stdio_usb improvements

[kilograham]

• Use shared IRQ if available to avoid 1ms timer.
• Allow use of stdio_usb with user's tinyusb setup if it has CDC

[kilograham]

fixes #838

[eustas]

Hi. This PR has added a check that callback is executed on the right core... But what is the way to setup default alarm pool to fire on core1? Thanks in advance.

[thewh1teagle]

Hi
How can I use it?
I tried to set

pico_enable_stdio_usb(server 1)

but got the warning

stdio USB was configured along with user use of TinyUSB device mode, but CDC is not enabled

Trying to make the PI as HID + Serial at the same time

[lurch]

All of the Raspberry Pi engineers are currently on holiday, so if none of the examples at https://github.com/raspberrypi/pico-examples/blob/master/README.md#usb-device help, you'll probably get a quicker response from https://forums.raspberrypi.com/

[peterharperuk]

Don't configure stdio to use usb. Use a UART instead

[thewh1teagle]

@peterharperuk
But I want to use the same cable for both HID + CDC (for serial) without special tools.

[thewh1teagle]

I found a way to do that and it works. the PICO act as HID + CDC device and I can read serial output in my PC in the same time.
example here

But I'm still looking for relevant documentation about that and maybe official example