docs: Update details on using ADCAll object for vref/vbat channels.

This commit is contained in:
Peter Hinch 2016-03-11 10:49:44 +00:00 committed by Damien George
parent 70f32f0f73
commit 85d3b6165a

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@ -18,6 +18,7 @@ class ADC -- analog to digital conversion
val = adc.read_core_vbat() # read MCU VBAT
val = adc.read_core_vref() # read MCU VREF
Constructors
------------
@ -77,6 +78,65 @@ Methods
The ADCAll Object
-----------------
Instantiating this changes all ADC pins to analog inputs. It is possible to read the
MCU temperature, VREF and VBAT without using ADCAll. The raw data can be accessed on
ADC channels 16, 17 and 18 respectively. However appropriate scaling will need to be applied.
.. only:: port_pyboard
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)