lsm303d: run sampling 200usec faster to avoid aliasing

this runs the sampling of the accelerometer 200usec faster than
requested and then throw away duplicates using the accelerometer
status register data ready bit. This avoids aliasing due to drift in
the stm32 clock compared to the lsm303d clock

src/drivers/lsm303d/lsm303d.cpp

@@ -196,6 +196,7 @@ static const int ERROR = -1;
 
 #define REG7_CONT_MODE_M		((0<<1) | (0<<0))
 
+#define REG_STATUS_A_NEW_ZYXADA		0x08
 
 #define INT_CTRL_M              0x12
 #define INT_SRC_M               0x13
@@ -217,6 +218,14 @@ static const int ERROR = -1;
 #define EXTERNAL_BUS 0
 #endif
 
+/*
+  we set the timer interrupt to run a bit faster than the desired
+  sample rate and then throw away duplicates using the data ready bit.
+  This time reduction is enough to cope with worst case timing jitter
+  due to other timers
+ */
+#define LSM303D_TIMER_REDUCTION				200
+
 extern "C" { __EXPORT int lsm303d_main(int argc, char *argv[]); }
 
 
@@ -289,9 +298,9 @@ private:
 
 	perf_counter_t		_accel_sample_perf;
 	perf_counter_t		_mag_sample_perf;
-	perf_counter_t		_accel_reschedules;
 	perf_counter_t		_bad_registers;
 	perf_counter_t		_bad_values;
+	perf_counter_t		_accel_duplicates;
 
 	uint8_t			_register_wait;
 
@@ -561,9 +570,9 @@ LSM303D::LSM303D(int bus, const char* path, spi_dev_e device, enum Rotation rota
 	_mag_read(0),
 	_accel_sample_perf(perf_alloc(PC_ELAPSED, "lsm303d_accel_read")),
 	_mag_sample_perf(perf_alloc(PC_ELAPSED, "lsm303d_mag_read")),
-	_accel_reschedules(perf_alloc(PC_COUNT, "lsm303d_accel_resched")),
 	_bad_registers(perf_alloc(PC_COUNT, "lsm303d_bad_registers")),
 	_bad_values(perf_alloc(PC_COUNT, "lsm303d_bad_values")),
+	_accel_duplicates(perf_alloc(PC_COUNT, "lsm303d_accel_duplicates")),
 	_register_wait(0),
 	_accel_filter_x(LSM303D_ACCEL_DEFAULT_RATE, LSM303D_ACCEL_DEFAULT_DRIVER_FILTER_FREQ),
 	_accel_filter_y(LSM303D_ACCEL_DEFAULT_RATE, LSM303D_ACCEL_DEFAULT_DRIVER_FILTER_FREQ),
@@ -622,7 +631,7 @@ LSM303D::~LSM303D()
 	perf_free(_mag_sample_perf);
 	perf_free(_bad_registers);
 	perf_free(_bad_values);
-	perf_free(_accel_reschedules);
+	perf_free(_accel_duplicates);
 }
 
 int
@@ -874,7 +883,9 @@ LSM303D::ioctl(struct file *filp, int cmd, unsigned long arg)
 
 				/* update interval for next measurement */
 				/* XXX this is a bit shady, but no other way to adjust... */
-				_accel_call.period = _call_accel_interval = ticks;
+				_call_accel_interval = ticks;
+
+                                _accel_call.period = _call_accel_interval - LSM303D_TIMER_REDUCTION;
 
 				/* if we need to start the poll state machine, do it */
 				if (want_start)
@@ -1388,7 +1399,10 @@ LSM303D::start()
 	_mag_reports->flush();
 
 	/* start polling at the specified rate */
-	hrt_call_every(&_accel_call, 1000, _call_accel_interval, (hrt_callout)&LSM303D::measure_trampoline, this);
+	hrt_call_every(&_accel_call, 
+                       1000,
+                       _call_accel_interval - LSM303D_TIMER_REDUCTION, 
+                       (hrt_callout)&LSM303D::measure_trampoline, this);
 	hrt_call_every(&_mag_call, 1000, _call_mag_interval, (hrt_callout)&LSM303D::mag_measure_trampoline, this);
 }
 
@@ -1466,20 +1480,6 @@ LSM303D::measure()
 
 	check_registers();
 
-	// if the accel doesn't have any data ready then re-schedule
-	// for 100 microseconds later. This ensures we don't double
-	// read a value and then miss the next value.
-	// Note that DRDY is not available when the lsm303d is
-	// connected on the external bus
-#ifdef GPIO_EXTI_ACCEL_DRDY
-	if (_bus == PX4_SPI_BUS_SENSORS && stm32_gpioread(GPIO_EXTI_ACCEL_DRDY) == 0) {
-		perf_count(_accel_reschedules);
-		hrt_call_delay(&_accel_call, 100);
-                perf_end(_accel_sample_perf);
-		return;
-	}
-#endif
-
 	if (_register_wait != 0) {
 		// we are waiting for some good transfers before using
 		// the sensor again.
@@ -1493,6 +1493,12 @@ LSM303D::measure()
 	raw_accel_report.cmd = ADDR_STATUS_A | DIR_READ | ADDR_INCREMENT;
 	transfer((uint8_t *)&raw_accel_report, (uint8_t *)&raw_accel_report, sizeof(raw_accel_report));
 
+        if (!(raw_accel_report.status & REG_STATUS_A_NEW_ZYXADA)) {
+		perf_end(_accel_sample_perf);
+		perf_count(_accel_duplicates);
+		return;
+        }
+
 	/*
 	 * 1) Scale raw value to SI units using scaling from datasheet.
 	 * 2) Subtract static offset (in SI units)
@@ -1681,7 +1687,7 @@ LSM303D::print_info()
 	perf_print_counter(_mag_sample_perf);
 	perf_print_counter(_bad_registers);
 	perf_print_counter(_bad_values);
-	perf_print_counter(_accel_reschedules);
+	perf_print_counter(_accel_duplicates);
 	_accel_reports->print_info("accel reports");
 	_mag_reports->print_info("mag reports");
         ::printf("checked_next: %u\n", _checked_next);