TinyTapeout 02 External Scanchain controller

scanchain-controller.py

from machine import Pin
import time

CLK_PERIOD_MS = 1
NUM_DESIGNS = 250  # should be higher

# from chip's perspective
# pinout here: https://projects.raspberrypi.org/en/projects/getting-started-with-the-pico/1
ext_clk = Pin(6, Pin.OUT)
ext_latch = Pin(7, Pin.OUT)
ext_scan = Pin(8, Pin.OUT)
ext_data_in = Pin(9, Pin.OUT)  # data driven by us into the chip
ext_data_out = Pin(10, Pin.IN)  # data captured by us from the chip


def scanchain_io(design_index, input_value):
    """
    design_index is the index of the design in the scan chain (0 to 249),
    input_value is the value to drive into the scan chain (0 to 255).

    Returns the digital value read from the design through the scanchain.
    """
    
    ext_data_in.value(0)
    ext_latch.value(0)
    ext_scan.value(0)
    ext_clk.value(0)

    # shift the input value into the scan chain
    for i in range(8):
        ext_data_in.value(1 if (input_value & (1 << i)) > 0 else 0)
        ext_clk.value(1)
        time.sleep_ms(CLK_PERIOD_MS)
        ext_clk.value(0)
        time.sleep_ms(CLK_PERIOD_MS)

    # now shift the data input the selected design
    for i in range(8 * design_index):
        ext_clk.value(1)
        time.sleep_ms(CLK_PERIOD_MS)
        ext_clk.value(0)
        time.sleep_ms(CLK_PERIOD_MS)

    # latch data into the dut
    ext_latch.value(1)
    time.sleep_ms(CLK_PERIOD_MS)
    ext_latch.value(0)
    time.sleep_ms(CLK_PERIOD_MS)

    # capture data by setting scan high and toggle the clock
    ext_scan.value(1)
    time.sleep_ms(CLK_PERIOD_MS)
    ext_clk.value(1)
    time.sleep_ms(CLK_PERIOD_MS)
    ext_clk.value(0)
    time.sleep_ms(CLK_PERIOD_MS)
    ext_scan.value(0)

    # drive data out through the chain
    for i in range(8 * (NUM_DESIGNS - design_index - 1)):
        ext_clk.value(1)
        time.sleep_ms(CLK_PERIOD_MS)
        ext_clk.value(0)
        time.sleep_ms(CLK_PERIOD_MS)

    out_value = 0
    for i in range(8):
        ext_clk.value(1)
        time.sleep_ms(CLK_PERIOD_MS)
        out_value |= ext_data_out.value() << i
        ext_clk.value(0)
        time.sleep_ms(CLK_PERIOD_MS)

    return out_value