I'm trying to implement the firmata protocol and having a bit of a difficult time deciphering the spec for writing digital pins:
I have noted the following parts of the spec of Firmata 2.3
* type command channel first byte second byte
------------------------------------------------------------------------------
* digital I/O message 0x90 port LSB(bits 0-6) MSB(bits 7-13)
and
/* two byte digital data format, second nibble of byte 0 gives the port number (e.g. 0x92 is the third port, port 2)
* 0 digital data, 0x90-0x9F, (MIDI NoteOn, but different data format)
* 1 digital pins 0-6 bitmask
* 2 digital pin 7 bitmask
*/
I'm having some difficulty interpreting the spec. I've looked at other implementations, but haven't been able to see the relationship between the spec and implementation.
So let's say I am wanting to turn on the Arduino LED (pin 13), I know it will be on the second port, port 1, so the first byte will be #{91}.
I'm getting confused about the bitmask for the second two bytes though. I know what a bitmask is, so I want to enable the right bit for the pin.
Why is the bitmask so large for the digital pins? I'm familiar with using bitmasks on the digital outputs of PLCs, which seems much different (one pin, one bit)
My thought is that pin 13 would be the 7th pin on port 1. Since I don't care about the other pins, I would mark the pin in the 2nd byte #{40} and I don't need any pins set for the third byte #{00}?
I don't think my interpretation of the bitmasks is correct, and it's probably where my error is
Am I on the right track for this? Is this the right command for setting a pin high or low?
After some strace debugging with the firmata test application, I discovered the simple command to turn on Pin 13 was:
#{912000}
and to turn it off:
#{910000}