mpu6000: change bus speed based on registers being accessed

this ensures we follow the datasheet requirement of 1MHz for general
registers and up to 20MHz for sensor and int status registers

src/drivers/mpu6000/mpu6000.cpp

@@ -161,6 +161,14 @@
 
 #define MPU6000_ONE_G					9.80665f
 
+/*
+  the MPU6000 can only handle high SPI bus speeds on the sensor and
+  interrupt status registers. All other registers have a maximum 1MHz
+  SPI speed
+ */
+#define MPU6000_LOW_BUS_SPEED				1000*1000
+#define MPU6000_HIGH_BUS_SPEED				10*1000*1000
+
 class MPU6000_gyro;
 
 class MPU6000 : public device::SPI
@@ -351,7 +359,7 @@ private:
 extern "C" { __EXPORT int mpu6000_main(int argc, char *argv[]); }
 
 MPU6000::MPU6000(int bus, spi_dev_e device) :
-	SPI("MPU6000", ACCEL_DEVICE_PATH, bus, device, SPIDEV_MODE3, 10000000),
+	SPI("MPU6000", ACCEL_DEVICE_PATH, bus, device, SPIDEV_MODE3, MPU6000_LOW_BUS_SPEED),
 	_gyro(new MPU6000_gyro(this)),
 	_product(0),
 	_call_interval(0),
@@ -991,6 +999,9 @@ MPU6000::read_reg(unsigned reg)
 
 	cmd[0] = reg | DIR_READ;
 
+        // general register transfer at low clock speed
+        set_frequency(MPU6000_LOW_BUS_SPEED);
+
 	transfer(cmd, cmd, sizeof(cmd));
 
 	return cmd[1];
@@ -1003,6 +1014,9 @@ MPU6000::read_reg16(unsigned reg)
 
 	cmd[0] = reg | DIR_READ;
 
+        // general register transfer at low clock speed
+        set_frequency(MPU6000_LOW_BUS_SPEED);
+
 	transfer(cmd, cmd, sizeof(cmd));
 
 	return (uint16_t)(cmd[1] << 8) | cmd[2];
@@ -1016,6 +1030,9 @@ MPU6000::write_reg(unsigned reg, uint8_t value)
 	cmd[0] = reg | DIR_WRITE;
 	cmd[1] = value;
 
+        // general register transfer at low clock speed
+        set_frequency(MPU6000_LOW_BUS_SPEED);
+
 	transfer(cmd, nullptr, sizeof(cmd));
 }
 
@@ -1139,6 +1156,10 @@ MPU6000::measure()
 	 * Fetch the full set of measurements from the MPU6000 in one pass.
 	 */
 	mpu_report.cmd = DIR_READ | MPUREG_INT_STATUS;
+
+        // sensor transfer at high clock speed
+        set_frequency(MPU6000_HIGH_BUS_SPEED);
+
 	if (OK != transfer((uint8_t *)&mpu_report, ((uint8_t *)&mpu_report), sizeof(mpu_report)))
 		return;