kopia lustrzana https://github.com/micropython/micropython
162 wiersze
7.5 KiB
ReStructuredText
162 wiersze
7.5 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) # create an ADCAll object
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adc = pyb.ADCAll(resolution, mask) # create an ADCAll object for selected analog channels
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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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val = adc.read_vref() # read MCU supply voltage
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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 masked ADC pins to analog inputs. The preprocessed 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 is handled according to reference voltage used (usually 3.3V).
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The temperature sensor on the chip is factory calibrated and allows to read the die temperature
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to +/- 1 degree centigrade. Although this sounds pretty accurate, don't forget that the MCU's internal
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temperature is measured. Depending on processing loads and I/O subsystems active the die temperature
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may easily be tens of degrees above ambient temperature. On the other hand a pyboard woken up after a
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long standby period will show correct ambient temperature within limits mentioned above.
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The ``ADCAll`` ``read_core_vbat()``, ``read_vref()`` and ``read_core_vref()`` methods read
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the backup battery voltage, reference voltage and the (1.21V nominal) reference voltage using the
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actual supply as a reference. All results are floating point numbers giving direct voltage values.
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``read_core_vbat()`` returns the voltage of the backup battery. This voltage is also adjusted according
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to the actual supply voltage. To avoid analog input overload the battery voltage is measured
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via a voltage divider and scaled according to the divider value. To prevent excessive loads
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to the backup battery, the voltage divider is only active during ADC conversion.
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``read_vref()`` is evaluated by measuring the internal voltage reference and backscale it using
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factory calibration value of the internal voltage reference. In most cases the reading would be close
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to 3.3V. If the pyboard is operated from a battery, the supply voltage may drop to values below 3.3V.
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The pyboard will still operate fine as long as the operating conditions are met. With proper settings
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of MCU clock, flash access speed and programming mode it is possible to run the pyboard down to
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2 V and still get useful ADC conversion.
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It is very important to make sure analog input voltages never exceed actual supply voltage.
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Other analog input channels (0..15) will return unscaled integer values according to the selected
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precision.
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To avoid unwanted activation of analog inputs (channel 0..15) a second prarmeter can be specified.
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This parameter is a binary pattern where each requested analog input has the corresponding bit set.
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The default value is 0xffffffff which means all analog inputs are active. If just the internal
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channels (16..18) are required, the mask value should be 0x70000.
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It is possible to access channle 16..18 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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Note that this example is only valid for the F405 MCU and all values are not corrected by Vref and
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factory calibration data.
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