In previous posts I've set up a Single Board Computer to survive power hits (
here) and run code at startup using
systemd (
here).
What next?
Let's run some code.
In the coming weeks, time permitting, I'll reuse elements of the code in some new projects.
Here is how I ran each test script.
- used raspi-config to enable I2C and SPI on the SBC (tutorial here)
- SSH'd into the SBC
- cd'd to the directory containing the file
- ran it: python3 [name of the script]
- Outputs were visible at DAC output pins and/or the SBC's via its terminal.
I2C--MCP4728
The
MCP4728 is an inexpensive 4-channel 12-bit I2C DAC IC.
The wiring used:
 |
| I2C wiring (SDA, SCL) is the same for other I2C parts used in this post. |
The RPI's pinout:
 |
| To orient this correctly, put the SBC on the bench and face its HDMI jack to the left. Useful webpage and site describing this is here. |
 |
| Pinout of the MCP4728: (p. 22 of data sheet) 1. Vdd: 3.3V 2 and 3 I2C. 4: tie to GND 5: EEPROM (not used for the examples; let it float) 6-9: outputs, 10: tie to GND |
 |
| MCP4728 Breakout Board. How to program the IC begins on page 23 of its datasheet. |
To make this go, I needed to use
uv to add
smbus2 (I cover uv basics in the
last post).
uv add smbus2
smbus abstracts the bit-banging needed to make I2C work--more
here.
The 4728 DAC IC supports two modes--"multi-write" allowing writes to any of its 4 channels independently and "fast-write" sending output data to all 4 channels in a single burst.
Python code for multi-write, in this case writing a single value to channel B that can be seen with a DVM or scope:
import smbus2 as smbus
#smbus2 can be imported from uv--smbus can't
########################
"""
example: writing value(s) to a single channel of the mcp4728 (B in this case)
even at 400K i2c, not a very fast process, so not good for everything.
OK for simple tasks.
"""
bus = smbus.SMBus(1)
DEVICE_ADDRESS = 0x60 #default addr.
def set_channel_b_voltage(value, vref=1, gain=0):
"""
Sets voltage for CHANNEL B only using Multi-Write Command.
value: 0-4095
vref: 1 (Internal 2.048V), 0 (VDD)
gain: 0 (x1), 1 (x2)
"""
value = max(0, min(4095, value))
# 1. Build Command Byte for Channel B
# Command: 0 1 0 0 0 (Multi-Write)
# Channel B: 0 1
# UDAC: 0 (Update now)
# Binary: 01000010 -> Hex: 0x42
command_byte = 0x42
# 2. Build Upper Data Byte (Config + D11-D8)
# Bits: VREF PD1 PD0 Gx D11 D10 D9 D8
# We assume PD1=0, PD0=0 (Normal Power)
upper_data_bits = (value >> 8) & 0x0F
config_bits = (vref << 7) | (0 << 5) | (gain << 4)
byte2 = config_bits | upper_data_bits
# 3. Build Lower Data Byte (D7-D0)
byte3 = value & 0xFF
# 4. Send Command
try:
# write_i2c_block_data(Address, CommandByte, [DataBytes])
bus.write_i2c_block_data(DEVICE_ADDRESS, command_byte, [byte2, byte3])
print(f"Channel B updated to {value}")
except OSError as e:
print(f"Error: {e}")
# --- Test ---
# Set Channel B to ~1.024V (Half Scale)
Same idea but using "fast-mode" to produce 4 distinct DC voltages at each output.
import smbus2 as smbus
import time
#smbus2 can be imported from uv--smbus can't
########################
"""
example: writing to a all four channels of the mcp4728
even at 400K i2c, not a very fast process, so not good for everything.
but--fast write is faster vs writing data to each channel individually.
"""
# Initialize I2C
bus = smbus.SMBus(1)
DEVICE_ADDRESS = 0x60
def set_all_channels(va, vb, vc, vd):
"""
Updates all 4 channels (A, B, C, D) simultaneously using Fast Write.
Values must be 0-4095.
