137 lines
4.4 KiB
ReStructuredText
137 lines
4.4 KiB
ReStructuredText
.. currentmodule:: pyb
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.. _pyb.DAC:
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class DAC -- digital to analog conversion
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=========================================
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The DAC is used to output analog values (a specific voltage) on pin X5 or pin X6.
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The voltage will be between 0 and 3.3V.
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*This module will undergo changes to the API.*
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Example usage::
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from pyb import DAC
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dac = DAC(1) # create DAC 1 on pin X5
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dac.write(128) # write a value to the DAC (makes X5 1.65V)
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dac = DAC(1, bits=12) # use 12 bit resolution
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dac.write(4095) # output maximum value, 3.3V
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To output a continuous sine-wave::
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import math
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from pyb import DAC
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# create a buffer containing a sine-wave
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buf = bytearray(100)
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for i in range(len(buf)):
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buf[i] = 128 + int(127 * math.sin(2 * math.pi * i / len(buf)))
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# output the sine-wave at 400Hz
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dac = DAC(1)
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dac.write_timed(buf, 400 * len(buf), mode=DAC.CIRCULAR)
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To output a continuous sine-wave at 12-bit resolution::
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import math
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from array import array
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from pyb import DAC
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# create a buffer containing a sine-wave, using half-word samples
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buf = array('H', 2048 + int(2047 * math.sin(2 * math.pi * i / 128)) for i in range(128))
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# output the sine-wave at 400Hz
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dac = DAC(1, bits=12)
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dac.write_timed(buf, 400 * len(buf), mode=DAC.CIRCULAR)
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Constructors
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------------
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.. class:: DAC(port, bits=8, *, buffering=None)
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Construct a new DAC object.
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``port`` can be a pin object, or an integer (1 or 2).
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DAC(1) is on pin X5 and DAC(2) is on pin X6.
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``bits`` is an integer specifying the resolution, and can be 8 or 12.
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The maximum value for the write and write_timed methods will be
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2\*\*``bits``-1.
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The *buffering* parameter selects the behaviour of the DAC op-amp output
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buffer, whose purpose is to reduce the output impedance. It can be
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``None`` to select the default (buffering enabled for :meth:`DAC.noise`,
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:meth:`DAC.triangle` and :meth:`DAC.write_timed`, and disabled for
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:meth:`DAC.write`), ``False`` to disable buffering completely, or ``True``
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to enable output buffering.
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When buffering is enabled the DAC pin can drive loads down to 5KΩ.
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Otherwise it has an output impedance of 15KΩ maximum: consequently
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to achieve a 1% accuracy without buffering requires the applied load
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to be less than 1.5MΩ. Using the buffer incurs a penalty in accuracy,
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especially near the extremes of range.
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Methods
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-------
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.. method:: DAC.init(bits=8, *, buffering=None)
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Reinitialise the DAC. *bits* can be 8 or 12. *buffering* can be
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``None``, ``False`` or ``True``; see above constructor for the meaning
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of this parameter.
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.. method:: DAC.deinit()
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De-initialise the DAC making its pin available for other uses.
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.. method:: DAC.noise(freq)
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Generate a pseudo-random noise signal. A new random sample is written
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to the DAC output at the given frequency.
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.. method:: DAC.triangle(freq)
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Generate a triangle wave. The value on the DAC output changes at the given
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frequency and ramps through the full 12-bit range (up and down). Therefore
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the frequency of the repeating triangle wave itself is 8192 times smaller.
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.. method:: DAC.write(value)
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Direct access to the DAC output. The minimum value is 0. The maximum
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value is 2\*\*``bits``-1, where ``bits`` is set when creating the DAC
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object or by using the ``init`` method.
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.. method:: DAC.write_timed(data, freq, *, mode=DAC.NORMAL)
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Initiates a burst of RAM to DAC using a DMA transfer.
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The input data is treated as an array of bytes in 8-bit mode, and
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an array of unsigned half-words (array typecode 'H') in 12-bit mode.
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``freq`` can be an integer specifying the frequency to write the DAC
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samples at, using Timer(6). Or it can be an already-initialised
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Timer object which is used to trigger the DAC sample. Valid timers
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are 2, 4, 5, 6, 7 and 8.
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``mode`` can be ``DAC.NORMAL`` or ``DAC.CIRCULAR``.
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Example using both DACs at the same time::
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dac1 = DAC(1)
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dac2 = DAC(2)
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dac1.write_timed(buf1, pyb.Timer(6, freq=100), mode=DAC.CIRCULAR)
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dac2.write_timed(buf2, pyb.Timer(7, freq=200), mode=DAC.CIRCULAR)
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Constants
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---------
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.. data:: DAC.NORMAL
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NORMAL mode does a single transmission of the waveform in the data buffer,
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.. data:: DAC.CIRCULAR
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CIRCULAR mode does a transmission of the waveform in the data buffer, and wraps around
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to the start of the data buffer every time it reaches the end of the table.
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