Here's a schematic of my project:

enter image description here

The purpose is to make laptop work as keyboard for desktop PC. Nano uses Serial.read() to get keystrokes from laptop and forwards key press and release data to Leonardo (using I2C) acting as USB-HID keyboard.

Nano's setup looks like:

void setup() {
  Serial.write("Hello");  // https://serialport.io/docs/api-stream#serialpor|

Leonardo's setup looks like:

void setup() {

I have tested both boards individually, but not the whole setup. Whenever either board connects to USB port it powers up due to +5 volt from USB. Now that I have i2c wires in place, I am confused -

if I connect leonardo & desktop first [and i2c-master (Nano) isn't powered on yet], what will happen at Wire.begin(4)? Will it wait (1s, 5s, forever) until i2c-master comes online?

  • 2
    why don't you write a sketch to test this? – jsotola Apr 13 at 16:57

Generally you should not provide power to any pin of a device, which is powered off. That can cause current flowing through clamping diodes to ground and might destroy the pins hardware or the device (also it can lead to weird situations, where the device is somewhat powered through the IO pin, which might lead to instability).

You can either cut the connection between the devices, if only one of them is powered (this should be possible with some extra components), or you can power both devices from one USB port. For the other port you would cut the V+ (positive supply voltage) line of the USB port/cable (this makes sure, that you don't get a bad current between both computers on the USB power line, if they provide a bit different voltage). Since the ground wire still connects both computers and the Arduinos, the communication will still work correctly.

if I connect leonardo & desktop first [and i2c-master (Nano) isn't powered on yet], what will happen at Wire.begin(4)?

The Wire.begin() line won't get stuck, since it doesn't do much on the lines and it doesn't know, if something is connected on the other end, until some transaction happens on the bus. But it might be, that any transaction from the powered Arduino (the master) might cause a blocked I2C bus (since at some places the library waits for the lines to get high. The connection through the clamping diodes might hold the line at low. I'm not sure about that). Then you would need to reset both Arduinos, to make sure, that the bus is in a healthy state.

The solution from above (powering both Arduinos from one USB port) will also make sure, that both Arduinos start at the same time and thus won't block the bus through the clamping diodes.

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