"""
# 1. Clamp values to 12-bit range (0-4095)
values = [va, vb, vc, vd]
data_payload = []
for val in values:
val = max(0, min(4095, val))
# 2. Split 12-bit value into two bytes
# Byte 1: 0 0 0 0 D11 D10 D9 D8 (Assuming Normal Power Mode)
# We shift the value right by 8 bits to get the top nibble
# and mask with 0x0F to ensure the command bits (top 4) are 0000.
upper_byte = (val >> 8) & 0x0F
# Byte 2: D7 D6 D5 D4 D3 D2 D1 D0
lower_byte = val & 0xFF
data_payload.extend([upper_byte, lower_byte])
# 3. Send Data
# write_i2c_block_data(Address, Register/Cmd, [Data List])
# The MCP4728 Fast Write command doesn't use a standard register address.
# The "Command" IS the first byte of the data stream.
# We strip the first byte off our payload to use as the "Cmd" argument,
# and pass the rest as the list of data bytes.
try:
bus.write_i2c_block_data(DEVICE_ADDRESS, data_payload[0], data_payload[1:])
except OSError as e:
print(f"I2C Error: {e}")
# --- Example Usage ---
try:
print("Starting 4-Channel Waveform...")
while True:
# Example: Set different static voltages
# A=0V, B=1V, C=2V, D=Full Scale (approx)
set_all_channels(0, 2000, 3000, 4095)
time.sleep(1)
# Example: Invert them
set_all_channels(4095, 3000, 2000, 0)
time.sleep(1)
except KeyboardInterrupt:
# Turn off outputs on exit
set_all_channels(0, 0, 0, 0)
print("\nStopped.")
I was curious how the IC performed. Can the 4728 produce decent, quickly modulating AC? Here's code that created a triangle wave using a lookup table.
import smbus2 as smbus
import time
"""
needs smbus2 module--add that first.
uses a lookup table to create_triangle_table
a triangle output on Chan A
Requires 400K I2C
"""
# --- Configuration ---
DEVICE_ADDRESS = 0x60
BUS_NUMBER = 1
DAC_RESOLUTION = 4096
# Multi-Write Command Constants
CMD_MULTI_WRITE = 0x40 # 0100 0000
# Channel Select bits (shifted to match DAC1, DAC0 position)
CH_A = 0x00 # 0000 0000 (Channel A)
CH_B = 0x02 # 0000 0010 (Channel B)
# --- I2C Setup ---
try:
bus = smbus.SMBus(BUS_NUMBER)
except Exception as e:
print(f"Error opening I2C bus: {e}")
exit(1)
def create_triangle_table(steps=1000):
"""
Creates a full cycle triangle wave lookup table (0 -> 4095 -> 0).
"""
table = []
# Rising edge
for i in range(steps // 2):
val = int((i / (steps // 2)) * (DAC_RESOLUTION - 1))
table.append(val)
# Falling edge
for i in range(steps // 2):
val = int((1 - (i / (steps // 2))) * (DAC_RESOLUTION - 1))
table.append(val)
return table
def send_value_channel_a(value):
"""
Sends a specific 12-bit value to CHANNEL A using Multi-Write.
"""
# Clamp value
value = max(0, min(4095, int(value)))
# --- Construct Multi-Write Command ---
# Byte 1: Command (01000) | Channel (00 for A) | UDAC (0)
# Binary: 0100 0000 -> Hex: 0x40
cmd_byte = CMD_MULTI_WRITE | CH_A
# Byte 2: Vref(1) | PD(00) | Gain(0) | Upper Data (D11-D8)
# Vref=1 (Internal 2.048V), Gain=0 (x1)
# 0x80 is the bitmask for Vref=1
upper_nibble = (value >> 8) & 0x0F
byte2 = 0x80 | upper_nibble
# Byte 3: Lower Data (D7-D0)
byte3 = value & 0xFF
try:
bus.write_i2c_block_data(DEVICE_ADDRESS, cmd_byte, [byte2, byte3])
except OSError:
pass
def estimate_max_update_rate(samples=500):
"""
Benchmarks I2C speed on Channel A.
"""
print("Calibrating I2C speed...")
start = time.time()
for _ in range(samples):
send_value_channel_a(2048)
duration = time.time() - start
rate = samples / duration
print(f"Max Update Rate: {rate:.1f} Hz")
return rate
def run_triangle_wave(frequency):
"""
Generates a triangle wave on Channel A at the specified frequency.
