AHRS400 Series User’s Manual
Page 16
Doc# 7430-0004-01 Rev. A
3.7
Timing
The maximum AHRS data update rate is 70 samples per second.
In some applications, using the DMU’s digital output requires a precise
understanding of the internal timing of the device. The processor internal to
the DMU runs in a loop - collecting data from the sensors, processing the
data, and then collecting more data. The data is reported to the user through
a parallel process. In continuous mode, the system processor activity is
repeatable and accurate timing information can be derived based purely on
the overall loop rate.
The unit goes through three processes in one data cycle. First, the sensors
are sampled. Second, the unit processes the data for output. After
processing the data, the DMU will make another measurement while
presenting the current measurement for output. Third, the unit actually
transfers the data out; either over the RS-232 port, or onto the analog
outputs.
In the case of the analog output, the data is presented immediately on the
analog output pins after the data processing step is over. In the cas e of the
digital data, the data is transferred only if the previous data packet is
cleared. The DMU continues to take data, so that in practice, roughly every
third measurement will be available over the RS-232 interface.
A time tag is attached to each data packet. The time tag is simply the value
of a free running counter at the time the A/D channels are sampled. The
clock counts down from 65535 to 0, and a single tick corresponds to 0.79
microseconds. The timer rolls over approximately every 50 millis econds.
You can use this value to track relative sampling time between data packets,
and correlate this with external timing.
3.8
Temperature Sensor
The DMU has an onboard temperature sensor. The temperature sensor is
used to monitor the internal temperature of the DMU to allow for
temperature calibration of the sensors. The temperature sensor is specified
to be within ± 2% accurate over the DMU operating temperature range.
The DMU reads and outputs the temperature sensor voltage with 12-bit
precision.
The DMU will output the temperature sensor voltage in the digital data
packet scaled as follows:
V
temp
(V) = data *
5/4096
where
data
is the 16-bit unsigned integer sent as the temperature
information in the data packet. (The DMU uses two full bytes to express
the data, but it is really scaled to 12 bits.)
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