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Diffstat (limited to 'watch-library/watch/watch_adc.h')
| -rw-r--r-- | watch-library/watch/watch_adc.h | 80 |
1 files changed, 75 insertions, 5 deletions
diff --git a/watch-library/watch/watch_adc.h b/watch-library/watch/watch_adc.h index d4620365..77a8bfb7 100644 --- a/watch-library/watch/watch_adc.h +++ b/watch-library/watch/watch_adc.h @@ -24,12 +24,82 @@ ////< @file watch_adc.h /** @addtogroup adc Analog Input - * @brief This section covers functions related to the SAM L22's analog-to-digital converter, as well as - * configuring and reading values from the three analog-capable pins on the 9-pin connector. + * @brief This section covers functions related to the SAM L22's analog-to-digital converter, + * as well as configuring and reading values from the five analog-capable pins on the + * 9-pin connector. */ /// @{ -/** @brief Enables the ADC peripheral, and configures the selected pin for analog input. - * @param pin One of pins A0, A1 or A2. +/** @brief Enables the ADC peripheral. You must call this before attempting to read a value + * from an analog pin. */ -void watch_enable_analog(const uint8_t pin); +void watch_enable_adc(); + +/** @brief Configures the selected pin for analog input. + * @param pin One of pins A0-A4. + */ +void watch_enable_analog_input(const uint8_t pin); + +/** @brief Reads an analog value from one of the pins. + * @param pin One of pins A0-A4. + * @return a 16-bit unsigned integer from 0-65535 representing the sampled value, unless you + * have changed the number of samples. @see watch_set_num_analog_samples for details + * on how that function changes the values returned from this one. + **/ +uint16_t watch_get_analog_pin_level(const uint8_t pin); + +/** @brief Sets the number of samples to accumulate when measuring a pin level. Default is 16. + * @param samples A power of 2 <= 1024. Specifically: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 + or 1024. Any other value will be ignored. + * @details The SAM L22's ADC has a resolution of 12 bits. By default, the watch configures + * the ADC to take 16 samples of the analog input and accumulate them in the result + * register; this effectively gives us a 16-bit resolution, at the cost of taking 16 + * ADC cycles to complete a measurement. If you are measuring a slowly changing signal + * like a thermistor output or an ambient light sensor this is probably fine, even + * desirable. If you are measuring something a bit more fast-paced, like an analog + * accelerometer, you may wish to exchange precision for speed. In this case you may + * call this function to configure the ADC to accumulate fewer samples. HOWEVER! Note + * that this may change the range of values returned from watch_get_analog_pin_level: + * - For watch_set_num_analog_samples(1), the returned value will be 12 bits (0-4095). + * - For watch_set_num_analog_samples(2), the returned value will be 13 bits (0-8191). + * - For watch_set_num_analog_samples(4), the returned value will be 14 bits (0-16383). + * - For watch_set_num_analog_samples(8), the returned value will be 15 bits (0-32767). + * For sampling values over 16, the returned value will still be 16 bits (0-65535); the + * ADC will automatically divide the measured value by whatever factor is necessary to fit + * the result in 16 bits. + * @see watch_get_analog_pin_level + **/ +void watch_set_num_analog_samples(uint16_t samples); + +/** @brief Sets the length of time spent sampling, which allows measurement of higher impedance inputs. + * Default is 1. + * @param cycles The number of ADC cycles to sample, between 1 and 64. + * @see this article by Thea Flowers: https://blog.thea.codes/getting-the-most-out-of-the-samd21-adc/ + * which is where I learned all of this. + * @details To measure an analog value, the SAM L22 must charge a capacitor to the analog voltage + * presented at the input. This takes time. Importantly, the higher the input impedance, + * the more time this takes. As a basic example: if you are using a thermistor tied to + * VCC to measure temperature, the capacitor has to charge through the thermistor. The + * higher the resistor value, the higher the input impedance, and the more time we need + * to allow for the measurement. By default, the ADC is configured to run on a 500 kHz + * clock with a sample time of one cycle. This is appropriate for an input impedance up + * to about 28kΩ. Setting the sampling time to 4 cycles allows for an input impedance up + * to 123kΩ. Setting the sampling time to the maximum of 64 cycles theoretically allows + * for input impedance up to 2 MΩ. (I based these numbers on the calculator in the linked + * blog post; it also has a ton of great info on the SAM D21 ADC, which is similar to the + * SAM L22's). + **/ +void watch_set_analog_sampling_length(uint8_t cycles); + +/** @brief Disables the analog circuitry on the selected pin. + * @param pin One of pins A0-A4. + */ +void watch_disable_analog_input(const uint8_t pin); + +/** @brief Disables the ADC peripheral. + * @note You will need to call watch_enable_adc to re-enable the ADC peripheral. When you do, it will + * have the default settings of 16 samples and 1 measurement cycle; if you customized these + * parameters, you will need to set them up again. + **/ +void watch_disable_adc(); + /// @} |