"""
TABLE_SIZE = 2048
wave_table = create_triangle_table(TABLE_SIZE)
max_rate = estimate_max_update_rate()
# Calculate Step Size
step_size = (TABLE_SIZE * frequency) / max_rate
print(f"\n--- Outputting {frequency} Hz Triangle Wave on Channel A ---")
print(f"Step Size: {step_size:.4f}")
print("Press CTRL+C to stop.")
current_index = 0.0
try:
while True:
idx = int(current_index) % TABLE_SIZE
val = wave_table[idx]
send_value_channel_a(val)
current_index += step_size
except KeyboardInterrupt:
print("\nStopping...")
send_value_channel_a(0) # Turn off
if __name__ == "__main__":
try:
freq_input = float(input("Enter desired frequency (Hz): "))
run_triangle_wave(freq_input)
except ValueError:
print("Please enter a valid number.")
The output was not great....a 100hz Triangle wave looked "stair steppy":
 |
| At 100hz, stair-step but usable (maybe). At higher frequencies the DAC's output became heavily distorted and unusable.... |
The performance was held back by the overhead imposed by Python as well as speed limits of I2C.
Increasing the SBC's I2C speed to 400K didn't help much. That's done on an RPi by editing "config.txt":
sudo vi /boot/firmware/config.txt
#edit these lines:
#dtparam=i2c_arm=on
#dtparam=i2c_baudrate=400000
Did 400K and address the MCP4728 using "fast-mode" help? Not really. The code below, to create 4 triangle waveforms 90 degrees out of phase with each other, "worked" but again its output was again too blocky and looked downright awful at frequencies above about 250hz.
import smbus2 as smbus
import time
#uv can only import smbus2.
"""
create a triangle lookup table and send 4 signals 90 degrees out of phase
to each of mcp4728's 4 channels.
Works, but output is stairstepped--even at 400k i2c we are pushing
the limits of what this tech can do.
Looks better at sub-audio frequences (eg, as a quadrature LFO)
Prob needs an RC HP filter for each channel....
"""
# --- Configuration ---
DEVICE_ADDRESS = 0x60
BUS_NUMBER = 1
TABLE_SIZE = 2048
DAC_MAX = 4095
# --- I2C Setup ---
try:
bus = smbus.SMBus(BUS_NUMBER)
except Exception as e:
print(f"Error opening I2C bus: {e}")
exit(1)
def create_triangle_table(steps):
"""Generates a lookup table for a 0-4095-0 triangle wave."""
table = []
half_steps = steps // 2
# Rising (0 -> 4095)
for i in range(half_steps):
val = int((i / half_steps) * DAC_MAX)
table.append(val)
# Falling (4095 -> 0)
for i in range(half_steps):
val = int((1 - (i / half_steps)) * DAC_MAX)
table.append(val)
return table
def write_channel(channel_idx, value):
"""
Writes to a SINGLE channel using the Multi-Write Command.
channel_idx: 0=A, 1=B, 2=C, 3=D
"""
value = max(0, min(4095, int(value)))
# --- Command Byte Construction ---
# Bits: 0 1 0 0 0 DAC1 DAC0 UDAC
# Multi-Write Code: 01000 (0x08 << 3 = 0x40)
# Channel: Shifted left by 1 bit
# UDAC: 0 (Update Immediately)
cmd_byte = 0x40 | (channel_idx << 1)
# --- Data Bytes ---
# Byte 2: Vref(1) | PD(00) | Gain(0) | Upper Data
# Vref=1 (Internal 2.048V) -> 0x80
upper_nibble = (value >> 8) & 0x0F
byte2 = 0x80 | upper_nibble
# Byte 3: Lower Data
byte3 = value & 0xFF
try:
bus.write_i2c_block_data(DEVICE_ADDRESS, cmd_byte, [byte2, byte3])
except OSError:
pass
def run_quad_phase_wave(frequency):
wave_table = create_triangle_table(TABLE_SIZE)
# Calculate Phase Offsets (indices)
offset_0 = 0
offset_90 = TABLE_SIZE // 4 # 512
offset_180 = TABLE_SIZE // 2 # 1024
offset_270 = (TABLE_SIZE * 3) // 4 # 1536
# Estimate speed for loop timing
# Since we do 4 separate writes now, it will be slower.
# Let's assume approx 300-400 full updates/sec without sleep.
estimated_updates_per_sec = 350.0
step_size = (TABLE_SIZE * frequency) / estimated_updates_per_sec
print(f"\n--- Outputting {frequency} Hz Quad-Phase Waves ---")
print("Sequential Mode: Updating A -> B -> C -> D")
print("Verify voltages with Multimeter/Scope.")
