NU32v2: Using the LS7183 Quadrature Clock Converter

The LS7183 quadrature clock converter is a chip used to convert quadrature signals from encoders into up and down counts that can be sent to a microchip.

Overview

Rotary encoders create quadrature signals on two channels (A and B) corresponding to the speed and direction of a motor. Some encoders produce up to a million counts per revolution. For very fast quadrature signals there is no way that a software quadrature decoder can avoid overrun errors. Instead we may use the LS7183 chip to translate the A and B quadrature signals into brief pulses indicating up-count or down-count. The up and down counts can be configured as external input signals to increment counters on the PIC32. By reading the counters on the PIC, we can determine the angle of the motor.

Details

The resistor bias is used to control the pulse width of the up and down counts. The mode (1X, 2X, 4X) is used to multiply the quadrature clock rate by 1, 2 or 4. The mode is selected by connecting MODE to

• VSS (GND) -> 1X
• VDD (3.3V) -> 2X
• Float (don't connect to anything; in practice, though, you may have to connect it to a floating input of the PIC32 to get reliable behavior) -> 4X

The UP and DOWN count pins should be connected to external counters on the PIC32.

Sample Code

The full code can be downloaded here. The code configures 2 external counters (see Counters and Timers) that will increment with each pulse received from the LS7183. The example sends the encoder position via RS232 to your PC.

The main piece of code shown below reads the UP and DOWN counters. With a variable called bigcount, it adds the the recent UP counts, subtracts the down counts and adjusts for roll-over of the counters. The full code placed this code in the main while loop, but this code can be placed in an ISR if you want to read the encoder at a given rate.

count0 = ReadTimer4();  // in your routine this must be done at least every 32000
	                // encoder counts to avoid rollover ambiguity
count1 = ReadTimer5(); 	

bigcount += count0 - last0; // add on the recent up-counts, since the last time
	
if (count0 < last0){
  bigcount += 65536; 		// count0 only increments, so if it got lower 
  		                //it must have rolled over
}

last0 = count0;

bigcount -= count1 - last1; // we're not worrying about rollover of the 32 bit bigcount total
	
if (count1 < last1){
  bigcount -= 65536;
}
	
last1 = count1;