PX4 Cube locks up if UART is accessed inside hal.scheduler timer process

Hello -

I am writing an external IMU driver for the PX4 Cube that communicates via UART. Everything works smoothly until I try and access the UART read() function (multiple times per iteration to read the entire buffer) from inside a hal.scheduler timer process, at which point the PX4 completely dies. I have attempted this with uartC and uartE and get the same result each time. I can access the UART just fine from within the AP_InertialSensor::update() function, but unfortunately ArduPilot is set up so that I need to call _notify_new_gyro_raw_sample/_notify_new_accel_raw_sample from within the timer process for the code to work. Has anyone run into this issue before, and if so, how did you work around it?

Thanks for your help!

@tridge is probably best to answer this one.
Can you share what imu you’re working with?

It’s a VectorNav VN-100/VN-110, but honestly the only relevant information is that I can’t access the UART within a timer loop. The problem persists if I try and do this with any UART device.

I figured it out. Turns out the UARTDriver read() function has a catch (returns -1/0xFF) to restrict it to the thread in which its object was instantiated. This prevented it from working within any of the timer threads I tried and lead to problems that would crash the entire program.

hi William,
i meet the same problem with you,could you point how to solve the problem?

hi William,
as you mentioned above,i removed return -1 as below,is it right?
int16_t UARTDriver::read()
if (lock_read_key != 0 || _uart_owner_thd != chThdGetSelfX()){
//return -1;
if (!_initialised) {
return -1;

uint8_t byte;
if (!_readbuf.read_byte(&byte)) {
    return -1;
if (!_rts_is_active) {

return byte;


i have one IMU sensor which is on UART, how can i integrate it to the ardupilot?

you have to add driver for the imu.

Ok. Actually I am very new to this. Can you guide me or any reference how to write the driver. As i have seen the ardupilot site where driver article is there. But it looks complex. Can you guide me for the same?

Sorry, I have never done this. Tridgell seems to be doing uavcan imu. You can wait for his code to come out