How PIOs work on the RP23xx family

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This was recommended to me by a friend and is a great explanation of how they work, something i hadn’t really dug into in the past.

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PIOs are a very significant feature of the RP2350 and RP2040. In my application (grblHAL CNC motion controller), PIOs allow the RP2350 at 150 mhz to perform as well as the iMXRT1062 (an M7 Cortex ARM on the Teensy 4.1) at 600 mhz when generating step pulse trains with acceleration/deceleration. This is with almost no jitter because the PIO doesn’t get interrupted. grblHAL on STM32 and ESP32 have significantly more jitter.

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Yeah, PIOs are great. I did a twin AiP33628 seven segment four digit LED drive with PIOs recently, only 13 lines of pioasm, and no flicker no matter what the RP2040’s main CPU is doing.

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Any more info on this? Sounds very cool.

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Sure. I thought I had posted something to Embedded Slack, but it has got too old and disappeared.

It’s a digital clock in my office, and another in bedroom. They arrived as a cheap “soldering kit”, a PCB with SMT RGB LEDs and a plastic case with diffuser. Search for GY21261 for the kit PDF.

I didn’t like the firmware as described in the kit documentation. It was closed source. The time had to be set, no WiFi. I didn’t have a build chain for the microcontroller. I decoded the protocol with a DSO, and got help from someone on Slack who had found the datasheet (thanks, whoever that was, it’s expired content now!).

So I attached an RP2040 as a Pico W, with four I/O pins leading to the socket of the original microcontroller.

It was a rapid prototyping approach. CircuitPython. A bytearray framebuffer. Two rp2pio.StateMachine instances, one for each AiP33628, bound to different pins. The “draw the segments” function calls background_write on the pipes. The PIO code does all the work. It looks like this;

def init_draw():
    """
    configure rp2040 pio state machines

    emit a start pulse on the data line
    discard two bits of data from msb
    emit 29 bits of data msb first with clocking
    hold the clock high when done
    emit a stop pulse on the data line
    """
    global sm_m, sm_h

    pipe = adafruit_pioasm.assemble(
        """
.program pipe
.side_set 1

    pull           side 0  [8]
    set  pins 1    side 0  [1]
    set  pins 0    side 0
    out  y 2       side 0
    set  y 29      side 0

loop:
    nop            side 0
    out  pins 1    side 0
    nop            side 1
    jmp  y-- loop  side 1
    set  pins 0    side 1  [4]
    set  pins 1    side 1  [1]
    set  pins 0    side 1
    nop            side 0
        """
    )

    # minimum 30khz to prevent flicker
    # maximum 3mhz to prevent latching failure and false lit segments
    sm_h = rp2pio.StateMachine(
        pipe,
        frequency=3_000_000,
        first_sideset_pin=board.GP1,  # pico.2 -- u4.1
        first_set_pin=board.GP0,  # pico.1 -- u4.2
        first_out_pin=board.GP0,
        wait_for_txstall=False,
        out_shift_right=False,
    )

    sm_m = rp2pio.StateMachine(
        pipe,
        frequency=2_900_000,
        first_sideset_pin=board.GP3, # pico.5 -- u4.16
        first_set_pin=board.GP2,  # pico.4 -- u4.5
        first_out_pin=board.GP2,
        wait_for_txstall=False,
        out_shift_right=False,
    )

it has become a general purpose digital display since then, mostly showing the NTP synchronised local time, but during experiments can be scripted from command line of desktop to show any four digit number in red, green, or blue. Automatic recovery from reboot of wireless router was the thing that took the longest to resolve.

2 Likes
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Hi @quozl ,

Are you able to share more code, or would you happen to have a copy of the datasheet?

I’ve bought 2 of these ‘upgraded’ kits with an ESP8266 on them - but they still run the old MCU with the terrible interface - I’d love to make them general purpose displays by replacing the MCU.

Any help would be massively appreciated!

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Thanks, but no plans to release more of the code; it is not general purpose, but a subsystem for my own use and needs.

You can find the AiP33628 datasheet on the ‘net fairly easily. It isn’t in English, but the technical details are usable without translation. The datasheet is by Wuxi I-CORE Electronics Co., Ltd., document 214072, revision A2 dated 2021-08.

Can you give more detail of the variant with ESP8266 onboard? I’ve not seen that.

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Thanks for this! I had a search and hadn’t found it, these magic numbers helped narrow down the results a lot!

The ones I’ve got are from Aliexpress, they seem to have flooded the site from multiple sellers are the new flavour-of-the-month clock kit. Seems like it’s fundamentally the same circuit with the same terrible code, just with a Chinese only web UI and NTP bolted on with an ESP01 module.

I can’t seem to post the link due to forum rules, but if you search there for ESP8266 Clock Kit you’ll see them.

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Thanks, I’ve found the product. An expected enhancement, given the price of the ESP01 module. My use of Raspberry Pi Pico W to replace the MCU was not as cost effective, but the PIO feature was very helpful in removing the last irritating bit-banging problem, of the display flickering every time CircuitPython was doing anything else.

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I’m hoping I can get decent enough performance with an ESP8266/ESP32 since I already have client code written using arduino libraries for that architecture, but it’s great to know the PIO route will get me there if needed - haven’t dipped my toes in the water with PIOs or CircuitPython yet, so might be a great excuse!

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FWIW, you can use Arduino IDE and Libraries for the RP2350. The Earl Philhower core can be selected in the Arduino IDE and quite a few boards based on the chip are supported. I have yet to run into any problems with it.