class ADC – analog to digital conversion¶
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
Create an ADC object associated with the given pin. This allows you to then read analog values on that pin.
Read the value on the analog pin and return it. The returned value will be between 0 and 4095.
Read analog values into
bufat a rate set by the
bufcan be bytearray or array.array for example. The ADC values have 12-bit resolution and are stored directly into
bufif its element size is 16 bits or greater. If
bufhas only 8-bit elements (eg a bytearray) then the sample resolution will be reduced to 8 bits.
timershould 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,
timercan 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.
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
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)