from machine import Pin
from time import sleep_us


MOSI_PIN = 2
MISO_PIN = 3
SCLK_PIN = 4
CS_PIN = 5

mosi = Pin(MOSI_PIN, Pin.OUT)
miso = Pin(MISO_PIN, Pin.IN)
sclk = Pin(SCLK_PIN, Pin.OUT)
cs = Pin(CS_PIN, Pin.OUT)


HALF_PERIOD_US = 100


sclk.value(0)
cs.value(1)
mosi.value(0)


def spi_transfer_byte(tx_byte):

    rx_byte = 0

    cs.value(0)

    sleep_us(HALF_PERIOD_US)

    for bit_index in range(7, -1, -1):

        out_bit = (tx_byte >> bit_index) & 1

        mosi.value(out_bit)

        sleep_us(HALF_PERIOD_US)

        sclk.value(1)

        in_bit = miso.value()

        rx_byte = (rx_byte << 1) | in_bit

        sleep_us(HALF_PERIOD_US)

        sclk.value(0)

    sleep_us(HALF_PERIOD_US)

    cs.value(1)

    mosi.value(0)

    return rx_byte


def spi_transfer_string(text):

    received_chars = []

    for ch in text:

        tx_byte = ord(ch)

        if tx_byte > 127:
            raise ValueError("Only ASCII characters are allowed")

        rx_byte = spi_transfer_byte(tx_byte)

        print(
            "TX: 0x%02X   RX: 0x%02X"
            % (tx_byte, rx_byte)
        )

        received_chars.append(chr(rx_byte))

    return "".join(received_chars)



print()
print("Software SPI Loopback")
print("---------------------")
print("SPI Mode 0")
print("CPOL = 0")
print("CPHA = 0")
print("Bit order = MSB First")
print()
print("MOSI = GP2")
print("MISO = GP3")
print("SCLK = GP4")
print("CS   = GP5")
print()


while True:

    text = input("Enter ASCII string: ")

    if text == "":
        print("Please enter at least one character.")
        print()
        continue

    try:

        received = spi_transfer_string(text)

        print()
        print("Sent     :", text)
        print("Received :", received)

        if received == text:
            print("Loopback test: PASS")
        else:
            print("Loopback test: FAIL")

        print()

    except ValueError as error:

        print(error)
        print()