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Using only 7 bits presents a slight problem for the left/right (L/R) mode. I want to continue using the direct duty value input, but I'll need to know when to change directions. I handled this by using the direction bit as the 8th data input. When in L/R mode, bit P1.7 is used to determine this. So, the input value 0-127 will be CCW full speed (0) to stop (127). Values 128-255 will be CW stop to full speed. In actual usage, I'll truncate the values to operate from 0-100 on each side of center (I'll explain this later).

The Circuit
After determining the process, I can now lay out the hardware. Looking at the circuit (right-click and open in a new window for a larger view) you can see the inputs on the left, and the outputs on the right. I chose high power MOSFETs because of their high current capacity. You can run a rather large motor without the need for heat sinks. However, since the gate voltages on the FETs are +/- 10 volts, I couldn't drive them directly (when are they going to invent a 12V microprocessor!). So I used small NPN and PNP transistors to drive the FETs. I could have used logic level devices, but I wanted the capability to control a 5 amp motor, and there aren't any logic level, p-channel FETs available to handle large currents. In place of the IRFZ34, I suppose I could have simplified the circuit by using an IRLZ44, which is a high current logic level n-FET but hey, I wanted to be consistent. And besides, I didn't have any in stock. A very nice way to drive the FETs is to use opto isolators. Again, I didn't have any on hand, and I was coming up on deadline. So, I stayed with the original plan.

Even so, using standard FETs meant that the system was current limited due to the p-channel devices. Because of the smaller active area in the p-FET, it can't handle as much current as an n-FET. I could have used four n-channel FETs. In order to do that, I would need to generate a high voltage gate pulse for the high side switches, but I didn't want to do that.

Instead, to control the motor, I turn on the p-FET and do all the chopping with the n-FET. This doesn't increase the current capacity of the p-FETs, but not switching them does reduce the temperature increase that would be caused by the transition from off to on and back off.

The Firmware
I set up a chopper to operate at a 1ms cycle by programming TR1 to interrupt every 1ms. At the start of the cycle, the port is read to input the speed value, and that turns on the correct FETs based on the how the direction bit is set. The motor_on: loop waits and decrements the value so that each unit is equal to about 10 microseconds. So a value of, say, 16, will keep the program in the loop for 160 usecs as the value counts down. After that time, the motor is 'chopped' and it sits in the motor_chop: loop and waits for the timer interrupt to occur. In this way, the motor is on 160 usec out of a possible 1000 usec, so the duty cycle is 160/1000 or 16%. This is shown in the waveform below.

As noted previously, the n-channel FETs can handle more current, and thus can handle more heat. Every time you turn a device on and off, heat is generated as the gate voltage slews from 0 to Vgate and back to 0. The steeper the slope, the less heat is generated, but nothing is instantaneous. Doing the chopping with the n-FET allows the circuit to handle larger motors. The motor I used for development draws 1 amp, and I was able to run it without any heat sinks -- the FETs didn't even get warm.

Here's the code. When the program starts, it sets the stack, configures the ports and starts the interrupt timer. It then goes into a continuous loop. The first thing it does is inputs the data from Port 1. The direction bit is ANDed out and the data is compared to 0 to see if the motor should stop. If so, it goes to the stop: routine to turn off the motor drivers and see if the brake enable bit is set. If it is, it calls the brake: routine that turns on both n-FETS, effectively shorting out the motor.

If the data is not 0, the mode0 bit is checked to see if the controller is in left/right mode, or not. If not, it checks the direction bit to see which pair of FETs to turn on, and jumps to the motor_on: loop. Once there, it wastes some time and counts down the value that's in the accumulator. The NOPs are there to create a 10 usec delay for each unit of the speed value. After the value reaches 0, it falls out of the loop, turns off both n-FETs (the p-FET remains on) and enters motor_chop: where it waits for the timer to interrupt and start the cycle all over again.

For left/right mode, it again checks the direction bit, but for this mode, it is really the eighth bit of the speed value. For values of 80h and above, the motor is considered in forward mode, so it jumps to the go_foward: routine where it acts just like the regular direction mode. However, if the value is less than 80h, the motor goes in the reverse direction. Since I don't want the motor to start at max speed at 7Fh, I compliment the accumulator first. This makes a value of 0 the high speed.

Notice that I used HEX values to explain the left/right mode. This was done to make the principle easier to understand. If you recall, I want to be able to input a decimal value between 0 and 100 to get direct duty cycle. Seven bits gives me 128 steps, but I just use 0-100 for the control input. Values 100-127 are considered the same. In fact, I had a little problem with the actual full speed value. There is quite a bit of overhead required to make the calculations, so when I got to 96% duty, the program overshot the interrupt. This made the system go to 50% duty. I tried to trim the calculations as much as possible, but in the end, I simply forced an "on full" if the value is 95% or greater. I could have reduced this error to about half by increasing the microcontroller clock to 24 MHz, and really working at the code, but this wasn't an issue with me, so I left it.

I ran out of time for this issue, but next time I hope to have the R/C interface added to the controller.

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Lesson 1:
T o o l
1.1. Programmer
1.2. Edsim 51
1.3. MIDE-51
1.4. ATMEL ISP

Lesson 2:
Input Output
2.1.LED
2.2.Swicht
2.3.7 Segmen
2.4.LCD Character
2.5.ADC
2.6.DAC
2.7.Motor Stepper
2.8.Keypad

Lesson 3:
Timer Counter
3.1.Basic
3.2.Mode 0
3.3.Mode 1
3.4.Mode 2
3.5.Mode 3

Lesson 4:
Serial Comm.
4.1.Basic
4.2.LED
4.3.Rotate LED
4.2 ADC
4.3.LCD

Lesson 5:
Interuption
5.1.Basic
5.2.Timer
5.2.External