Nuts and Volts - September 2007

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In playback mode, the green LED is lit and we pull a new set of servo values from the EEPROM. Note that the GET_EE2 function returns a word, but we’re placing the result in a byte (pos⁰). The compiler is smart enough to put the LSB of the returned value into pos⁰. We can get the MSB of the return manually by copying __PARAM². When a function returns a word, the LSB is in __PARAM¹; the MSB is in __PARAM².

After retrieving a movement record, we decrement the records count and check for button presses. We can stop the playback cycle or, by pressing the y axis button, put it into a pause mode. Assuming no button presses, we check to see if the playback cycle is complete. If not, then the EE pointer is advanced to the next set of position values. When we do get to the end, the program checks the loop-back jumper. When this is installed, the records count is reloaded, the EE pointer sent back to the beginning, and the playback continues. With no loop-back jumper, the mode is returned to idle and playback stops.

The pause mode doesn’t do anything except monitor button presses. We can either resume the cycle by pressing the y axis button again, or stop it by pressing the x axis button.

Play_Paused:
  IF btnTmr(1) = BtnOK THEN
    runMode = M_PLAY
    btnTmr(1) = 0
    GOTO Main
  ENDIF
  IF btnTmr(0) = BtnOK THEN
    runMode = M_IDLE
    btnTmr(0) = 0
  ENDIF
  GOTO Main

And there we have it — a two-channel servo controller that plays live, records, and plays back. Figure 6 shows my final board. The six-pin header on the right edge is where I connected my DB- 15 joystick adapter (using 0.025” post-header sockets). Note that the SX28 proto board has busses for Vdd and Vss so connecting the pull-ups and pull-downs for the joystick connection is very easy at this point.

Connections beneath the board are made with wire-wrapping wire, except for the power connections to the joysticks. Here’s a bit of a tip for your toolbox: Get a roll of blue painter’s tape. It’s great for holding wires and components while soldering, and won’t leave a sticky mess when you pull it off — I always have it on hand when I’m building circuit boards.

The board does work as well as I’d hoped, but one

STAMP APPLICATIONS

■ FIGURE 6. Completed Waldo.

thing I will do is order an LTC1298 and replace the ADC0832 with it. I didn’t happen to have an LTC1298 in my supply, so I went with what I had. It’s a little pricey (about $11), but I think the additional resolution will be worth having, and the pin-out exactly matches the ADC0832. Next up for me is to attach a neat little pan-tilt servo head that I bought from Lynxmotion and make it dance!