Prototyping an ATMEGA168 And Xbee Based Robot Controller

I have recently started work prototyping some ideas for a simple wireless remote controller for my robots.

Originally I was going to purchase some thumb joysticks available from Sparkfun to use for prototyping. Luckily I had the idea to use an old Playstation One Dual Shock controller that has been collecting dust for ages in my closet. It was kind enough to donate its innards for my purposes. Surprisingly the thumb sticks found inside the Playstaion controller are identical to the Sparkfun ones.

I went ahead and etched two breakout boards for the liberated thumb joysticks and have begun to interface them with one of my ATMEGA168 breakout boards. The software is really quite simplistic. It is just a matter of doing two analog to digital conversions for each axis of the joystick, forming a packet with the joystick readings and transmitting the packet out the micro's USART to the Xbee module.

Determining Encoder Rotational Direction Using An ATMEGA168

While working on a DC motor controller project I needed to decode quadrature encoder signals.

Basically a quadrature encoder has two signal lines, A and B. Rotational direction of the encoder can be determined by the phase difference between these signals. 

A simple way to detect the phase difference of the signals with the ATMEGA168 is to generate an interrupt on the falling edge of signal A and look at the value of signal B. If signal B is high of the falling edge of signal A the encoder is turning one way. And if signal B is low on the falling edge of signal A the encoder is turning the other way.

AVR PWM Signal Generation Using The ATMEGA644P's Timer

While working on a recent project I needed to generate a PWM signal from my ATMEGA644P in order to operate a SN754410 motor driver as a speed controller.

To generate the PWM I used the ATMEGA's 8-bit Timer/Counter 2 configured for fast PWM mode. Fast PWM works as follows:

The timer will continually count up from 'bottom' to 'top'. When 'top' is reached the counter will then wrap around and begin counting up from 'bottom' again. In this case we are using an 8 bit timer so 'bottom' will be equal to 0 and top will be equal to 255 decimal or FF hex. When the counter reaches the 'compare' value an output pin will be set. This pin will then reset when the counter wraps around to 'bottom'. We are able to vary the value of 'compare' in order to generate a fixed frequency PWM signal with a variable duty cycle.

In order to set up the timer its registers need to be configured as follows:

Set the Data direction for timer pin set as output

I am using timer 2 output A so pin 7 port D (PD7 OC2A/PCINT31) needs to be set as output:

DDRD |= (1 << PD7)

Set waveform generation mode

Waveform generation mode is set in the two registers TCCR2A and TCCR2B, Timer/Counter Control Registers A and B. I need Fast PWM counting from 0 to 0xFF so I set the registers as:

TCCR2A |= (1 << WGM0)
TCCR2A |= (1 << WGM1)

Set compare output mode

Compare output mode is set in the TCCR2A register:

TCCR2A |= (1 << COM2A0)
TCCR2A |= (1 << COM2A1)

Clock selection

Clock selection is set in the TCCR2B register. I am not concerned about the frequency of the PWM signal so I just select the AVR's clock as without any prescalling.

TCCR2B |= (1 << CS20)

Set the output compare

Lastly the fast PWM's duty cycle needs to be set by adjusting the compare value. This is set in the OCR2A, Output Compare Register A. This register can be written to in any point in software to vary the duty cycle of the output PWM signal.

OCR2A = duty_cycle

PCB Toner Transfer Experimentation