current_index = 0.0
try:
while True:
# Base Index
base_idx = int(current_index)
# Calculate values for all 4 channels
val_a = wave_table[(base_idx + offset_0) % TABLE_SIZE]
val_b = wave_table[(base_idx + offset_90) % TABLE_SIZE]
val_c = wave_table[(base_idx + offset_180) % TABLE_SIZE]
val_d = wave_table[(base_idx + offset_270) % TABLE_SIZE]
# Send them sequentially
# 0=A, 1=B, 2=C, 3=D
write_channel(0, val_a)
write_channel(1, val_b)
write_channel(2, val_c)
write_channel(3, val_d)
current_index += step_size
except KeyboardInterrupt:
print("\nStopping...")
for i in range(4):
write_channel(i, 0)
if __name__ == "__main__":
try:
freq = float(input("Enter frequency (Hz) [Try 1.0 for multimeter testing]: "))
run_quad_phase_wave(freq)
except ValueError:
print("Invalid number.")
Next let's move on to an 8-bit I2C ADC:
I2C--PCF8591
The
PCF8591 is a super affordable analog to digital converter (ADC) with 4 inputs one DAC output. As a proof of concept I created some code for simple ADC and DC tests.
PCF8591 as ADC:
The BOB had a pot, I could turn that to see outputs change on the SBC (I was SSH'd into the SBC, so this was visible in the same terminal I used to run the code):
import smbus2 as smbus
import time
#reads input 3 of the PCF8591 and displays every 500ms
#on most bob's that's the built in pot.
#make sure to have BOB jumper P6 in place.
bus = smbus.SMBus(1)
ADDRESS = 0x48
# We use 0x43 instead of 0x03
# 0x40 (Enable DAC Output) + 0x03 (Channel 3)
# Many modules require the DAC to be enabled to clock the ADC correctly.
COMMAND = 0x43
def read_ain3_reliable():
# 1. Send Control Byte
bus.write_byte(ADDRESS, COMMAND)
# 2. Dummy Read (Clears the buffer)
bus.read_byte(ADDRESS)
# 3. Real Read (Gets fresh data)
return bus.read_byte(ADDRESS)
print("Reading Potentiometer on AIN3 (Reliable Mode)...")
print("Press CTRL+C to stop.")
try:
while True:
pot_val = read_ain3_reliable()
# Convert to Voltage
voltage = (pot_val / 255.0) * 3.3
# simple bar chart
bar = '#' * int(pot_val / 5)
print(f"Value: {pot_val:<3} | Voltage: {voltage:.2f}V | {bar}", end='\r')
time.sleep(0.1)
except KeyboardInterrupt:
print("\nStopped.")
....and coded a simple "read all 4 inputs" script:
import smbus2 as smbus
import time
#make sure to uv add smbus2
#reads voltages present on the 4 PCV8591 inputs.
bus = smbus.SMBus(1)
ADDRESS = 0x48
def read_all_channels():
results = []
# We cycle through channels 0 to 3
for channel in range(4):
# 1. Write Control Byte to select the channel
# Control Byte: 0x40 (Enable Output) | channel_index
bus.write_byte(ADDRESS, 0x40 | channel)
# 2. Dummy Read (clears the previous conversion from the buffer)
bus.read_byte(ADDRESS)
# 3. Real Read (gets the current value)
value = bus.read_byte(ADDRESS)
results.append(value)
return results
print("PCF8591 input Scanner...")
print("Change input voltages and see which column changes!")
print("AIN0 AIN1 AIN2 AIN3")
print("---------------------------")
while True:
vals = read_all_channels()
# Print formatted columns
print(f"{vals[0]:<7} {vals[1]:<7} {vals[2]:<7} {vals[3]:<7}", end='\r')
time.sleep(0.2)
PCF8591 as DAC:
The PCF8591 had a single channel 8 bit DAC; I wrote some code to make its output a 10hz ramp (which clipped, but, whatever. I'll fix it later--this was good enough for now).
