machine: add USBDevice.Attach and USBDevice.Detach by sago35 · Pull Request #5563 · tinygo-org/tinygo

sago35 · 2026-07-28T13:13:15Z

This PR adds machine.USBDev.Attach() and machine.USBDev.Detach(), exposing
USB soft-connect control (the D+ pull-up) to user code.

Motivation

The USB device is attached to the bus automatically during startup, before
user code has had a chance to finish its USB configuration (usb.VendorID /
usb.ProductID / usb.Product, additional interfaces registered through
machine.ConfigureUSBEndpoint, ...). For composite devices such as keyboards
this means the host may enumerate a half-configured device: identifiers and
interfaces set from main() can arrive after the host has already read the
descriptors.

With this API, an application or library can stay detached until its
configuration is complete, so the host enumerates the finished device exactly
once:

func init() {
      machine.USBDev.Detach() // stay invisible to the host for now
}

func main() {
      usb.Product = "my-keyboard"
      // ... register HID interfaces, set VID/PID, ...
      machine.USBDev.Attach() // now let the host enumerate
}

It can also be used to force re-enumeration without replugging the cable
(Detach() → Attach()).

Prior art

Soft-connect control is a well-established primitive in embedded USB stacks,
exposed for exactly these use cases (delaying enumeration until configuration
is complete, and forcing re-enumeration from software):

Arduino: USBDevice.detach() / USBDevice.attach() (AVR and SAMD
cores).
TinyUSB: tud_disconnect() / tud_connect().
Zephyr: usb_dc_detach() / usb_dc_attach() in the device-controller
driver API.
STM32 HAL: HAL_PCD_DevDisconnect() / HAL_PCD_DevConnect().

The Attach/Detach naming follows the USB specification's bus-state wording
(a device is "attached" to the bus), which is also where the SAMD register
bit name (CTRLB.DETACH) comes from — and matches the Arduino API, easing
migration for users coming from that ecosystem.

Implementation

One-register operation per chip family, following each reference manual:

┌────────────────────┬─────────────────────────────┐
│        Chip        │          Mechanism          │
├────────────────────┼─────────────────────────────┤
│ atsamd21, atsamd51 │ USB_DEVICE.CTRLB DETACH bit │
├────────────────────┼─────────────────────────────┤
│ nrf52840           │ USBD.USBPULLUP              │
├────────────────────┼─────────────────────────────┤
│ rp2040, rp2350     │ SIE_CTRL PULLUP_EN bit      │
└────────────────────┴─────────────────────────────┘

The ESP32 variants are not covered: their USB is a Serial/JTAG peripheral
with no equivalent soft-connect concept. Existing behavior is unchanged —
the methods are purely additive.

Testing

Compile-tested a program calling both methods for all five chip families
(xiao-ble, xiao, wioterminal, pico, pico2).
Verified on hardware with a XIAO nRF52840 Sense running a Vial keyboard
firmware (tinygo-keyboard)
that detaches in a package init() and attaches after configuration:
the host enumerates the device once, with the complete configuration
(CDC + keyboard/mouse/raw HID interfaces and a custom ProductID).**

github-actions · 2026-07-28T13:30:38Z

Size difference with the dev branch:

Binary size difference

 flash                          ram
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rdon-key

I left one comment about using the RP2040/RP2350 atomic set/clear aliases for SIE_CTRL.PULLUP_EN.

deadprogram · 2026-08-17T01:25:43Z

Hello @sago35 thanks for working on this.

Here are a couple of items extracted from an automated code review.

nRF52840: Detach() isn't sticky. The IRQ handler unconditionally re-enables the pull-up on every USBEVENT/EVENTCAUSE_READY:

// machine_nrf52840_usb.go:105-107
initEndpoint(0, usb.ENDPOINT_TYPE_CONTROL)
nrf.USBD.USBPULLUP.Set(1)

Unlike the other four chips, the pull-up here is owned by the ISR rather than Configure(), so a later power-ready event can silently re-attach. Could a detached bool on USBDevice guard that line? (The Detach-in-init → Attach-in-main flow you tested wouldn't necessarily hit it.)

Missing platforms. usb.go also builds for stm32f4/f7/h7, and esp32c3/c6/xx declare their own USBDevice, so machine.USBDev.Attach() won't compile there. stm32 isn't mentioned in the description but is the easy win — dctlSDIS (machine_stm32_otgfs_usb.go:98) and DCTL_SDIS (machine_stm32h7_usb.go:120) already exist and are already toggled in Configure. For esp32, machine_esp32xx_usb.go:65-68 sets the no-op-stub precedent (SetStallEPIn etc.), so no-op methods there would keep user code portable.

