Cec protocol using PIO

[sapfeer0k]

Hi, I have implemented a similar project a year ago , and I used PIO for bitbanging. I am going to share the code (although it's micropython) , may be you will port it to c .

@asm_pio(set_init=PIO.OUT_HIGH)
def image_view_on(): # step is 0.1ms
    wrap_target()

    set(x, 3) #4x 1
    set(y, 4) #5x 0

    # start bit
    set(pins, 0) [31]
    set(pins, 0) [3]
    set(pins, 1) [7]
    # 1 - 4 bit
    label('sender')
    set(pins, 0) [5]
    set(pins, 1) [16]
    jmp(x_dec, 'sender')
    # 5 - 8 + EOM bit
    label('receiver')
    set(pins, 0) [14]
    set(pins, 1) [8]
    jmp(y_dec, 'receiver')
    # Ack bit
    set(pins, 0) [5]
    set(pins, 1) [15] # + 2 following lines
    # second frame sents 00000100 1
    set(x, 4) #5x 0
    set(y, 1) #2x 0
    # 1 - 5 bits
    label('data_1')
    set(pins, 0) [14]
    set(pins, 1) [8]
    jmp(x_dec, 'data_1')
    # send 6th bit 1
    set(pins, 0) [5]
    set(pins, 1) [16]
    # send 7-8 bits 0
    label('data_2')
    set(pins, 0) [14]
    set(pins, 1) [8]
    jmp(y_dec, 'data_2')
    # EOM bit
    set(pins, 0) [5]
    set(pins, 1) [16]
    # Ack bit
    set(pins, 0) [5]
    set(pins, 1) [17]
    irq(0)
    pull()
    wrap()

Let me know your opinion. Thanks

[gkoh]

Thanks for raising this discussion @sapfeer0k.
During prototyping I tried to implement the core, generic, CEC bit handler in PIO, however I kept running out of instructions.
I eventually determined the only solution was to split the PIO state machine into two pieces:

• one to handle transmit
• one to handle receive
• ... and load the correct PIO code when needed

I was able to implement transmit in the requisite PIO space.
However, receive was an unsolved challenge:

• I was unable to make it fit in 32 instructions and detect/fail on timing errors

Thank you for code, which handles the transmit side quite well. And indeed I may add a separate implementation to do transmit in PIO.
One reason I haven't (yet) is portability. The current CEC handler is in C and really only requires:

• edge configurable interrupts
• sub-millisecond timer
  this means it can be ported to almost any micro-controller.

One question, I don't see in your code where you detect the ACK from the target.
Am I misreading it?

[sapfeer0k]

Hi @gkoh ,

I also have faced the same problem with the PIO instruction length limitation. And I had to split reception / transmission PIO code. (Although I don't use reception, but it is almost the same)

Your observation about the missing ACK detection is correct. I simply ignored that part , because I was unable to make it work reliably.
I was quite new to RP2040 and embedded programming at that time (it is actually my first project with microcontrollers). I have no tools to debug efficiently (like osciloscope / analyzers).
The reason I have implemented this - I wanted to fix remote wake up for my LG TV (somehow it is broken and TV loses WI-FI connection after a day in a sleep mode). I have created a WI-FI (WOL packet) -> CEC bridge on Pico and it serves me well and consumes less energy that the TV in standby (https://imgur.com/a/Zvz6nfc).

[gkoh]

Doing this as your first project and without an oscilloscope is quite impressive, good work!
I definitely needed to pull out my oscilloscope to confirm signal timing.

If you want to continue improving it, I can suggest a few changes:

• change the step time to 0.15ms
  • this will reduce instruction count (and timing will be +0.05ms out, within the 0.2ms spec)
  • so start bit will be set(pins, 0) [25] for a 3.75ms low
  • this saves ... 1 instruction, but we only have 32
• consider using PULL to get your x and y values from the higher level code
  • if done right, you could collapse the header and data block transmit into one loop
  • this could save quite a few instructions

As for the ACK bit ... it's not really needed if your TV does CEC properly. It is needed if the message is lost/ignored and need to retry transmit.

I'd be interested in what you tried with the ACK bit handling and what made it unreliable.

Oh, one more question: How are you powering your Pico? Just USB?