ATTiny45 Quadrature Decoder

Renoster has 4 Lynxmotion GHM-15 gear head motors each with a hall effect quadrature encoder. The quadrature encoders outputs two square waves 90 degrees apart (see Fig. 1) as the motor turns. Using these signals one can determine the speed and direction of the motor and from that the robots relative movement.

Acquiring the encoded signals can be a challenge. Each encoder requires two free IO lines on your MCU, since the signals can trigger relatively quick it is preferred that at least one of the inputs is an interrupt. Renoster's REV 2.3 controller board made use of an IO extender to read these, unfortunately several things conspired together to make this a less than ideal solution. To overcome these challenges the TinyQED was created.

TinyQED is a simple ATTiny45 based widget that counts the signals from the encoders and makes them available via I2C.

Sparkfun's BatchPCB service was used to create the PCB.

Implementing the I2C slave interface was a bit of a challenge but as with everything AVR, AVRFreaks had the answer in a thread where Dan Gates implemented I2C slave code from Don Blake, which formed the starting point for TinyQED.

AVR Studio Project Files: TinyQED.zip

/***************************************************************************
* File              : TinyQED
* Compiler          : AVRstudio 4
* Revision          : 1.0
* Date              : Tuesday, June 11, 2007
* Revised by        : Adriaan Swanepoel
*                   : Adapted from Dan Gates I2C analogue slave
*                     and Jim Remington's Quadrature encoder for the O 
*
* Target device	: ATtiny45
*
* Connections
*                               ATTiny45
*                 +--------------------------------+
*                 | 1 pb5 reset              VCC 8 | VCC
*         Motor A | 2 pb3                    pb2 7 | SCL
*         Motor B | 3 pb4                    pb1 6 | 
*             GND | 4 GND                    pb0 5 | SDA
*                 +--------------------------------+
****************************************************************************/
 
#include <stdlib.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/eeprom.h>
#include "usiTwiSlave.h"
 
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
 
union doublebyte
{
  unsigned int value;
  unsigned char bytes[2];
};
 
union doublebyte enc_pos;
unsigned char enc_dir;
unsigned char enc_last=0;
unsigned char enc_now;
unsigned char EEMEM slaveaddress = 0x36;  //Default address
 
/******************************************************************************
 *
 * 
 *
 * Description:	
 * ARGS:		none
 * RETURN:		none
 *
 *****************************************************************************/
ISR (PCINT0_vect)
{
	enc_now = (PINB & (3 << 3)) >> 3;                //read the port pins and shift result to bottom bits
	enc_dir = (enc_last & 1) ^ ((enc_now & 2) >> 1); //determine direction of rotation
 
	if(enc_dir == 0) 
		enc_pos.value++; 
 
		else   
 
			enc_pos.value--;	//update encoder position
 
	enc_last = enc_now;			//remember last state
}
 
/******************************************************************************
 *
 * setaddress
 *
 * Description:	Updates the I2C address in the internal eeprom
 * ARGS:		The new I2C address
 * RETURN:		none
 *
 *****************************************************************************/
void setaddress(unsigned char address)
{
	eeprom_write_byte(&slaveaddress, (uint8_t*)address);
}
 
/******************************************************************************
 *
 * readaddress
 *
 * Description:	Reads the preprogrammed I2C address from the internal eeprom
 * ARGS:		none
 * RETURN:		The devices I2C address
 *
 *****************************************************************************/
unsigned char readaddress()
{
	return eeprom_read_byte((uint8_t*)&slaveaddress);
}
 
/******************************************************************************
 *
 * main
 *
 * Description:	Where it all starts...
 * ARGS:		none
 * RETURN:		none
 *
 *****************************************************************************/
int main(void)
{
	unsigned char temp;
	enc_pos.value = 0;
 
	PCMSK |= (1 << PCINT3); // tell pin change mask to listen to pin2
	GIMSK |= (1 << PCIE);   // enable PCINT interrupt in the general interrupt mask
 
	sei();
 
	cbi(DDRB, DDB3);        // PB3 set up as input
	cbi(DDRB, DDB4);        // PB4 set up as input
	sbi(PORTB, PB3);	      // Set PB3 internal pullup
	sbi(PORTB, PB4);	      // Set PB4 internal pullup
 
	usiTwiSlaveInit(readaddress());
 
	for (;;)
	{
		if (usiTwiDataInReceiveBuffer())
		{
			temp = usiTwiReceiveByte();
 
			//which register requested
			//1..9   Reserved Commands
			//10..19 Reserved Data
			switch (temp)
			{
				//Reset the counter
				case 1 : enc_pos.value = 0; 
					 break; 
 
				//Center counter value
				case 2 : enc_pos.value = 32767;
					 break;
 
				//Set address
				case 3 : setaddress(usiTwiReceiveByte());
					 break;
 
				//Send the counter
				case 10: usiTwiTransmitByte(enc_pos.bytes[0]);
					 usiTwiTransmitByte(enc_pos.bytes[1]); 
					 break;
 
				default : //Do nothing
					 break;
			}	  
		}
 
		asm volatile ("NOP"::);
	}
}