Servo - July 2009

Making the Motor Cable

To use the LEGO NXT servo motor with the robot board, I needed a simple way to connect it to the sensor inputs and motor driver outputs. I don’t like to chew up cables, especially odd cables like the six-pin RJ11. So, I looked for a way to interface this motor to my board that was nice and clean. Microchip’s standard PIC programmer cable is a six-pin RJ11 and this was the only cable of that type that I had, so I searched for something else that used that cable. On the SparkFun site ( www.sparkfun.com), I found the Olimex ICSP breakout board (part Dev-08108) which had a short six-pin RJ11 cable and two other connectors that would take two different pin pitch connectors. I used the connector with the .1” pin pitch to connect the signal lines to my board. Figure 3 shows how I used this breakout board to connect the RJ11 cable to my board.

The RJ11 cable did not exactly match the plug on the motor. I needed a slight modification to get everything working right. Figure 4 shows what the motor connector looks like and the pin-out of the signals that are going to be used — which is all of them.

If you look at any RJ11 cable, you’ll see that the retainer clip (the thing that goes click when it is snapped in) is centered in the Olimex cable. That clip isn’t centered on the LEGO NXT servo motor. This required some basic plastic surgery (pardon the pun). I modified the cable by removing the retainer clip and carving out some of the connector with a file. This is very difficult to explain in words, so look at Figure 5 to see what I’m talking about. The bold black line in Figure 5 shows the final form of the connector; the red line shows the part that was removed to get it to fit into the LEGO servo motor connector. It is possible that I could have left part of the retainer clip still on the cable, but even without this the cable fits quite snugly into the jack. You only need to modify one end of the cable. The other end fits as expected into the Olimex ICSP breakout board.

As previously mentioned, all six signal wires are used to run the motor and get the quadrature feedback to the robot board.

Everything is Connected. Now What?

We have the motor PWM inputs connected to a 754410 one amp H-bridge chip that is controlled by two of the PWM outputs of the ATMEGA168 chip. We’re only using one channel here since I’ve only got one motor. It isn’t a very exciting robot, but it is still cool to see one’s labor rewarded with a correctly functioning project. I built this project using avr-gcc 4. 3.0 in the Eclipse Europa IDE using the Eclipse AVR plug-in. You should be able to port it to your favorite environment with little effort.

We all know how PWM works to adjust the motor speed, so I won’t go into detail. The function InitMotors() in the source code atm168.c shows how this is done. The

Figure 3. Olimex PIC ICSP breakout board.

complete source code for this project can be downloaded from the www.servomagazine.com site under Mr. Roboto as atm168.zip. Remember that it is C code written in avr-gcc — a free compiler for the Atmel AVR microcontrollers. When you look at the SetMotorSpeed() function, you might wonder why I change bits in the TCCR0A register when I change directions (see Listing 1). I do this because we can save an I/O line from the ATMEGA168 by using a single output to enable both H-bridges (1,2EN and 3,4EN)