[C.CC USERS] The operation of "digital.input" and Plumbing debugging

[Steve Pretty]

I am interested in trying to write some extra "functional blocks" to use 
with plumbing - things like timing elements and counters. I am thinking 
of simulating the link of blocks you would have in an IEC 61131 
Programmable Logic Controller. I was  looking at using the digital.input 
process (in the digital.module) as a part of this idea.  In the 
documentation comments, it states:

This procedure will output a LEVEL (either LOW or HIGH) whenever 
the**pin changes value.

I am not convinced the process works as described.  I tried the code:

PAR
   digital.input(2, v!)
   digital.output(13, v?)

If digital.input works as described, I would expect the LED to follow 
the button - i.e. come on when the button is pressed, and go off when it 
is released - but it does not. What happens is the LED comes on the 
first time the button is pressed, but stays on thereafter, whatever you 
do with the button.

I then created a PROC using the very low level digital.read to get the 
value of pin 2 - this version of code works as expected.

I think what is happening here is that digital.input process is 
interrupt driven. It is designed to work with the external interrupt 
pins of the microcontroller (arduino uno pins 2 and 3). The interrupt 
mode can be set to LOW, CHANGE, RISING or FALLING.  I have not yet 
figured out where (or if) you set the mode - but it looks to me that you 
are using RISING mode, rather than CHANGE.  If that is the case, 
digital.input will always read the pin after the rising edge, so can 
only ever send a HIGH, never a low.

This is consistent with the behaviour of the button programme in the 
Plumbing book:

button.press --> pin.toggle

Which expands to

digital.input --> debounce --> level.to.signal --> pin.toggle

With this programme, the LED comes on when the button is pressed the 
first time - stays on when button is released (showing there is no 
message from digital.input on button release). The second time the 
button is pressed the LED goes off.

I am guessing you will want to change the comment rather than having to 
rework the book examples. It would, however, be useful to have a 
digital.input that would send a HIGH and a LOW.


While I was wrestling with this issue, I became aware of how tricky it 
is to debug code in this environment (wish I had an in circuit emulator 
to hand!)  My instinct was to add serial.write statements to the code to 
show what was coming out of Digital.input - but alas, I have not been 
able to get the Printing.module to work.  Something is sent - I do get 
some random characters appearing in occplug.  I am guessing that the 
serial port in not correctly configured.  Do I have to use the 
serial.start proc to initialise things - if so, which pin(s) to I have 
to configure, and how do I know what Baud rate is required?  (The serial 
link is obviously working OK for the code loading phase).


Final thought for the day - I have read about a tool that you were once 
working on called POPExplorer - basically a visual programing 
environment for occam. Is there any ongoing work on that - it would make 
an interesting front end for plumbing.

Regards

Steve