143 lines
5.7 KiB
ReStructuredText
143 lines
5.7 KiB
ReStructuredText
.. currentmodule:: pyb
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.. _pyb.ADC:
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class ADC -- analog to digital conversion
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=========================================
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.. only:: port_pyboard
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Usage::
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import pyb
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adc = pyb.ADC(pin) # create an analog object from a pin
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val = adc.read() # read an analog value
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adc = pyb.ADCAll(resolution) # creale an ADCAll object
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val = adc.read_channel(channel) # read the given channel
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val = adc.read_core_temp() # read MCU temperature
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val = adc.read_core_vbat() # read MCU VBAT
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val = adc.read_core_vref() # read MCU VREF
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Constructors
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------------
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.. only:: port_pyboard
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.. class:: pyb.ADC(pin)
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Create an ADC object associated with the given pin.
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This allows you to then read analog values on that pin.
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Methods
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-------
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.. only:: port_pyboard
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.. method:: ADC.read()
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Read the value on the analog pin and return it. The returned value
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will be between 0 and 4095.
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.. method:: ADC.read_timed(buf, timer)
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Read analog values into ``buf`` at a rate set by the ``timer`` object.
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``buf`` can be bytearray or array.array for example. The ADC values have
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12-bit resolution and are stored directly into ``buf`` if its element size is
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16 bits or greater. If ``buf`` has only 8-bit elements (eg a bytearray) then
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the sample resolution will be reduced to 8 bits.
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``timer`` should be a Timer object, and a sample is read each time the timer
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triggers. The timer must already be initialised and running at the desired
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sampling frequency.
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To support previous behaviour of this function, ``timer`` can also be an
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integer which specifies the frequency (in Hz) to sample at. In this case
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Timer(6) will be automatically configured to run at the given frequency.
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Example using a Timer object (preferred way)::
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adc = pyb.ADC(pyb.Pin.board.X19) # create an ADC on pin X19
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tim = pyb.Timer(6, freq=10) # create a timer running at 10Hz
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buf = bytearray(100) # creat a buffer to store the samples
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adc.read_timed(buf, tim) # sample 100 values, taking 10s
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Example using an integer for the frequency::
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adc = pyb.ADC(pyb.Pin.board.X19) # create an ADC on pin X19
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buf = bytearray(100) # create a buffer of 100 bytes
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adc.read_timed(buf, 10) # read analog values into buf at 10Hz
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# this will take 10 seconds to finish
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for val in buf: # loop over all values
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print(val) # print the value out
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This function does not allocate any memory.
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The ADCAll Object
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-----------------
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.. only:: port_pyboard
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Instantiating this changes all ADC pins to analog inputs. The raw MCU temperature,
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VREF and VBAT data can be accessed on ADC channels 16, 17 and 18 respectively.
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Appropriate scaling will need to be applied. The temperature sensor on the chip
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has poor absolute accuracy and is suitable only for detecting temperature changes.
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The ``ADCAll`` ``read_core_vbat()`` and ``read_core_vref()`` methods read
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the backup battery voltage and the (1.21V nominal) reference voltage using the
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3.3V supply as a reference. Assuming the ``ADCAll`` object has been Instantiated with
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``adc = pyb.ADCAll(12)`` the 3.3V supply voltage may be calculated:
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``v33 = 3.3 * 1.21 / adc.read_core_vref()``
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If the 3.3V supply is correct the value of ``adc.read_core_vbat()`` will be
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valid. If the supply voltage can drop below 3.3V, for example in in battery
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powered systems with a discharging battery, the regulator will fail to preserve
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the 3.3V supply resulting in an incorrect reading. To produce a value which will
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remain valid under these circumstances use the following:
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``vback = adc.read_core_vbat() * 1.21 / adc.read_core_vref()``
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It is possible to access these values without incurring the side effects of ``ADCAll``::
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def adcread(chan): # 16 temp 17 vbat 18 vref
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assert chan >= 16 and chan <= 18, 'Invalid ADC channel'
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start = pyb.millis()
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timeout = 100
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stm.mem32[stm.RCC + stm.RCC_APB2ENR] |= 0x100 # enable ADC1 clock.0x4100
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stm.mem32[stm.ADC1 + stm.ADC_CR2] = 1 # Turn on ADC
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stm.mem32[stm.ADC1 + stm.ADC_CR1] = 0 # 12 bit
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if chan == 17:
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stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x200000 # 15 cycles
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stm.mem32[stm.ADC + 4] = 1 << 23
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elif chan == 18:
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stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x1000000
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stm.mem32[stm.ADC + 4] = 0xc00000
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else:
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stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x40000
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stm.mem32[stm.ADC + 4] = 1 << 23
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stm.mem32[stm.ADC1 + stm.ADC_SQR3] = chan
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stm.mem32[stm.ADC1 + stm.ADC_CR2] = 1 | (1 << 30) | (1 << 10) # start conversion
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while not stm.mem32[stm.ADC1 + stm.ADC_SR] & 2: # wait for EOC
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if pyb.elapsed_millis(start) > timeout:
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raise OSError('ADC timout')
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data = stm.mem32[stm.ADC1 + stm.ADC_DR] # clear down EOC
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stm.mem32[stm.ADC1 + stm.ADC_CR2] = 0 # Turn off ADC
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return data
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def v33():
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return 4096 * 1.21 / adcread(17)
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def vbat():
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return 1.21 * 2 * adcread(18) / adcread(17) # 2:1 divider on Vbat channel
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def vref():
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return 3.3 * adcread(17) / 4096
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def temperature():
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return 25 + 400 * (3.3 * adcread(16) / 4096 - 0.76)
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