import sys
import time
from machine import Pin

# Define SPI pins (Master role)
# GP2: MOSI (Master Out Slave In)
# GP3: MISO (Master In Slave Out)
# GP4: SCLK (Clock)
# GP5: CS (Chip Select, Active-Low)
MOSI = Pin(2, Pin.OUT, value=0)
MISO = Pin(3, Pin.IN, Pin.PULL_DOWN)
SCLK = Pin(4, Pin.OUT, value=0)  # CPOL = 0 (Clock idle low)
CS = Pin(5, Pin.OUT, value=1)  # CS active-low (Idle high)

# Using CPOL=0, CPHA=0 scheme


def spi_transfer_byte(b):
  CS.value(0)  # Assert Chip Select (Active Low)
  time.sleep_us(10)

  received_byte = 0
  for i in range(7, -1, -1):
    # Set MOSI bit (MSB first)
    bit = (b >> i) & 1
    MOSI.value(bit)

    time.sleep_us(10)
    SCLK.value(1)  # Rising edge (Sample/Shift)

    # Read MISO bit back
    miso_bit = MISO.value()
    received_byte |= miso_bit << i

    time.sleep_us(10)
    SCLK.value(0)  # Falling edge

  CS.value(1)  # Deassert Chip Select
  time.sleep_us(10)
  return received_byte


print("Software SPI Master Ready. Enter text:")

while True:
  line = sys.stdin.readline()
  if not line:
    continue
  text = line.strip()
  if not text:
    continue

  print(f"Transmitting via SPI: {text}")
  received_chars = []

  for char in text:
    b = ord(char)
    rx_b = spi_transfer_byte(b)
    received_chars.append(chr(rx_b))

  decoded_str = "".join(received_chars)
  print(f"Received back via MISO loopback: {decoded_str}")