Enable/disable connection check made with DTR

[MrGreensWorkshop]

This gives users the option to disable DTR check.
this fixes issues #906, #921

Problem:

tud_cdc_connected is using DTR bit which is used in stdio_usb.c If DTR is not set by host (PC terminal or other device), we can't send anything to host.

src/class/cdc/cdc_device.h

static inline bool tud_cdc_connected (void)
{
  return tud_cdc_n_connected(0);
}

src/class/cdc/cdc_device.h

bool tud_cdc_n_connected(uint8_t itf)
{
  // DTR (bit 0) active  is considered as connected
  return tud_ready() && tu_bit_test(_cdcd_itf[itf].line_state, 0);
}

how to fix

PICO_STDIO_USB_CONNECTION_WITHOUT_DTR option added to config file for who needs to enable.
It doesn't change current behavior because it's disabled by default. Just gives user an option to enable it.

Usage

add this to your CMakeLists.txt file and DTR check will be disabled.
add_definitions(-DPICO_STDIO_USB_CONNECTION_WITHOUT_DTR=1)

[lurch]

I guess this might affect the behaviour of the PICO_STDIO_USB_CONNECT_WAIT_TIMEOUT_MS and PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS options? 🤷

[MrGreensWorkshop]

@lurch
It depends on future plans for stdio_usb_connected so I respect that and change the code.

I think you mean when PICO_STDIO_USB_CONNECTION_CHECK_WITH_DTR and those flags combined ?

how about this? when both flags are enabled adding sleep

bool stdio_usb_connected(void) {
#if PICO_STDIO_USB_CONNECTION_CHECK_WITH_DTR
    // this actually checks DTR
    return tud_cdc_connected();
#else
    #if PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS != 0
         sleep_ms(PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS);
    #endif
    return true;
#endif
}

[lurch]

I dunno, it's entirely up to @kilograham , but I was just wondering if it might be e.g. worth doing a compile-time #warning if PICO_STDIO_USB_CONNECT_WAIT_TIMEOUT_MS is non-zero, and PICO_STDIO_USB_CONNECTION_CHECK_WITH_DTR is zero? 🤷

And IMHO you probably don't want a sleep inside of stdio_usb_connected, given how often it gets called?

[MrGreensWorkshop]

to be clear PICO_STDIO_USB_CONNECTION_CHECK_WITH_DTRis default enabled. so sleep will be called if PICO_STDIO_USB_CONNECTION_CHECK_WITH_DTR disabled and if PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS is set other than 0.

in here, when stdio_usb_connected called, it is always returns true so, if PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS != 0 I though its best wait it in stdio_usb_connected.

        do {
            if (stdio_usb_connected()) {
#if PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS != 0
                sleep_ms(PICO_STDIO_USB_POST_CONNECT_WAIT_DELAY_MS);
#endif
                break;
            }
            sleep_ms(10);
        } while (!time_reached(until));

[MrGreensWorkshop]

@kilograham Thanks,
I added a fix,

1. There was a typo on comments. -> fixed
2. PICO_STDIO_USB_CONNECTION_WITHOUT_DTR was enabled by default -> disabled

if you want to revert let me know.

[kilograham]

nice, thanks