ADC: added an enable_channel() API

enable_channel() must be used before the Ch6() method is called to
enable gathering of data on the required channels

libraries/AP_ADC/AP_ADC.h

@@ -37,9 +37,14 @@ class AP_ADC
 
 	   The function returns the amount of time (in microseconds)
 	   since the last call to Ch6().
+
+	   NOTE: You must enable the channels using enable_channel()
+	   before use
 	*/
 	virtual uint32_t Ch6(const uint8_t *channel_numbers, uint16_t *result) = 0;
 
+	virtual void enable_channel(const uint8_t ch) = 0;
+
   private:
 };
 

libraries/AP_ADC/AP_ADC_ADS7844.cpp

@@ -100,6 +100,7 @@ void AP_ADC_ADS7844::read(void)
 		return;
 	}
 
+	// build a sequence of commands to use to fetch the channels
 	for (ch = 0; ch < 8; ch++) {
 		if (enable_mask & (1<<ch)) {
 			enable_cmd[num_enabled++] = adc_cmd[ch];
@@ -187,6 +188,13 @@ void AP_ADC_ADS7844::Init( AP_PeriodicProcess * scheduler )
 
 }
 
+
+// enable a channel
+void AP_ADC_ADS7844::enable_channel(uint8_t ch)
+{
+	enable_mask |= (1<<ch);
+}
+
 // Read one channel value
 float AP_ADC_ADS7844::Ch(uint8_t ch_num)
 {
@@ -220,10 +228,6 @@ uint32_t AP_ADC_ADS7844::Ch6(const uint8_t *channel_numbers, uint16_t *result)
 	uint32_t sum[6];
 	uint8_t i;
 
-	for (i=0; i<6; i++) {
-		enable_mask |= (1<<channel_numbers[i]);
-	}
-
 	// ensure we have at least one value
 	for (i=0; i<6; i++) {
 		while (_count[channel_numbers[i]] == 0) /* noop */;

libraries/AP_ADC/AP_ADC_ADS7844.h

@@ -29,6 +29,7 @@ class AP_ADC_ADS7844 : public AP_ADC
 
 	// Read 6 sensors at once
 	uint32_t Ch6(const uint8_t *channel_numbers, uint16_t *result);
+	void enable_channel(uint8_t ch);
 	bool	filter_result;
 
 	private:

libraries/AP_ADC/examples/AP_ADC_test/AP_ADC_test.pde

@@ -1,4 +1,4 @@
-
+/// -*- tab-width: 4; Mode: C++; c-basic-offset: 4; indent-tabs-mode: nil -*-
 /*
   Example of APM_ADC library.
   Code by Jordi Muñoz and Jose Julio. DIYDrones.com
@@ -18,6 +18,8 @@ AP_TimerAperiodicProcess  adc_scheduler;
 unsigned long timer;
 AP_ADC_ADS7844 adc;
 
+static const uint8_t channel_map[6] = { 1, 2, 0, 4, 5, 6};
+
 void setup()
 {
 	Serial.begin(115200, 128, 128);
@@ -30,9 +32,12 @@ void setup()
 	adc.Init(&adc_scheduler);   // APM ADC initialization
 	delay(1000);
 	timer = millis();
+
+	for (uint8_t ch=0; ch<6; ch++) {
+		adc.enable_channel(channel_map[ch]);
+	}
 }
 
-static const uint8_t channel_map[6] = { 1, 2, 0, 4, 5, 6};
 float v;
 uint32_t last_usec = 0;
 

libraries/AP_InertialSensor/AP_InertialSensor_Oilpan.cpp

@@ -50,6 +50,11 @@ AP_InertialSensor_Oilpan::AP_InertialSensor_Oilpan( AP_ADC * adc ) :
 void AP_InertialSensor_Oilpan::init( AP_PeriodicProcess * scheduler)
 {
   _adc->Init(scheduler);
+
+  // tell the ADC driver that we will be needed some channels
+  for (uint8_t ch=0; ch<6; ch++) {
+	  _adc->enable_channel(_sensors[ch]);
+  }
 }
 
 bool AP_InertialSensor_Oilpan::update()