Trying to understand NFC ASK Modulation

It is possible for sure, but its not really a project I would recommend taking on without an oscilloscope.

So if your looking for an excuse to purchase an MXO4 scope, id say go for it ;) 

I am Rx/Tx USB-PD with the pio on a PI pico right now which is differential Manchester, so that could provide some inspiration  

It is possible for sure, but its not really a project I would recommend taking on without an oscilloscope.

So if your looking for an excuse to purchase an MXO4 scope, id say go for it ;) 

I am Rx/Tx USB-PD with the pio on a PI pico right now which is differential Manchester, so that could provide some inspiration  

Agree, unfortunately this is a project that's very unlikely to work, because there's several hurdles here; not just the hardware design and building of the receiver, but also the decoding complexity, it won't be a nice friendly stream of usable data all the time either, there will be noise to deal with so it needs some signal processing as well (can be partially in hardware and partially in software). It's all a major project, not a weekend thing. 

If the Pico can be replaced with a PC and sound card, then that could make life easier to better understand the challenge and implement on the PC first perhaps, rather than coding for the Pico initially. But the signal would need to be mixed down to see it (e.g. with any SDR project hardware).

Yes that was in the back of my mind as I don't have a suitable scope... more like hoping those who have already purchased an MX04 scope, or equivalent, may be looking for another excuse (i.e. project) to get some more play time with their expensive toy.

A slightly similar (but different) project was a heartbeat monitor strap, they transmit ASK-like pulses at a much lower frequency (5 kHz) which in theory sounds as if it should make the project easier, but with low frequencies come other issues, especially dealing with noise. I built the front end to it last year, but didn't get around to blogging it. For that, I used op-amp buffers basically, but because the signal is weak (compared to NFC, the heartbeat straps need to be monitorable from a slightly larger distance, and they run from a coin cell so not a lot of power), it needed heavy filtering to extract the signal from the noise. I didn't use SDR, but just used sharp filters. I did manage to get the signal (receivable to almost a couple of metres!, which was more than enough for its use-case) in a decent form, but other stuff got in the way and I never got to complete it. Also my pickup coil was massive : ) About the size of a coke-can.

I don't have the schematic (I didn't create one, because I was just experimenting, and thought I was going to write the circuit down soon, but didn't) but could dig up the stripboard I created it on, would need some reverse-engineering by someone!

Well this project doesn't require anything fancy, right now I am using a 555 timer as a level translator and $10 logic analyzer! 

I am transmitting the same source advertisement packet from a Pico

Pico sending a good crc packet,

A source request

Transmitting usb-pd with the pico is quite simple, for example here is my python code for sending a goodcrc message

def goodCRC():
    # If preamble can be 65 BMC bits then it could be done here
    # Right now, PIOSM takes care of 64 BMC bits preamble
    preamble = []#[0b1010101010]*7
    sop = [symtable['S1']]*3 + [symtable['S2']]
    header = b'\x01\x01'
    data = array.array('i',[])
    payload = header+bytes(data)
    
    crc = binascii.crc32(payload)
    payload += crc.to_bytes(4,'little')
    
    txsyms = preamble + sop
    txsyms += usbpd4b5bencode(payload)
    txsyms += [symtable['EOP'],0]

    pdtx(txsyms)

The receive side is a bit more complicated... 

I think it will be a helpful project for someone just wanting to experiment and learn about usb-pd signaling 

This looks great!

Thanks! Its been quite enjoyable experimenting with the pico, its like I'm building an my first power supply again many years later its fun try to new things!

To send the same message as my power supply, the code was also quite simple too

def usbpdbroadcast():
    # If preamble can be 65 BMC bits then it could be done here
    # Right now, PIOSM takes care of 64 BMC bits preamble
    preamble = []#[0b1010101010]*7
    sop = [symtable['S1']]*3 + [symtable['S2']]
    header = b'\xa1\x69'
    data = array.array('i',[0x0a01912c,0x0002d12c,0x0003c12c,0x0004b12c,0x0005a0fa,0x000640e1])
    payload = header+bytes(data)
    
    crc = binascii.crc32(payload)
    payload += crc.to_bytes(4,'little')
    
    txsyms = preamble + sop
    txsyms += usbpd4b5bencode(payload)
    txsyms += [symtable['EOP'],0]

    pdtx(txsyms)
    
    
symtable = {
    0: 0b11110,
    1: 0b01001,
    2: 0b10100,
    3: 0b10101,
    4: 0b01010,
    5: 0b01011,
    6: 0b01110,
    7: 0b01111,
    8: 0b10010,
    9: 0b10011,
    10: 0b10110,
    11: 0b10111,
    12: 0b11010,
    13: 0b11011,
    14: 0b11100,
    15: 0b11101,
    'S1': 0b11000,
    'S2': 0b10001,
    'S3': 0b00110,
    'RST1': 0b00111,
    'RST2': 0b11001,
    'EOP': 0b01101}

Hopefully soon I will have the 2 talking to each other, I think I have most of the pieces... 

It's great that MicroPython is being used, it's ideal for experimentation like this.

MicroPython is not bad at all for data manipulation for protocols! I was stuck on a CRC issue a while back (I cant recall the specifics), and it was possible to find a normal Python library, and edit it slightly to function with MicroPython, so there's actually a wealth of potentially useful code that can be used with it, with a bit of tweaking occasionally.

I hope MicroPython gets closer to real Python so that microcontroller projects benefit more from a lot of regular Python libraries.

I discovered this interesting project online... which uses lower frequencies etc. Schematics etc included.

github.com/.../pico-dco