Support » Pololu TReX Jr User’s Guide » 4. RC/Analog in Detail »
4.c. General RC Information
When the channel inputs are RC servo pulses, the update rate of the channel values is determined by the pulse-train frequency of your RC receiver. Typically this is 50 Hz (each channel updates every 20 ms). The TReX Jr can handle channel pulse-train frequencies of 10 Hz to 125 Hz.
The TReX Jr measures the width of RC servo pulses with 12-bit resolution and an accuracy of 1 us. The raw channel value is provided as a 12-bit number in units of 0.4 us, so a raw channel value of 2500 can be physically interpreted as a pulse width of 2500×0.4 =1500 us. Servo pulse widths typically range from 1 ms to 2 ms, with a neutral value of 1.5 ms. This would correspond to raw channel values of 2500, 5000, and 3750, respectively. The TReX Jr can handle pulse widths as short as 0.5 ms (raw channel value 1250) and as long as 2.5 ms (raw channel value 6250). If a channel reading is considered to be an error, the raw channel value will be 0xFFFF (65535 in decimal). Errors result from pulse widths outside the acceptable range and from pulse trains with frequencies outside of the acceptable range. If a signal ceases on a channel, that channels value will stop updating until approximately 150 ms elapse, at which point the channel’s raw value will become 0xFFFF. It will remain as such until valid signals are once again detected on the line.
As a safety feature, you can designate channels as required using the “required channels” configuration parameter. If a required channel goes for more than 500 ms without receiving a valid RC servo pulse and the TReX Jr is in RC mode, the TReX Jr will enter safe-start mode (see Section 4.e). By default, the TReX Jr only requires channel one.
If you are not using all five channels, you can optionally ignore unused channels via the “ignored channels” configuration parameter. Ignored channels are treated as fixed at their neutral values (no matter what signals their lines receive).