ADC: only read channels that are actually being used

on the APM2 we usually use none of the ADC channels, although we may
use the airspeed sensor. This change means we detect which channels
are being read, and only do the SPI transfers for those ones. That
saves us about 100usec per timer interrupt (ie. about 10% of our CPU)

libraries/AP_ADC/AP_ADC_ADS7844.cpp

@@ -63,6 +63,11 @@ static volatile uint32_t _sum[8];
 // how many values we've accumulated since last read
 static volatile uint16_t _count[8];
 
+// a mask of what channels are actually being read. If a channel has
+// never been read, then don't bother gathering it. That saves us a
+// lot of cycles in the timer call
+static uint8_t enable_mask;
+
 static uint32_t last_ch6_micros;
 
 // TCNT2 values for various interrupt rates,
@@ -87,15 +92,35 @@ static inline unsigned char ADC_SPI_transfer(unsigned char data)
 void AP_ADC_ADS7844::read(void)
 {
 	uint8_t ch;
+	unsigned char enable_cmd[9];
+	uint8_t num_enabled = 0;
+
+	if (enable_mask == 0) {
+		// no channels to read
+		return;
+	}
+
+	for (ch = 0; ch < 8; ch++) {
+		if (enable_mask & (1<<ch)) {
+			enable_cmd[num_enabled++] = adc_cmd[ch];
+		}
+	}
+	enable_cmd[num_enabled] = 0;
+
+	num_enabled = 0;
 
 	bit_clear(PORTC, 4);							// Enable Chip Select (PIN PC4)
-	ADC_SPI_transfer(adc_cmd[0]);						// Command to read the first channel
+	ADC_SPI_transfer(enable_cmd[0]);					// Command to read the first channel
 
 	for (ch = 0; ch < 8; ch++) {
 		uint16_t v;
 
+		if ((enable_mask & (1<<ch)) == 0) {
+			continue;
+		}
+		
 		v = ADC_SPI_transfer(0) << 8;	         // Read first byte
-		v |= ADC_SPI_transfer(adc_cmd[ch + 1]);  // Read second byte and send next command
+		v |= ADC_SPI_transfer(enable_cmd[++num_enabled]);  // Read second byte and send next command
 
 		if (v & 0x8007) {
 			// this is a 12-bit ADC, shifted by 3 bits.
@@ -168,6 +193,8 @@ float AP_ADC_ADS7844::Ch(uint8_t ch_num)
 	uint16_t count;
 	uint32_t sum;
 
+	enable_mask |= (1<<ch_num);
+
 	// ensure we have at least one value
 	while (_count[ch_num] == 0)  /* noop */ ;
 
@@ -193,6 +220,10 @@ 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 */;