STM32W108CB datasheet(135/179 Pages) STMICROELECTRONICS

135 / 179 page

STM32W108CB, STM32W108HB

Analog-to-digital converter

Doc ID 16252 Rev 2

135/179

Note:

ADC sample timing is the same whether the STM32W108 is using the 24 MHz crystal

oscillator or the 12 MHz high-speed RC oscillator. This facilitates using the ADC soon after

the CPU wakes from deep sleep, before switching to the crystal oscillator.

9.1.7

Operation

Setting the ADC_EN bit in the ADC_CFG register enables the ADC; once enabled, it

performs conversions continuously until it is disabled. If the ADC had previously been

disabled, a 21 µs analog startup delay is imposed before the ADC starts conversions. The

delay timing is performed in hardware and is simply added to the time until the first

conversion result is output.

When the ADC is first enabled, and or if any change is made to ADC_CFG after it is

enabled, the time until a result is output is double the normal sample time. This is because

the ADC’s internal design requires it to discard the first conversion after startup or a

configuration change. This is done automatically and is hidden from software except for the

longer timing. Switching the processor clock between the RC and crystal oscillator also

causes the ADC to go through this startup cycle. If the ADC was newly enabled, the analog

delay time is added to the doubled sample time.

If the DMA is running when ADC_CFG is modified, the DMA does not stop, so the DMA

buffer may contain conversion results from both the old and new configurations.

The following procedure illustrates a simple polled method of using the ADC. After

completing the procedure, the latest conversion results is available in the location written to

by the DMA. This assumes that any GPIOs and the voltage reference have already been

configured.

Allocate a 16-bit signed variable, for example analogData, to receive the ADC output.

(Make sure that analogData is half-word aligned – that is, at an even address.)

Disable all ADC interrupts – write 0 to INT_ADCCFG.

Set up the DMA to output conversion results to the variable, analogData.

Reset the DMA – set the ADC_DMARST bit in ADC_DMACFG.

Define a one sample buffer – write analogData’s address to ADC_DMABEG, set

ADC_DMASIZE to 1.

Write the desired offset and gain correction values to the ADC_OFFSET and

ADC_GAIN registers.

Start the ADC and the DMA.

Write the desired conversion configuration, with the ADC_EN bit set, to ADC_CFG.

Clear the ADC buffer full flag – write INT_ADCULDFULL to INT_ADCFLAG.

Start the DMA in auto wrap mode – set the ADC_DMAAUTOWRAP and

ADC_DMALOAD bits in ADC_DMACFG.

Wait until the INT_ADCULDFULL bit is set in INT_ADCFLAG, then read the result from

analogData.

To convert multiple inputs using this approach, repeat steps 4 through 6, loading the desired

input configurations to ADC_CFG in step 5. If the inputs can use the same offset/gain

correction, just repeat steps 5 and 6.

9.1.8

Calibration

Sampling of internal connections GND, VREF/2, and VREF allow for offset and gain

calibration of the ADC in applications where absolute accuracy is important. Measurement

of the regulated supply VDD_PADSA provides an accurate means of calibrating the ADC as

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