTR comes back. CircuitPython's TinyUSB path resets right away, so I removed it in 8cd3687.

Pico W, no delay Result

1200 touch, then UF2 copy 10/10, ROM drive 0.5-0.7 s after close

open at 115200, or held at 1200 5/5, no reset

power off 3/3, back in CircuitPython

Same on Pico 2 W:

Pico 2 W, no delay Result

1200 touch, then UF2 copy 10/10, ROM drive 0.5-0.9 s after close

open at 115200, or held at 1200 5/5, no reset

power off 3/3, back in CircuitPython

Same on Feather nRF52840:

Feather nRF52840, no delay Result

1200 touch, then UF2 copy 10/10, FTHR840BOOT drive 0.5-0.7 s after close

open at 115200, or held at 1200 5/5, no reset

power off 3/3, back in CircuitPython

Drop the 100 ms recheck. The TinyUSB ports reset as soon as DTR drops at 1200 baud, and this now does the same. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

mikeysklar and others added 2 commits October 12, 2026 06:15

mikeysklar requested a review from dhalbert October 12, 2026 23:37

mikeysklar mentioned this pull request Oct 12, 2026

Zephyr port: three fixes for Raspberry Pi boards #11573

Merged

dhalbert previously approved these changes Oct 13, 2026

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mikeysklar mentioned this pull request Oct 13, 2026

Give Zephyr boards a USB serial number #11579

Merged

dhalbert requested a review from tannewt October 13, 2026 15:30

tannewt requested changes Oct 13, 2026

View reviewed changes

Comment thread ports/zephyr-cp/supervisor/usb.c

Comment thread ports/zephyr-cp/README.md Outdated

Comment thread ports/zephyr-cp/boards/rp2_boot_mode.dtsi Outdated

Merge remote-tracking branch 'origin/main' into zephyr-cp-bootloader-…

58da085

…touch

mikeysklar mentioned this pull request Oct 15, 2026

Let Raspberry Pi Pico boards reboot into their UF2 bootloader from an app adafruit/adaboot#9

Merged

mikeysklar dismissed dhalbert’s stale review via b661f23 October 15, 2026 22:22

mikeysklar requested review from dhalbert and tannewt October 15, 2026 22:28

tannewt requested changes Oct 16, 2026

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mikeysklar mentioned this pull request Oct 16, 2026

zephyr-cp: add ItsyBitsy nRF52840; add dynamic buses on CLUE and ItsyBitsy #11612

Open

zephyr-cp: reset on the 1200-bps touch right away

8cd3687

Drop the 100 ms recheck. The TinyUSB ports reset as soon as DTR drops at 1200 baud, and this now does the same. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

mikeysklar requested a review from tannewt October 16, 2026 22:27

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Zephyr port: open the serial port at 1200 baud to enter the bootloader - #11570

Zephyr port: open the serial port at 1200 baud to enter the bootloader#11570
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mikeysklar:zephyr-cp-bootloader-touch

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	if (!dtr) {
	cdc_line_coding_t coding;
	// Use whichever CDC is itf 0.
	tud_cdc_get_line_coding(&coding);

	if (coding.bit_rate == 1200) {
	reset_to_bootloader();
	}
	} else {

Pico W, no delay	Result
1200 touch, then UF2 copy	10/10, ROM drive 0.5-0.7 s after close
open at 115200, or held at 1200	5/5, no reset
power off	3/3, back in CircuitPython

Pico 2 W, no delay	Result
1200 touch, then UF2 copy	10/10, ROM drive 0.5-0.9 s after close
open at 115200, or held at 1200	5/5, no reset
power off	3/3, back in CircuitPython

Feather nRF52840, no delay	Result
1200 touch, then UF2 copy	10/10, FTHR840BOOT drive 0.5-0.7 s after close
open at 115200, or held at 1200	5/5, no reset
power off	3/3, back in CircuitPython

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