import smbus2 as smbus
import time
# --- Configuration ---
DEVICE_ADDRESS = 0x48
BUS_NUMBER = 1
# Control Byte: 0x40
# Bit 6 (1) = Analog Output Enable (Turn on DAC)
# Bits 1-0 (00) = Channel 0 (doesn't matter for DAC, but good default)
CMD_ENABLE_DAC = 0x40
try:
bus = smbus.SMBus(BUS_NUMBER)
except Exception as e:
print(f"Error opening I2C bus: {e}")
exit(1)
def set_dac_value(value):
"""
Writes a value (0-255) to the PCF8591 DAC.
Protocol: [Address] [Control Byte] [Data Byte]
"""
# Clamp value to 8-bit range
value = int(max(0, min(255, value)))
try:
# write_byte_data sends: Address -> Register(Cmd) -> Value
bus.write_byte_data(DEVICE_ADDRESS, CMD_ENABLE_DAC, value)
except OSError:
pass # Ignore I2C errors to keep the wave running
def estimate_speed(samples=500):
"""
Benchmarks how fast your Pi can talk to this specific chip.
"""
print("Calibrating I2C speed...")
start = time.time()
for _ in range(samples):
set_dac_value(128)
duration = time.time() - start
rate = samples / duration
print(f"Max Update Rate: {rate:.1f} Hz")
return rate
def run_ramp_wave(target_freq):
# 1. Benchmark
max_rate = estimate_speed()
# 2. Calculate Step Size
# Total range is 256 steps (0 to 255)
# Total steps needed per second = 256 * frequency
# We can only do 'max_rate' updates per second.
step_size = (256 * target_freq) / max_rate
print(f"\n--- Outputting {target_freq} Hz Ramp Wave ---")
print(f"Step Increment: {step_size:.4f}")
print("Press CTRL+C to stop.")
current_val = 0.0
try:
while True:
# Send current integer value
set_dac_value(current_val)
# Increment
current_val += step_size
# Reset if we hit the top (Sawtooth shape)
if current_val >= 256:
current_val -= 256
except KeyboardInterrupt:
print("\nStopping...")
set_dac_value(0) # Turn off output
if __name__ == "__main__":
run_ramp_wave(10) # 10 Hz Target
DAC output:
I2C OLED
DiWHY digital projects usually aren't complete without a cheap OLED. Could I get that to work? Yep.
First I needed to add some app packages to Linux....
sudo apt-get update
sudo apt-get install libjpeg-dev zlib1g-dev libfreetype6-dev liblcms2-dev libopenjp2-7 libtiff5
Then I added
luma.oled using uv--I used luma to drive the OLED since it didn't consume a lot of memory:
uv add luma.oled
Code used:
import time
from luma.core.interface.serial import i2c
from luma.core.render import canvas
from luma.oled.device import ssd1306
# Initialize I2C connection (Port 1 is standard for Pi)
serial = i2c(port=1, address=0x3C)
# Initialize the SSD1306 device
# Note: Ensure width/height match your specific display (usually 128x64 or 128x32)
device = ssd1306(serial, width=128, height=64)
print("Display initialized. Press Ctrl+C to exit.")
try:
while True:
# The 'canvas' allows you to draw on the screen
# It automatically clears the screen at the start of the block
# and displays the result at the end of the block.
with canvas(device) as draw:
# Draw a white rectangle border
draw.rectangle(device.bounding_box, outline="white", fill="black")
# Draw text
draw.text((30, 25), "Hello DiWHY ", fill="white")
# Prevent the script from consuming 100% CPU
time.sleep(1)
except KeyboardInterrupt:
# Clear screen on exit
device.cleanup()
print("Exiting...")
 |
| "seems working" |
SPI--MCP3002 ADC
For most projects I use SPI: better quality ADC reads and DAC writes vs. I2C.
uv add spidev
The
MCP3002 is a 10-bit 2-channel affordable IC; as a proof of concept I wrote a simple "read both channels and throw it into the terminal" script as a proof-of-concept.
 |
| I hate breadboarding.... |
import spidev
import time
# SPI setup
spi = spidev.SpiDev()
spi.open(0, 0) # Bus 0, Device 0 (CE0)
spi.max_speed_hz = 1000000 # 1 MHz
def read_mcp3002(channel):
"""
Reads data from MCP3002 (10-bit ADC).
