Re: Wireless - switchless kernal mod

On Tue, Apr 3, 2018 at 7:44 PM, Mike Naberezny <mike@naberezny.com> wrote:
>
> I used an AVR pin change interrupt to detect an edge on a past project.  The
> code was in assembly.  The AVR has a large number of working registers so I
> reserved one of them for the pin change interrupt service routine.  The very
> first instruction of the ISR, before preserving registers or anything else,
> was to read the port into that dedicated register.  This was to capture the
> state of the pin as close in time as possible to when the edge occurred.
> The ISR would then compare the value to detect the edge and either continue
> or bail out.
>
> Source code:
> https://github.com/mnaberez/vwradio/blob/b06c3a95f9b345dc4330cc3a8751b3f6244829ba/avr_volume/firmware/m62419fp_spi.asm#L50-L61
>
> This is only an approximation of edge detection.  Several processor cycles
> will occur between the edge and that first instruction of the ISR capturing
> the port value.  The pin may change state again during that time.  In my
> project, the signal I was sampling was slow enough that this was acceptable.
>
> Regards,
> Mike

Nice clever approach! I'm familiar with the syntax but I never handled
these microcontrollers with only assembly. So I'll definitely go along
the C route even if I hate soldering bodge wires.

On Tue, Apr 3, 2018 at 8:49 PM, Jim Brain <brain@jbrain.com> wrote:
> I would recommend not starting/stopping.  Just set up a free running
> counter, with the accuracy you need, and then do:
>
>
> ISR() {  //counter 0 compare match, runs at 10uS
>     if(ctr < 255)
>         ctr++;
> }
>

I didn't want to use interrupts for measuring time thinking they might
interfere with external interrupts but I see there are separate bits
for each interrupt. So timer interrupts don't inhibit external
interrupts.

Instead of 10uS I used 50uS. I wanted to be more verbose about the
status of transfer so I created two separate state machines, one for
bit transfer and another for byte transfers. Possibly there are
redundant parts in the code but I tested in the Atmel Studio and it
seems to be working.

Here is the Attiny85 code : https://pastebin.com/7YWL0e9G

I also wrote the kernal selector program. https://pastebin.com/a17v1sJB

Gave it a try and guess what :) It didn't work.

I grabbed my logic analyzer and sampled /OE line while kernal selector
is running (it disables interrupts and only accesses kernal to clear
the screen upon launching). There seems to be lots of accesses even if
I think I did cut access to kernal.

Back to the starting point :)

Regards,

Nejat