Pololu TReX Jr User Manual

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for the mode selected by the “channel input source” parameter (this parameter’s default value is RC
mode).

▪ Red and green LEDs blink in unison around once per second: The TReX Jr is waiting to see if it
should enter learning mode. This happens when the TReX Jr is in RC or analog mode, the mix
jumper is in place, and the serial out (SO) pin is grounded. This last condition can arise if the TReX
Jr is connected serially to an unpowered microcontroller. Ungrounding the SO pin will cause the
TReX Jr to startup as normal; removing the mix jumper will put the TReX Jr into learning-mode, at
which point it will either start trying to learn the channels or will wait for a valid signal on the input
lines. If it waits, it will flash the red and green LEDs in unison four times faster while waiting.

▪ Red and green LEDs alternate around once per second: The TReX Jr is waiting to see if it should
enter firmware-upgrade mode. This happens when the TReX Jr is in serial mode (i.e. there is no
mode jumper in place), the mix jumper is in place, and the serial out (SO) pin is grounded. This last
condition can arise if the TReX Jr is connected serially to an unpowered microcontroller.
Ungrounding the SO pin will cause the TReX Jr to startup as normal; removing the mix jumper will
put the TReX Jr into firmware-upgrade mode, which will cause the red and green LEDs to start
alternating four times faster.

◦ While Running Normally:

▪ Red LED on solid, green LED might flicker: The red LED solidly on indicates that serial is in
control of the motors (i.e. serial override is active or the TReX Jr is in serial mode). While serial is in
control of the motors, the green LED acts as a serial-packet-received indicator. It lights when a
command byte is received and stays lit until the last byte of the command packet is received. The
flicker rate of the green LED will depend on the baud rate you’re using and the rate at which you’re
sending serial commands to the TReX Jr.

▪ Green LED on solid, red LED might flicker: The green LED solidly on indicates that the channel
inputs are in control of the motors (i.e. the TReX Jr is in RC or analog mode and serial override is not
active). While the channel inputs are in control of the motors, the red LED acts as a serial-packet-
received indicator. It lights when a command byte is received and stays lit until the last byte of the
command packet is received. The flicker rate of the red LED will depend on the baud rate you’re
using and the rate at which you’re sending serial commands to the TReX Jr.

▪ Green LED is rapidly flashing, red LED might flicker: The TReX Jr will not let the channel inputs
start controlling the motors until safe-start conditions are first met (see Section 4.e). Specifically, it is
waiting for a valid signal on all required channels, for the motor signals to request no motor
movement, and for the serial override signal to not be requesting serial override mode. This state
occurs when the TReX Jr first starts running in RC or analog mode, and can be retriggered if an RC
signal is ever lost on a required channel. This state will never occur if the TReX Jr is in serial mode.
While in this state, the red LED acts as a serial-packet-received indicator as described in the bullet
point above.

◦ While in Learning Mode (see Section 3.e):

▪ Red and green LEDs blinking rapidly in unison: The TReX Jr is in RC mode and it is waiting for a
valid RC signal on at least one channel before it begins learning the channel calibration values.

▪ Red LED on solid, green LED off: This is the first stage of the learning sequence in which the
TReX Jr is attempting to learn the neutral values of each channel. This phase should last around two
seconds. Do not touch the RC/analog sticks while the red LED is on or you could cause learning to
fail.

▪ Status LEDs flash rapidly five times: When the TReX Jr is through learning the neutral values, it
will flash the status LEDs five times to indicate which channels had neutrals successfully learned and
which channels failed neutral learning. A green flash indicates success for the corresponding channel;
a red flash indicates failure. If all five channels fail to learn neutral values, the learning process stops,

Pololu TReX Jr User's Guide

3. Getting Started

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