Digispark: check if the USB host is connected

Question

I am trying to detect if the digispark device is actually connected to a host (i.e. the USB line is "active") vs. just powered. The issue: if I send a keypress while the data line is disconnected, the code hangs perpetually in sendKeyPress. My theory is that it keeps looping in while waiting for USB to be ready, which makes sense.

 while (!usbInterruptIsReady()) {
 // Note: We wait until we can send keyPress
 // so we know the previous keyPress was
 // sent.
 usbPoll();
 _delay_ms(5);
 }

I tried to use the macro usbInterruptIsReady() before calling sendKeyStroke in the main, but with no success. I would expect to see a function that keeps track of this state, but apparently there is none.

Also: it doesn't seem like what I am trying to accomplish being extremely weird, so my expectation is that someone else could have stumbled upon this and figured out a reasonable workaround. I would like to avoid setting up a watchdog timer.

Code below for example hangs when the device is powered but not connected to a PC. Thanks

Background: https://hackaday.io/project/191762-automatic-workstation-locker

#include "DigiKeyboard.h"
#include "Arduino.h"
void setup() {
 pinMode(LED_BUILTIN, OUTPUT);
 DigiKeyboard.update();
 DigiKeyboard.sendKeyStroke(0);
}
void loop() {
 static uint8_t bit = 0;
 bit = 1 - bit;
 digitalWrite(LED_BUILTIN, bit);
 DigiKeyboard.update();
 DigiKeyboard.sendKeyStroke(KEY_Q);
 delay(2000);
}

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Answer 1

The Digispark does not have a proper hardware USB implementation. It is possible that you simply can't do what you are attempting to do. A search I did indicated you are not the only person attempting to do this.

You may want to switch to one of the Arduinos that have a hardware USB interface like the Leonardo or Micro.

I should point out that the Digispark is not an Arduino, and it has its own forum where you may possibly get an answer from their own community.

Answer 2

Might be a bit late but I also had the same need for a project and managed to get it working with the usbInterruptIsReady() macro. That macro checks a global value somewhere that is used by other things, so you need to poll it for some time before it gives a valid result.

Here's a simple sketch that seems to work reliably and detects if a host is connected to USB:

// Include the Keyboard library regardless of USB availability.
#include "DigiKeyboard.h"
// Variable to store the state of USB connectivity.
bool is_usb = false;
void setup()
{
 // Start counting time.
 uint32_t start = millis();
 // Wait until 1000ms have passed or USB host has been detected.
 // I have tested with less than 1000ms timeout but it was unreliable.
 while (((millis() - start) <= 1000) && !is_usb)
 {
 // Check USB availability.
 is_usb = usbInterruptIsReady();
 // Use the Keyboard library delay function to keep calling usbPoll().
 DigiKeyboard.delay(5);
 }
 // For all Keyboard calls, always check first if the host is connected,
 // otherwise the connection might fail and you'll get in one of the many
 // infinite loops in the Digispark libraries.
 if (is_usb)
 {
 // Send the "clear" key stroke in setup() instead of loop() to allow
 // better keyboard control.
 DigiKeyboard.sendKeyStroke(0);
 }
}

• the busybee: OT: boolean == false is a code smell, better use !boolean.
• ÉmileBV: Calling that specific thing a code smell is wrong. The real issue is I didn't consistently do an explicit comparison. I will edit the answer to be more consistent.