+ <appendix>
+ <title>Calibration</title>
+ <para>
+ There are only two calibrations required for a TeleMetrum board, and
+ only one for TeleDongle and TeleMini. All boards are shipped from
+ the factory pre-calibrated, but the procedures are documented here
+ in case they are ever needed. Re-calibration is not supported by
+ AltosUI, you must connect to the board with a serial terminal program
+ and interact directly with the on-board command interpreter to effect
+ calibration.
+ </para>
+ <section>
+ <title>Radio Frequency</title>
+ <para>
+ The radio frequency is synthesized from a clock based on the 48 MHz
+ crystal on the board. The actual frequency of this oscillator
+ must be measured to generate a calibration constant. While our
+ GFSK modulation
+ bandwidth is wide enough to allow boards to communicate even when
+ their oscillators are not on exactly the same frequency, performance
+ is best when they are closely matched.
+ Radio frequency calibration requires a calibrated frequency counter.
+ Fortunately, once set, the variation in frequency due to aging and
+ temperature changes is small enough that re-calibration by customers
+ should generally not be required.
+ </para>
+ <para>
+ To calibrate the radio frequency, connect the UHF antenna port to a
+ frequency counter, set the board to 434.550MHz, and use the 'C'
+ command in the on-board command interpreter to generate a CW
+ carrier. For TeleMetrum, this is best done over USB. For TeleMini,
+ note that the only way to escape the 'C' command is via power cycle
+ since the board will no longer be listening for commands once it
+ starts generating a CW carrier.
+ </para>
+ <para>
+ Wait for the transmitter temperature to stabilize and the frequency
+ to settle down. Then, divide 434.550 MHz by the
+ measured frequency and multiply by the current radio cal value show
+ in the 'c s' command. For an unprogrammed board, the default value
+ is 1186611. Take the resulting integer and program it using the 'c f'
+ command. Testing with the 'C' command again should show a carrier
+ within a few tens of Hertz of the intended frequency.
+ As with all 'c' sub-commands, follow this with a 'c w' to write the
+ change to the parameter block in the on-board DataFlash chip.
+ </para>
+ <para>
+ Note that any time you re-do the radio frequency calibration, the
+ radio frequency is reset to the default 434.550 Mhz. If you want
+ to use another frequency, you will have to set that again after
+ calibration is completed.
+ </para>
+ </section>
+ <section>
+ <title>TeleMetrum Accelerometer</title>
+ <para>
+ The TeleMetrum accelerometer we use has its own 5 volt power
+ supply and
+ the output must be passed through a resistive voltage divider to match
+ the input of our 3.3 volt ADC. This means that unlike the barometric
+ sensor, the output of the acceleration sensor is not ratio-metric to
+ the ADC converter, and calibration is required. Explicitly
+ calibrating the accelerometers also allows us to load any device
+ from a Freescale family that includes at least +/- 40g, 50g, 100g,
+ and 200g parts. Using gravity,
+ a simple 2-point calibration yields acceptable results capturing both
+ the different sensitivities and ranges of the different accelerometer
+ parts and any variation in power supply voltages or resistor values
+ in the divider network.
+ </para>
+ <para>
+ To calibrate the acceleration sensor, use the 'c a 0' command. You
+ will be prompted to orient the board vertically with the UHF antenna
+ up and press a key, then to orient the board vertically with the
+ UHF antenna down and press a key. Note that the accuracy of this
+ calibration depends primarily on how perfectly vertical and still
+ the board is held during the cal process. As with all 'c'
+ sub-commands, follow this with a 'c w' to write the
+ change to the parameter block in the on-board DataFlash chip.
+ </para>
+ <para>
+ The +1g and -1g calibration points are included in each telemetry
+ frame and are part of the header stored in onboard flash to be
+ downloaded after flight. We always store and return raw ADC
+ samples for each sensor... so nothing is permanently "lost" or
+ "damaged" if the calibration is poor.
+ </para>
+ <para>
+ In the unlikely event an accel cal goes badly, it is possible
+ that TeleMetrum may always come up in 'pad mode' and as such not be
+ listening to either the USB or radio link. If that happens,
+ there is a special hook in the firmware to force the board back
+ in to 'idle mode' so you can re-do the cal. To use this hook, you
+ just need to ground the SPI clock pin at power-on. This pin is
+ available as pin 2 on the 8-pin companion connector, and pin 1 is
+ ground. So either carefully install a fine-gauge wire jumper
+ between the two pins closest to the index hole end of the 8-pin
+ connector, or plug in the programming cable to the 8-pin connector
+ and use a small screwdriver or similar to short the two pins closest
+ to the index post on the 4-pin end of the programming cable, and
+ power up the board. It should come up in 'idle mode' (two beeps),
+ allowing a re-cal.
+ </para>
+ </section>
+ </appendix>
projects / fw / altos / blobdiff