class ADC – analog to digital conversion¶

Usage:

import pyb

adc = pyb.ADC(pin)              # create an analog object from a pin
val = adc.read()                # read an analog value

adc = pyb.ADCAll(resolution)    # creale an ADCAll object
val = adc.read_channel(channel) # read the given channel
val = adc.read_core_temp()      # read MCU temperature
val = adc.read_core_vbat()      # read MCU VBAT
val = adc.read_core_vref()      # read MCU VREF

Constructors¶

class pyb.ADC(pin)¶: Create an ADC object associated with the given pin. This allows you to then read analog values on that pin.

Methods¶

adc.read()¶: Read the value on the analog pin and return it. The returned value will be between 0 and 4095.

adc.read_timed(buf, timer)¶

Read analog values into buf at a rate set by the timer object.

buf can be bytearray or array.array for example. The ADC values have 12-bit resolution and are stored directly into buf if its element size is 16 bits or greater. If buf has only 8-bit elements (eg a bytearray) then the sample resolution will be reduced to 8 bits.

timer should be a Timer object, and a sample is read each time the timer triggers. The timer must already be initialised and running at the desired sampling frequency.

To support previous behaviour of this function, timer can also be an integer which specifies the frequency (in Hz) to sample at. In this case Timer(6) will be automatically configured to run at the given frequency.

Example using a Timer object (preferred way):

adc = pyb.ADC(pyb.Pin.board.X19)    # create an ADC on pin X19
tim = pyb.Timer(6, freq=10)         # create a timer running at 10Hz
buf = bytearray(100)                # creat a buffer to store the samples
adc.read_timed(buf, tim)            # sample 100 values, taking 10s

Example using an integer for the frequency:

adc = pyb.ADC(pyb.Pin.board.X19)    # create an ADC on pin X19
buf = bytearray(100)                # create a buffer of 100 bytes
adc.read_timed(buf, 10)             # read analog values into buf at 10Hz
                                    #   this will take 10 seconds to finish
for val in buf:                     # loop over all values
    print(val)                      # print the value out

This function does not allocate any memory.

The ADCAll Object¶

Instantiating this changes all ADC pins to analog inputs. The raw MCU temperature, VREF and VBAT data can be accessed on ADC channels 16, 17 and 18 respectively. Appropriate scaling will need to be applied. The temperature sensor on the chip has poor absolute accuracy and is suitable only for detecting temperature changes.

The ADCAll read_core_vbat() and read_core_vref() methods read the backup battery voltage and the (1.21V nominal) reference voltage using the 3.3V supply as a reference. Assuming the ADCAll object has been Instantiated with adc = pyb.ADCAll(12) the 3.3V supply voltage may be calculated:

v33 = 3.3 * 1.21 / adc.read_core_vref()

If the 3.3V supply is correct the value of adc.read_core_vbat() will be valid. If the supply voltage can drop below 3.3V, for example in in battery powered systems with a discharging battery, the regulator will fail to preserve the 3.3V supply resulting in an incorrect reading. To produce a value which will remain valid under these circumstances use the following:

vback = adc.read_core_vbat() * 1.21 / adc.read_core_vref()

It is possible to access these values without incurring the side effects of ADCAll:

def adcread(chan):                              # 16 temp 17 vbat 18 vref
    assert chan >= 16 and chan <= 18, 'Invalid ADC channel'
    start = pyb.millis()
    timeout = 100
    stm.mem32[stm.RCC + stm.RCC_APB2ENR] |= 0x100 # enable ADC1 clock.0x4100
    stm.mem32[stm.ADC1 + stm.ADC_CR2] = 1       # Turn on ADC
    stm.mem32[stm.ADC1 + stm.ADC_CR1] = 0       # 12 bit
    if chan == 17:
        stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x200000 # 15 cycles
        stm.mem32[stm.ADC + 4] = 1 << 23
    elif chan == 18:
        stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x1000000
        stm.mem32[stm.ADC + 4] = 0xc00000
    else:
        stm.mem32[stm.ADC1 + stm.ADC_SMPR1] = 0x40000
        stm.mem32[stm.ADC + 4] = 1 << 23
    stm.mem32[stm.ADC1 + stm.ADC_SQR3] = chan
    stm.mem32[stm.ADC1 + stm.ADC_CR2] = 1 | (1 << 30) | (1 << 10) # start conversion
    while not stm.mem32[stm.ADC1 + stm.ADC_SR] & 2: # wait for EOC
        if pyb.elapsed_millis(start) > timeout:
            raise OSError('ADC timout')
    data = stm.mem32[stm.ADC1 + stm.ADC_DR]     # clear down EOC
    stm.mem32[stm.ADC1 + stm.ADC_CR2] = 0       # Turn off ADC
    return data

def v33():
    return 4096 * 1.21 / adcread(17)

def vbat():
    return  1.21 * 2 * adcread(18) / adcread(17)  # 2:1 divider on Vbat channel

def vref():
    return 3.3 * adcread(17) / 4096

def temperature():
    return 25 + 400 * (3.3 * adcread(16) / 4096 - 0.76)