This is the AVR Studio project and assembly language source code files for the latest version (v7.0) of my 64 channel serial servo controller.
This is the latest version of the ATMega168 version of the code which includes all of the new servo commands that I wrote about here including the multi-move command and the unit tests that I spoke of here. The controller allows you to set minimum, maximum and startup servo positions for each servo which can be saved into eeprom and used every time the controller is powered up.
This is the source code for the latest version of the 64 channel servo controller as detailed here.
This is an ATMega168 version of the controller that was originally developed for the ATtiny2313 but which was ported to the ATMega when I ran out of memory on the ATtiny.
The schematic required is similar to the one for the ATtiny2313, I’ll produce a new one for the ATMega168 when I get some time.
The 64 channel serial servo controller that I’ve been developing works pretty well for me but most of my development and testing was done in the AVR studio simulator. Once I actually started working with my hardware again I noticed a slight problem. Although my servo controller was operating to spec my spec was wrong.
The problem is that the data sheet I have for the Hitec HS-422 servo states that it requires a pulse length of between 900us and 2.
The time has finally come to put all of the code from the last three parts of this article together to form a complete serial configured, 64 channel, PWM servo controller for the ATTiny2313 and several CD74HCT238Es.
Here’s a recap of where we are and how we got here:
In part 1 I put together some basic PWM generation code that uses the 16-bit Timer1 to generate a rock solid PWM pulse train from some pulse duration data.
This is part three of a series of articles about the servo controller that I’m building for use in the hexapod robot that I intend to build. The first two articles in the series have presented the timer driven PWM generation code and the code used to take the configuration data that is managed by the serial port protocol and convert it into the data that is needed by the PWM generation code.
In part1 of this timer driven PWM servo controller I built some code which uses Timer1 in the ATTiny2313 to generate 64 PWM signals. The code in part 1 worked from hard-coded dummy data. The code presented here shows how we can create the data that the PWM generation code needs to run.
The timer driven PWM code presented here works in terms of two byte pulse durations and a 1 byte value that determines which pulses are still being generated.
As I mentioned recently, the original servo controller firmware that I posted was flawed and wasn’t suitable to use as a base for the more complex servo control code that I want to add to the controller. The new firmware follows the design that I spoke of here and relies on the ATTiny’s 16-bit timer to generate our PWM signals. Since we’re using a timer we don’t need to waste processor time with hard-coded timing loops full of ’nop’s as we did in the original firmware and this gives us much more time to do useful work.
Here’s the source code to the 64 channel ATtiny2313 servo controller. Note that you’ll need to use up to 8 CD74HCT238E, or equivalent, demultiplexer chips and that you can adjust the number of servos that you can control in steps of 8 using as many or as few CD74HCT238E chips as you want. If you only want 8 channels then you can do this without any demultiplexer chips; see here for source code to the 8 channel version.
The jitters and general instability of the hacked together simple servo controller (see here) for the ATtiny2313 were, it seems, down to the fact that the internal clock wasn’t stable enough and this caused enough timing issues to throw the PWM off enough to jiggle the servo position around rather than hold it steady. This morning my external 4MHz crystals arrived and after changing the processor’s fuse bits to accommodate the change in clock source everything was fine and the code started behaving itself.