Channel must be 0 or 1.
"""
if channel > 1 or channel < 0:
return -1
# datasheet Table 5-1 and Fig 5-1 [cite: 1706, 1707]
# We construct the control byte:
# Start Bit (1) | SGL/DIFF (1) | ODD/SIGN (channel) | MSBF (1)
# We position the Start bit to align with the SPI clocking.
# Sending 0x60 (0110 0000) places the start bit at bit 6.
# The (channel << 4) selects CH0 (0) or CH1 (1).
config_bits = 0x60 | (channel << 4)
# We send [Config, Dummy Byte]
# The device returns 10 bits of data.
resp = spi.xfer2([config_bits, 0x00])
# The result usually comes back across the two bytes.
# We need to mask and shift to get the 10-bit value.
# Based on alignment of 0x60, the valid data is in the last 10 bits.
# Parse the 10-bit result
result = ((resp[0] & 0x03) << 8) | resp[1]
return result
try:
print("Testing MCP3002 Inputs (Raw SPI)...")
while True:
adc_val_0 = read_mcp3002(0)
adc_val_1 = read_mcp3002(1)
# Convert to voltage (assuming 3.3V Vref)
voltage_0 = (adc_val_0 * 3.3) / 1023
voltage_1 = (adc_val_1 * 3.3) / 1023
print(f"CH0: {adc_val_0} ({voltage_0:.2f}V) | CH1: {adc_val_1} ({voltage_1:.2f}V)")
time.sleep(0.5)
except KeyboardInterrupt:
spi.close()
print("\nSPI Closed. Exiting...")
Output looked good:
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SPI--MCP4922
The
MCP4922 is an affordable 12-bit 2 channel DAC.
I see them cropping up in lots of DIY projects. Would it work with the indestructible SBC on the bench? You betcha.
import spidev
import time
import sys
"""
you need linux app python3-dev.
sudo apt-get update
sudo apt-get install python3-dev
you also need spidev, to add with UV:
uv add spidev
"""
# --- Config ---
SPI_BUS = 0
SPI_DEVICE = 0 # Ensure your CS wire is on Pin 24 (CE0)
spi = spidev.SpiDev()
try:
spi.open(SPI_BUS, SPI_DEVICE)
spi.max_speed_hz = 1000000
spi.mode = 0b00 # Explicitly set Mode 0 (CPOL=0, CPHA=0)
except Exception as e:
print(f"Error: {e}")
sys.exit(1)
def send_dac(channel, value):
# Channel 0 = A, 1 = B
# Config bits: Buffered, 1x Gain, Active
# A: 0011 (0x3) | B: 1011 (0xB)
# Note: I changed bit 12 to '1' (Active) and bit 13 to '1' (1x Gain)
# The previous script used 0x7 and 0xF which is also valid but let's be explicit.
config = 0x3000 if channel == 0 else 0xB000
data = config | (value & 0xFFF)
upper = (data >> 8) & 0xFF
lower = data & 0xFF
spi.xfer2([upper, lower])
print("--- DEBUG MODE: Slow Toggle ---")
print("Both channels should switch between 0V and 3.3V every 3 seconds.")
print("Measure Pin 14 (A) and Pin 10 (B) now.")
try:
while True:
print("Writing 0V (Low)...")
send_dac(0, 0) # Channel A -> 0
send_dac(1, 0) # Channel B -> 0
time.sleep(3)
print("Writing 3.3V (High)...")
send_dac(0, 4095) # Channel A -> Max
send_dac(1, 4095) # Channel B -> Max
time.sleep(3)
except KeyboardInterrupt:
spi.close()
Last thing I tried was 2 triangle waves with the 2nd 90 degrees out of phase.
