nrf: add sample time to ADC

This is important for inputs with a high input resistance. In those
cases, the sample time needs to be longer.

src/machine/adc.go

@@ -9,4 +9,5 @@ type ADCConfig struct {
 	Reference  uint32 // analog reference voltage (AREF) in millivolts
 	Resolution uint32 // number of bits for a single conversion (e.g., 8, 10, 12)
 	Samples    uint32 // number of samples for a single conversion (e.g., 4, 8, 16, 32)
+	SampleTime uint32 // sample time, in microseconds (µs)
 }

src/machine/machine_nrf52xxx.go

@@ -30,7 +30,6 @@ func (a ADC) Configure(config ADCConfig) {
 	var configVal uint32 = nrf.SAADC_CH_CONFIG_RESP_Bypass<<nrf.SAADC_CH_CONFIG_RESP_Pos |
 		nrf.SAADC_CH_CONFIG_RESP_Bypass<<nrf.SAADC_CH_CONFIG_RESN_Pos |
 		nrf.SAADC_CH_CONFIG_REFSEL_Internal<<nrf.SAADC_CH_CONFIG_REFSEL_Pos |
-		nrf.SAADC_CH_CONFIG_TACQ_3us<<nrf.SAADC_CH_CONFIG_TACQ_Pos |
 		nrf.SAADC_CH_CONFIG_MODE_SE<<nrf.SAADC_CH_CONFIG_MODE_Pos
 
 	switch config.Reference {
@@ -54,6 +53,22 @@ func (a ADC) Configure(config ADCConfig) {
 		// TODO: return an error
 	}
 
+	// Source resistance, according to table 89 on page 364 of the nrf52832 datasheet.
+	// https://infocenter.nordicsemi.com/pdf/nRF52832_PS_v1.4.pdf
+	if config.SampleTime <= 3 { // <= 10kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_3us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	} else if config.SampleTime <= 5 { // <= 40kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_5us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	} else if config.SampleTime <= 10 { // <= 100kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_10us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	} else if config.SampleTime <= 15 { // <= 200kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_15us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	} else if config.SampleTime <= 20 { // <= 400kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_20us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	} else { // <= 800kΩ
+		configVal |= nrf.SAADC_CH_CONFIG_TACQ_40us << nrf.SAADC_CH_CONFIG_TACQ_Pos
+	}
+
 	// Configure channel 0, which is the only channel we use.
 	nrf.SAADC.CH[0].CONFIG.Set(configVal)
 }