Micropython code for programming the MS5351M. Not yet tested.

main.py

import machine
import time
 
I2C_ADDR = 0x60 # Default Si5351A I2C address
I2C_SDA_PIN = 8   # RP2040 GP8
I2C_SCL_PIN = 9   # RP2040 GP9
 
def setup_si5351(i2c):
    def write_reg(reg, val):
        i2c.writeto_mem(I2C_ADDR, reg, bytes([val]))
 
    # 1. Disable all outputs
    write_reg(3, 0xFF)
 
    # 2. Power down all output drivers
    for reg in range(16, 24):
        write_reg(reg, 0x80)
 
    # 3. Calculate and configure PLLA registers for 624 MHz
    # Multiplier: a=24, b=24, c=25
    a_pll, b_pll, c_pll = 24, 24, 25
    p1_pll = 128 * a_pll + int(128 * b_pll / c_pll) - 512
    p2_pll = 128 * b_pll - c_pll * int(128 * b_pll / c_pll)
    p3_pll = c_pll
 
    write_reg(26, (p3_pll >> 8) & 0xFF)
    write_reg(27, p3_pll & 0xFF)
    write_reg(28, ((p1_pll >> 16) & 0x03) | (((p2_pll >> 16) & 0x03) << 4))
    write_reg(29, (p1_pll >> 8) & 0xFF)
    write_reg(30, p1_pll & 0xFF)
    write_reg(31, (((p3_pll >> 16) & 0x0F) << 4) | ((p2_pll >> 16) & 0x0F))
    write_reg(32, (p2_pll >> 8) & 0xFF)
    write_reg(33, p2_pll & 0xFF)
 
    # 4. Calculate and configure MultiSynth 0 registers for divide-by-40
    # Divider: a=40, b=0, c=1
    a_ms, b_ms, c_ms = 40, 0, 1
    p1_ms = 128 * a_ms + int(128 * b_ms / c_ms) - 512
    p2_ms = 128 * b_ms - c_ms * int(128 * b_ms / c_ms)
    p3_ms = c_ms
 
    write_reg(42, (p3_ms >> 8) & 0xFF)
    write_reg(43, p3_ms & 0xFF)
    write_reg(44, ((p1_ms >> 16) & 0x03) | (((p2_ms >> 16) & 0x03) << 4))
    write_reg(45, (p1_ms >> 8) & 0xFF)
    write_reg(46, p1_ms & 0xFF)
    write_reg(47, (((p3_ms >> 16) & 0x0F) << 4) | ((p2_ms >> 16) & 0x0F))
    write_reg(48, (p2_ms >> 8) & 0xFF)
    write_reg(49, p2_ms & 0xFF)
 
    # 5. Connect MS0 to CLK0, Power Up, Integer Mode, PLLA Source, 8mA drive
    # 0x4F = 0b01001111
    write_reg(16, 0x4F)
 
    # 6. Reset PLLA
    write_reg(177, 0x20)
 
    # 7. Enable CLK0
    write_reg(3, 0xFE)
 
    print("Success: Si5351A configured to output 15.6 MHz on CLK0 in RAM.")
 
def burn_to_nvram(i2c):
    """
    WARNING: The Si5351A NVRAM is One-Time Programmable (OTP).
    This permanently burns the current RAM configuration into the chip.
    """
    print("⚠️ WARNING: Initiating NVRAM burn sequence...")
    print("This is a One-Time Programmable (OTP) operation!")
    time.sleep(3) # Give user a chance to interrupt execution (Ctrl+C) if accidental
 
    def write_reg(reg, val):
        i2c.writeto_mem(I2C_ADDR, reg, bytes([val]))
 
    # Write 0xC0 to register 161 (NVM_WRITE) to trigger NVRAM burn
    write_reg(161, 0xC0)
    time.sleep(1) # Wait for NVM burn to complete
    print("✅ NVRAM burn sequence completed. This configuration is now permanent.")
 
if __name__ == "__main__":
    # Initialize I2C bus 0
    i2c = machine.I2C(0, scl=machine.Pin(I2C_SCL_PIN), sda=machine.Pin(I2C_SDA_PIN), freq=400000)
 
    # 1. Configure the RAM first to test the output
    setup_si5351(i2c)
 
    # 2. ⚠️ DANGER ZONE: Uncomment the line below ONLY AFTER verifying the 15.6 MHz 
    # output with an oscilloscope or frequency counter.
    # burn_to_nvram(i2c)