STM32: F4: Increase ADC sampling time for VBAT

[LMESTM]

Description

To get a valid VBAT measurement on F4 targets, it is required to increase
the sampling time to its maximum value.

This PR solves the issue reported here #4326

Status

READY

Tests

Tested on NUCLEO_F446RE target with VBAT measurements and non-regression OK

+-----------------------+---------------+-----------------------+--------+--------------------+-------------+
| target | platform_name | test suite | result | elapsed_time (sec) | copy_method |
+-----------------------+---------------+-----------------------+--------+--------------------+-------------+
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-analogin | OK | 12.65 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-analogout | OK | 11.8 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-businout | OK | 28.27 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-digitalio | OK | 13.17 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-i2c | OK | 14.54 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-interruptin | OK | 13.77 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-pwm | OK | 176.65 | shell |
| NUCLEO_F446RE-GCC_ARM | NUCLEO_F446RE | tests-api-spi | OK | 16.15 | shell |
+-----------------------+---------------+-----------------------+--------+--------------------+-------------+

More thoughts

Increasing the sample time may have impacts on performances (e.g. max number of samples per sec),
but this PR aims at getting valid sample first. There is way for improvement in MBED API to allow
configuration of this sampling time parameter. An issue will be entered accordingly.
Submitted #4692

[ohagendorf]

This has a huge impact on the normal usage of the ADC.
Why not put the long sample time only in the apropriate case when VBAT is sampled. During the normal usage the former config could be used:

    // Configure ADC channel
    sConfig.Rank         = 1;
    sConfig.SamplingTime = ADC_SAMPLETIME_15CYCLES;
    sConfig.Offset       = 0;

    switch (obj->channel) {
...
        case 18:
            sConfig.Channel = ADC_CHANNEL_VBAT;
            sConfig.SamplingTime = ADC_SAMPLETIME_480CYCLES;
            break;

[LMESTM]

@ohagendorf I was hoping to get some feedback :-) thanks for that - I will proceed as suggested.
Do you think that adding MBED API for tuning the sampling time can help users ?

[ohagendorf]

Maybe two features or three would be nice to have:

• setting sampling time
• setting resolution

But curently for a fast conversion there is another huge problem. We analysed the adc read function:

    float read() {
        lock();
        float ret = analogin_read(&_adc);
        unlock();
        return ret;
    }

and lock()+unlock() need much more time than analogin_read(). I had to look into the project documentation for the exact numbers but it was something around 1.5µs for the conversion and 5...6µs for calling lock()+unlock() (Nucleo_F446RE).
One idea would be to add another read function with a callback under timer control or use continuous mode. In this case a fast equidistant conversion corresponding to the maximum conversion rate of the hardware would be possible. Or in combination with the above function to set the sampling time with a slower timing.

[LMESTM]

thanks @ohagendorf
I will expose your requests in #4692 and I suggest you to follow-up there as well.

[adbridge]

@LMESTM So is there more work to come against this PR as a result of the above conversations ?

[LMESTM]

@adbridge No more work - the latest PR takes into account comments from @ohagendorf

[0xc0170]

/morph test

[mbed-bot]

Result: SUCCESS

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/morph test

Output

mbed Build Number: 761

All builds and test passed!