How to compare values? - Electronic Circuits Projects Diagrams Free

Lac · 26th January 2005, 11:23 AM

How can I compare a value from an ADC-reading to a set value? f.ex. the ADC has a range of 8 bit (0-255) and when the vaue is above a set value, f.ex. 200, one pin goes high, and when the value goes below 200 the pin goes low.

How can this be done in assembler, or what is the comands to comapre values. ie. higher than >, lower than <? thanks.

**Nigel Goodwin** · 26th January 2005, 11:48 AM

Quote:

Originally Posted by Lac

How can I compare a value from an ADC-reading to a set value? f.ex. the ADC has a range of 8 bit (0-255) and when the vaue is above a set value, f.ex. 200, one pin goes high, and when the value goes below 200 the pin goes low.

How can this be done in assembler, or what is the comands to comapre values. ie. higher than >, lower than <? thanks.

It all depends on the processor, on a PIC you use either subtraction, or XOR (for equality), and check the results on the flags affected. Other processors should have (will have!) similar features.

Lac · 26th January 2005, 12:07 PM

Jupp, its the 16LF877A chip, but how can I use the SUB and XOR commands to check if the value is higher than/lower than? I cant figure it out :? . thanks

Mike, K8LH · 26th January 2005, 01:13 PM

Theory and code examples can be found on PICList.com... Here's one (of many);

http://www.piclist.com/techref/microchip/compcon.htm

Regards, Mike - K8LH

**Nigel Goodwin** · 26th January 2005, 01:19 PM

Quote:

Originally Posted by Lac

Jupp, its the 16LF877A chip, but how can I use the SUB and XOR commands to check if the value is higher than/lower than? I cant figure it out :? . thanks

Check on the PICList, there's an entire section on comparisons.

But basically you do the subtraction and check the flags in the status register.

If you subtract 5 from 5, the answer is (obviously) zero, so the zero flag will be set.

If you subtract 6 from 5 the function overflows, and the carry will be affected (I can't remember which way off hand).

If you subtract 5 from 6, the answer is 1, and neither flag will be affected.

So by checking both C and Z, you can determine =, <>, <, >, <=, >=.

The PICList explains it all.

Lac · 26th January 2005, 02:26 PM

The piclist webpage is down? cant get in, loads for ever.

Mike, K8LH · 26th January 2005, 02:40 PM

It's coming up fine on my FireFox browswer... Let me check IE...

It's coming up ok in Internet Explorer too... Not sure why you're not getting in... Sorry...

Regards, Mike - K8LH

**Nigel Goodwin** · 26th January 2005, 03:45 PM

Works fine here with IE6 as well, the actual page is http://www.piclist.com/techref/microchip/compcon.htm but here's some examples off of it.

Code:

;if RAMx <= K
;Tony Nixon
  movf RAMx,w
  addlw 255 - K           ; eg if RAMx > 5 ... addlw d'250'
  skpnc
  goto Endif

;if K <= RAMx
;Tony Nixon
  movf RAMx,w
  addlw 255 - K + 1       ; eg if RAMx < 5 ... addlw d'251'
  skpc
  goto Endif

;if RAMx < K
; Tony Nixon
  movf RAMx,w
  addlw 255 - K + 1       ; eg if RAMx >= 5 ... addlw d'251'
  skpnc
  goto Endif

;if K < RAMx
; Tony Nixon
  movf RAMx,w
  addlw 255 - K           ; eg if RAMx <= 5 ... addlw d'250'
  skpc
  goto Endif

;if RAMx <= RAMy
; Scott Dattalo
  movf RAMx,w
  subwf RAMy,w
  skpc
  goto Endif

;if RAMy < RAMx
; Scott Dattalo
  movf RAMx,w
  subwf RAMy,w
  skpnc
  goto Endif

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26th January 2005, 11:23 AM	#1
Lac	How to compare values? How can I compare a value from an ADC-reading to a set value? f.ex. the ADC has a range of 8 bit (0-255) and when the vaue is above a set value, f.ex. 200, one pin goes high, and when the value goes below 200 the pin goes low. How can this be done in assembler, or what is the comands to comapre values. ie. higher than >, lower than <? thanks.

26th January 2005, 12:07 PM	#3
Lac	Jupp, its the 16LF877A chip, but how can I use the SUB and XOR commands to check if the value is higher than/lower than? I cant figure it out :? . thanks

26th January 2005, 01:13 PM	#4
Mike, K8LH	Theory and code examples can be found on PICList.com... Here's one (of many); http://www.piclist.com/techref/microchip/compcon.htm Regards, Mike - K8LH

26th January 2005, 02:26 PM	#6
Lac	The piclist webpage is down? cant get in, loads for ever.

26th January 2005, 02:40 PM	#7
Mike, K8LH	It's coming up fine on my FireFox browswer... Let me check IE... It's coming up ok in Internet Explorer too... Not sure why you're not getting in... Sorry... Regards, Mike - K8LH