What do you think?

sago35 · 2026-08-18T13:22:10Z

I will check later.

The USB device is attached to the bus automatically during startup, before user code has a chance to finish its USB configuration (device identifiers, extra HID interfaces, ...). Composite devices such as keyboards may therefore be enumerated by the host with an incomplete configuration. Attach and Detach expose the soft-connect control (DP pull-up) so that an application or library can detach in an init function, complete its configuration, and attach again to let the host enumerate the finished device. They can also be used to force re-enumeration without replugging the cable. Implemented for atsamd21, atsamd51, nrf52840, rp2040 and rp2350.

The USB IRQ handler re-enables the DP pull-up on every power-ready event, silently undoing an earlier Detach. Guard the pull-up write with a detached flag so the device stays off the bus until Attach is called.

- stm32f4, stm32f7, stm32h7: implement Attach and Detach using the DCTL soft-disconnect bit that Configure already toggles. - esp32c3, esp32c6, esp32s3: add no-op stubs to keep user code portable; the fixed-function USB Serial/JTAG controller has no software-controlled soft-connect.

Replace the direct soft-connect register writes in Configure with the equivalent Attach and Detach calls on atsamd21, atsamd51, rp2040, rp2350, stm32f4, stm32f7 and stm32h7.

sago35 · 2026-08-20T13:28:02Z

@deadprogram
Both points are addressed, split into three commits so they can be judged (or dropped) individually. The branch is also rebased onto the current dev.

nRF52840: sticky Detach (c16ff7b)

Added a usbDetached flag. Detach() sets it, Attach() clears it, and the IRQ handler now only re-enables the DP pull-up on a power-ready event when the flag is not set, so a later USBEVENT/EVENTCAUSE_READY can no longer silently re-attach the device. The flag is written before the register in both methods, so an event racing with Detach() still ends up detached.

Missing platforms (858a56f)

stm32f4 / stm32f7 / stm32h7: implemented Attach/Detach with the DCTL soft-disconnect bit (dctlSDIS / DCTL_SDIS). Note that I have no stm32 hardware at hand, so these are compile-tested only — but they reuse the exact soft-disconnect sequence Configure already performs on these chips, so they should behave the same.
esp32c3 / esp32c6 / esp32s3: added no-op stubs next to the existing SetStallEPIn etc. stubs, so machine.USBDev.Attach() stays portable. The fixed-function USB Serial/JTAG controller has no software-controlled soft-connect.

Refactor (e23373e, optional)

Replaced the direct soft-connect register writes inside Configure with the equivalent dev.Attach() / dev.Detach() calls on atsamd21, atsamd51, rp2040, rp2350 and the three stm32 families. This one is purely cosmetic — feel free to ask me to drop it if you prefer the explicit register writes in Configure.

One deliberate exception: the USBPULLUP.Set(0) at the start of the nRF52840 Configure is not replaced with Detach(), because that would set the sticky flag and prevent the IRQ handler from ever attaching during normal enumeration.

Verified by compiling a program that calls machine.USBDev.Detach() / Attach() for all affected targets: circuitplay-express, feather-m4, feather-nrf52840, pico, pico2, feather-stm32f405, nucleo-f722ze, nucleo-h753zi, esp32c3-12f, esp32s3-generic and xiao-esp32c6.

deadprogram · 2026-08-20T15:17:26Z

Thank you very much for adding this @sago35 now squash/merging.

rdon-key reviewed Jul 28, 2026

View reviewed changes

Comment thread src/machine/machine_rp2040_usb.go

sago35 mentioned this pull request Jul 28, 2026

RP2040/RP2350 provide atomic bitmask set/clear aliases #5564

Open

dgryski added this to the 0.42.0 milestone Aug 19, 2026

sago35 added 4 commits August 20, 2026 09:56

machine: make USB Detach sticky on nrf52840

c16ff7b

The USB IRQ handler re-enables the DP pull-up on every power-ready event, silently undoing an earlier Detach. Guard the pull-up write with a detached flag so the device stays off the bus until Attach is called.

machine: use Attach and Detach in USB Configure

e23373e

Replace the direct soft-connect register writes in Configure with the equivalent Attach and Detach calls on atsamd21, atsamd51, rp2040, rp2350, stm32f4, stm32f7 and stm32h7.

sago35 force-pushed the usbdev-attach-detach branch from 2a56d5e to e23373e Compare August 20, 2026 13:26

deadprogram merged commit 93bc654 into dev Aug 20, 2026
26 checks passed

deadprogram deleted the usbdev-attach-detach branch August 20, 2026 15:17

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machine: add USBDevice.Attach and USBDevice.Detach - #5563

machine: add USBDevice.Attach and USBDevice.Detach#5563
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