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Diffstat (limited to 'watch-library/watch/watch_adc.h')
| -rw-r--r-- | watch-library/watch/watch_adc.h | 47 |
1 files changed, 47 insertions, 0 deletions
diff --git a/watch-library/watch/watch_adc.h b/watch-library/watch/watch_adc.h index c9b6ad2a..a4f9edad 100644 --- a/watch-library/watch/watch_adc.h +++ b/watch-library/watch/watch_adc.h @@ -94,6 +94,53 @@ void watch_set_analog_num_samples(uint16_t samples); **/ void watch_set_analog_sampling_length(uint8_t cycles); +typedef enum { + ADC_REFERENCE_INTREF = ADC_REFCTRL_REFSEL_INTREF_Val, + ADC_REFERENCE_VCC_DIV1POINT6 = ADC_REFCTRL_REFSEL_INTVCC0_Val, + ADC_REFERENCE_VCC_DIV2 = ADC_REFCTRL_REFSEL_INTVCC1_Val, + ADC_REFERENCE_VCC = ADC_REFCTRL_REFSEL_INTVCC2_Val, +} watch_adc_reference_voltage; + + +/** @brief Selects the reference voltage to use for analog readings. Default is ADC_REFERENCE_VCC. + * @param reference One of ADC_REFERENCE_VCC, ADC_REFERENCE_VCC_DIV1POINT6, ADC_REFERENCE_VCC_DIV2 + * or ADC_REFERENCE_INTREF. + * @details In order to turn an analog voltage into a 16-bit integer, the ADC needs to compare the + * measured voltage to a reference point. For example, if you were powering the watch with + * VCC == 3.0V and you had two 10K resistors connected in series from 3V to GND, you could + * expect to get 3 volts when you measure the top of the voltage divider, 0 volts at the + * bottom, and 1.5 volts in the middle. If you read these values uising a reference voltage + * of ADC_REFERENCE_VCC, the top value would be about 65535, the bottom about 0, and the + * middle about 32768. However! If we used ADC_REFERENCE_VCC_DIV2 as our reference, we would + * expect to get 65535 both at the top and the middle, because the largest value the ADC can + * measure in this configutation is 1.5V (VCC / 2). + * + * By changing the reference voltage from ADC_REFERENCE_VCC to ADC_REFERENCE_VCC_DIV1POINT6 + * or ADC_REFERENCE_VCC_DIV2, you can get more resolution when measuring small voltages (i.e. + * a phototransistor circuit in low light). + * + * There is also a special reference voltage called ADC_REFERENCE_INTREF. The SAM L22's + * Supply Controller provides a selectable voltage reference (by default, 1.024 V) that you + * can select as a reference voltage for ADC conversions. Unlike the three references we + * talked about in the last paragraph, this reference voltage does not depend on VCC, which + * makes it very useful for measuring the battery voltage (since you can't really compare + * VCC to itself). You can change the INTREF voltage to 2.048 or 4.096 V by poking at the + * supply controller's VREF register, but the watch library does not support this use case. + **/ +void watch_set_analog_reference_voltage(watch_adc_reference_voltage reference); + +/** @brief Returns the voltage of the VCC supply in millivolts (i.e. 3000 mV == 3.0 V). If running on + * a coin cell, this will be the battery voltage. + * @details Unlike other ADC functions, this function does not return a raw value from the ADC, but + * rather scales it to an actual number of millivolts. This is because the ADC doesn't let + * us measure VCC per se; it instead lets us measure VCC / 4, and we choose to measure it + * against the internal reference voltage of 1.024 V. In short, the ADC gives us a number + * that's complicated to deal with, so we just turn it into a useful number for you :) + * @note This function depends on INTREF being 1.024V. If you have changed it by poking at the supply + * controller's VREF.SEL bits, this function will return inaccurate values. + */ +uint16_t watch_get_vcc_voltage(); + /** @brief Disables the analog circuitry on the selected pin. * @param pin One of pins A0-A4. */ |