.. currentmodule:: pyb .. _pyb.DAC: class DAC -- digital to analog conversion ========================================= The DAC is used to output analog values (a specific voltage) on pin X5 or pin X6. The voltage will be between 0 and 3.3V. *This module will undergo changes to the API.* Example usage:: from pyb import DAC dac = DAC(1) # create DAC 1 on pin X5 dac.write(128) # write a value to the DAC (makes X5 1.65V) dac = DAC(1, bits=12) # use 12 bit resolution dac.write(4095) # output maximum value, 3.3V To output a continuous sine-wave:: import math from pyb import DAC # create a buffer containing a sine-wave buf = bytearray(100) for i in range(len(buf)): buf[i] = 128 + int(127 * math.sin(2 * math.pi * i / len(buf))) # output the sine-wave at 400Hz dac = DAC(1) dac.write_timed(buf, 400 * len(buf), mode=DAC.CIRCULAR) To output a continuous sine-wave at 12-bit resolution:: import math from array import array from pyb 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(1, bits=12) dac.write_timed(buf, 400 * len(buf), mode=DAC.CIRCULAR) Constructors ------------ .. class:: pyb.DAC(port, bits=8, *, buffering=None) Construct a new DAC object. ``port`` can be a pin object, or an integer (1 or 2). DAC(1) is on pin X5 and DAC(2) is on pin X6. ``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 :meth:`DAC.noise`, :meth:`DAC.triangle` and :meth:`DAC.write_timed`, and disabled for :meth:`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. Methods ------- .. method:: 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. .. method:: DAC.deinit() De-initialise the DAC making its pin available for other uses. .. method:: DAC.noise(freq) Generate a pseudo-random noise signal. A new random sample is written to the DAC output at the given frequency. .. method:: 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. .. method:: 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. .. 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)