Raspberry Pi I2C Quickstart

Below are my notes concerning the control of a Sparkfun MCP4725 12-bit DAC over I2C with a Raspberry Pi.

Rasbperry Pi Setup

1. Enable the I2C interface if isn't already with raspi-config. Verify that the I2C device file(s) are present in /dev/ with ls /dev | grep i2c. (I had two files: i2c-1 and i2c-2.)
2. Install the i2c-tools package for debugging I2C interfaces.

sudo apt update && sudo apt install -y i2c-tools

i2cdetect

Attach the DAC to the Raspberry Pi. The pinout is simple:

Next, run the command i2cdetect -y 1. This will check for a device on bus 1 (/dev/i2c-1) and automatically accept confirmations:

leb@raspberrypi:~/$ i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
60: 60 -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Each I2C device must have a unique 7-bit address, i.e. 0x00 to 0x7f. The ranges [0x00, 0x07] and [0x78, 0x7f] are reserved. The above output indicates the DAC is at address 0x60. (Rows are the value of the first hexadecimal number of the address, columns are the second.)

i2cset

i2cset is a command line tool that is part of i2c-tools and that is used to write data to I2C devices. I can set the voltage output of the DAC to 0 as follows:

i2cset -y 1 0x60 0x40 0x00 0x00 i

The arguments mean the following:

• -y : Auto-confirm
• 1 : Use the device on bus 1
• 0x60 : Use the device at address 0x60
• 0x40 : This is a command byte
• 0x00 0x00 : These two data bytes specify the DAC output level
• i : This is the write mode. i means I2C block write: https://docs.kernel.org/i2c/smbus-protocol.html#i2c-block-write

Command byte

The command byte is explained on pages 23 and 25 of the MCP4725 datasheet. From most-significant to least-significant bits, the bits mean:

1. C2 : command bit
2. C1 : command bit
3. C0 : command bit
4. X : unused
5. X : unused
6. PD1 : Power down select
7. PD0 : Power down select
8. X : unused

According to Table 6-2 and Figure 6-2, C2, C1, C0 = 0, 1, 0 identifies the command to write to the DAC register and NOT also to the EEPROM. In normal operation, the power down bits are 0, 0 (page 28).

So, to write to the DAC register, we want to send 0b01000000 which in hexadecimal is 0x40.

Data bytes to voltage

The data bytes are explained in Figure 6-2 of the datasheet. The first byte contains bits 11-4, and the second byte bits 3-0 in the most-significant bits:

D11 D10 D9 D8 D7 D6 D5 D4 | D3 D2 D1 D0 X X X X

12-bits are used because this is a 12-bit DAC. The mapping between bytes and voltage is:

where V_max is the voltage supplied to the chip's Vcc pin (3.3V in my case). The output step size is \( \Delta V = V_{max} / 4096 \) or about 0.8 mV.

Control via Python

This is modified from Sparkfun's tutorial and uses the smbus Python bindings. Be aware that the tutorial example has a bug in how it prepares the list of bytes to send to the DAC.

import smbus


OUTPUT_MAX: int = 4095
V_MAX = 3.3


def send(output: float, channel: int = 1, device_address: int = 0x60, command_byte: int = 0x40):
    assert output > 0.0 and output <= 1.0, "Output voltage must be expressed as fraction of the maximum in the range [0.0, 1.0]"

    bus = smbus.SMBus(channel)

    output_bytes = int(output * OUTPUT_MAX) & 0xfff
    data_byte_0: int = (output_bytes & 0xff0) >> 4  # First data byte
    data_bytes: list[int] = [data_byte_0, (output_bytes & 0xf) << 4]  # Second data byte

    bus.write_i2c_block_data(device_address, command_byte, data_bytes)


if __name__ == "__main__":
    output: float = 0.42
    send(output)

    print(f"Estimated output: {output * V_MAX}")

Misc.

Basic Calculator bc

This is a command line calculator and can be used for hexadecimal, binary, and decimal conversions. Install with apt install bc.

# Convert 0x40 to binary
echo "ibase=16; obase=2; 40" | bc

# Convert 0x40 to decimal
echo "ibase=16; 40" | bc

Note that hexadecimal values must be uppercase, e.g. 0xC7, not 0xc7!