class DAC – digital to analog conversion

Important

The DAC Class is available only on the STM32-based Kookaberry, and not on the RP2040 / RP2350.

A digital-to-analog converter (DAC) is a data converter which generates an analogue output from a digital input. A DAC converts a limited number of discrete digital codes to a corresponding number of discrete analog output values.

See this on-line explanation.

A DAC is used to output analogue signals (a specific voltage) on a given GPIO pin. The voltage will be in the range 0V to 3.3V corresponding to digital value of

0 to 255 for 8-bit resolution, or
0 to 4095 if the DAC has 12-bit resolution.

On the STM32 Kookaberry only pins P4 and P5 have DAC capability. No DAC capability is provided on the RP2040 / RP2350 Kookaberry because of microcomputer hardware limitations.

Example usage:

from machine import DAC

dac = DAC('P4')         # create a 8-bit DAC on Pin P4
dac.write(128)          # write a value to the DAC (sets P4 to 1.65V)

To output a continuous sine-wave at 12-bit resolution:

import math
from array import array
from machine import DAC

# create a buffer containing a sine-wave, using half-word samples
buf = array('H', 2048 + int(2047 * math.sin(2 * math.pi * i / 128)) for i in range(128))

# output the sine-wave at 400Hz
dac = DAC('P4', bits=12)
dac.write_timed(buf, 400 * len(buf), mode=DAC.CIRCULAR)

DAC Constructors

class machine.DAC(pin, bits=8, \*, buffering=None)

Construct a new DAC object.

pin can be a machine.Pin object, or string (‘P4’ or ‘P5’).

bits is an integer specifying the resolution, and can be 8 or 12. The maximum value for the write and write_timed methods will be 2**``bits``-1.

The buffering parameter selects the behaviour of the DAC op-amp output buffer, whose purpose is to reduce the output impedance. It can be None to select the default (buffering enabled for DAC.noise(), DAC.triangle() and DAC.write_timed(), and disabled for DAC.write()), False to disable buffering completely, or True to enable output buffering.

When buffering is enabled the DAC pin can drive loads down to 5KΩ. Otherwise it has an output impedance of 15KΩ maximum: consequently to achieve a 1% accuracy without buffering requires the applied load to be less than 1.5MΩ. Using the buffer incurs a penalty in accuracy, especially near the extremes of range.

DAC Methods

DAC.init(bits=8, \*, buffering=None)

Reinitialise the DAC.

bits can be 8 or 12.

buffering can be None, False or True; see above constructor for the meaning of this parameter.

DAC.deinit(): De-initialise the DAC making its pin available for other uses.

DAC.noise(freq): Generate a pseudo-random noise signal. A new random sample is written to the DAC output at the given frequency.

DAC.triangle(freq): Generate a triangle wave. The value on the DAC output changes at the given frequency and ramps through the full 12-bit range (up and down). Therefore the frequency of the repeating triangle wave itself is 8192 times smaller.

DAC.write(value): Direct access to the DAC output. The minimum value is 0. The maximum value is 2**``bits``-1, where bits is set when creating the DAC object or by using the init method.

DAC.write_timed(data, freq, \*, mode=DAC.NORMAL)

Initiates a burst of RAM to DAC using a DMA transfer. The input data is treated as an array of bytes in 8-bit mode, and an array of unsigned half-words (array typecode ‘H’) in 12-bit mode.

freq can be an integer specifying the frequency to write the DAC samples at, using Timer(6). Or it can be an already-initialised Timer object which is used to trigger the DAC sample. Valid timers are 2, 4, 5, 6, 7 and 8.

mode can be DAC.NORMAL or DAC.CIRCULAR.

Example using both DACs at the same time:

dac1 = DAC(1)
dac2 = DAC(2)
dac1.write_timed(buf1, pyb.Timer(6, freq=100), mode=DAC.CIRCULAR)
dac2.write_timed(buf2, pyb.Timer(7, freq=200), mode=DAC.CIRCULAR)