With a couple of inverting op amps in series, this forms a super simple, super low parts count
quadrature function generator--try doing this entirely in the analog domain, right?
import spidev
import time
import sys
"""
you need linux app python3-dev.
sudo apt-get update
sudo apt-get install python3-dev
you also need spidev, to add with UV:
uv add spidev
"""
# --- Configuration ---
SPI_BUS = 0
SPI_DEVICE = 0 # Uses CE0 (Pin 24)
SPI_SPEED = 4000000 # 4 MHz (Plenty fast for Python)
# MCP4922 Configuration Bits
# Bit 15: 0=A, 1=B
# Bit 14: Buf (1=Buffered)
# Bit 13: Gain (1=1x, 0=2x)
# Bit 12: SHDN (1=Active, 0=Shutdown)
# Config for Channel A: 0 1 1 1 (0x7)
# Note: Unbuffered (0) is fine too, but Buffered (1) drives loads better.
config_A = 0x7000
# Config for Channel B: 1 1 1 1 (0xF)
config_B = 0xF000
# DAC Resolution
DAC_MAX = 4095
TABLE_SIZE = 2048
# --- Setup SPI ---
spi = spidev.SpiDev()
try:
spi.open(SPI_BUS, SPI_DEVICE)
spi.max_speed_hz = SPI_SPEED
except Exception as e:
print(f"Error opening SPI: {e}")
sys.exit(1)
def create_triangle_table(steps):
"""Generates a lookup table for a 0-4095-0 triangle wave."""
table = []
half_steps = steps // 2
# Rising (0 -> 4095)
for i in range(half_steps):
val = int((i / half_steps) * DAC_MAX)
table.append(val)
# Falling (4095 -> 0)
for i in range(half_steps):
val = int((1 - (i / half_steps)) * DAC_MAX)
table.append(val)
return table
def write_dac(channel_config, value):
"""
Sends a value to the MCP4922.
channel_config: config_A or config_B base bits
value: 0-4095
"""
# 1. Combine Config bits with 12-bit Value
# The command is 16 bits total.
# Top 4 bits are Config, Bottom 12 are Data.
data = channel_config | (value & 0xFFF)
# 2. Split into two bytes
upper_byte = (data >> 8) & 0xFF
lower_byte = data & 0xFF
# 3. Send via SPI
# xfer2 keeps CS low for the transaction
spi.xfer2([upper_byte, lower_byte])
def benchmark_spi(samples=1000):
"""Checks how fast Python can drive the SPI loop."""
print("Benchmarking SPI speed...")
start = time.time()
for _ in range(samples):
# Write both channels to simulate real load
write_dac(config_A, 2048)
write_dac(config_B, 2048)
duration = time.time() - start
rate = samples / duration
print(f"Max Update Rate (Dual Channel): {rate:.1f} Hz")
return rate
def run_quadrature_waves(frequency):
wave_table = create_triangle_table(TABLE_SIZE)
# Measure system speed to calculate step size
max_rate = benchmark_spi()
# Calculate Step Size
# We need to traverse 'frequency' full tables per second.
step_size = (TABLE_SIZE * frequency) / max_rate
# Calculate Phase Offset (90 degrees = 1/4 table)
offset_90 = TABLE_SIZE // 4
print(f"\n--- Generating {frequency} Hz Waves (90 deg offset) ---")
print("Connect Scope to Pin 14 (Ch A) and Pin 10 (Ch B)")
print(f"Step Size: {step_size:.4f}")
current_idx = 0.0
try:
while True:
# Calculate Indices
idx_a = int(current_idx) % TABLE_SIZE
idx_b = (idx_a + offset_90) % TABLE_SIZE
# Lookup Values
val_a = wave_table[idx_a]
val_b = wave_table[idx_b]
# Write to DACs
write_dac(config_A, val_a)
write_dac(config_B, val_b)
# Increment
current_idx += step_size
except KeyboardInterrupt:
print("\nStopping...")
write_dac(config_A, 0)
write_dac(config_B, 0)
spi.close()
if __name__ == "__main__":
try:
freq = float(input("Enter frequency (Hz): "))
run_quadrature_waves(freq)
except ValueError:
print("Invalid number.")
Are we there yet? Sure.
OUTTRO:
Thank goodness for AI or I'd still be writing and debugging the code fragments in today's post--for simple things like today's POC code AI assistance was invaluable.
But now the boss wants me code 1000 more scripts in half the time it took to write 5 of them--it's a damn slippery slope.
In other news--I'm moving my workshop so that means packing everything up, labeling, figuring out space in the new space, plugging everything back in, seeing what didn't make it. So no more posts for a bit while I figure all this out.
As the dentist says--you know the drill.
See ya next time.
