When no USB host is reading, flushAndWait() spins 50K iterations per
FIFO-full event. With putchar calling WriteByte per byte, the cumulative
delay starves I2C and other peripherals, freezing displays.
Add a txStalled flag: the first FIFO-full triggers one flushAndWait
attempt. If it fails (no host), txStalled is set and all subsequent
writes return immediately with no spin — just a register read and a
bool check. When a host reconnects, SERIAL_IN_EP_DATA_FREE goes back
to 1, bypassing the stall path and clearing the flag automatically.
When no USB host is reading (e.g. board not connected to a serial
monitor), WriteByte and Write would spin for up to 200k iterations
per byte waiting for the FIFO to drain. This stalled the entire
application, freezing unrelated peripherals like I2C displays.
Reduce flushTimeout from 200,000 to 50,000 iterations (~3ms) which
is enough for 2-3 USB frames when a host is connected, but short
enough that serial output won't freeze the application when no host
is reading. Serial output is best-effort; callers like putchar
already ignore write errors.
Applies to both ESP32-S3 and ESP32-C3 which share the same USB
Serial/JTAG controller design.
This improves the ESP32-C3 USBDevice implementation by using interrupts
to properly handle data RX/TX.
It also stubs out USB interfaces for HID functions, since those cannot
be implemented on ESP32-C3 due to using a JTAG-USB interface.
Signed-off-by: deadprogram <ron@hybridgroup.com>
deadprogram
Dima Jolkin