Post by thread:UP/DOWN counter using a PIC?

I have a question for all the PIC experts out there: I am trying to interface a quadrature decoder to a PIC which currently drives some stepping motors. I can easily figure out how to build an external up/down counter and "roll my own" absolute position decoder- but has anybody successfully used the PIC itself to do this? I was thinking along the lines of using one externally clocked counter to count all pulses, and another to count only those in one direction. Subtracting these two give a relative number of steps in a particular direction. Any comments? David \|||/ o o David Darknell -------+--+--oOO-(_)-OOo--+--+----------------------------------------- |--|__|--|___|--|__|--| spam_OUTdavidTakeThisOuTxia.com .....davidKILLspam@spam@papdev.com "In a world without need of fences, who needs Gates." Author unknown _______________________________________________________________________

david@PAPDEV.COM wrote: > > I have a question for all the PIC experts out there: I am trying to > interface a quadrature decoder to a PIC which currently drives some > stepping motors. I can easily figure out how to build an external up/down > counter and "roll my own" absolute position decoder- but has anybody > successfully used the PIC itself to do this? > > I was thinking along the lines of using one externally clocked counter to > count all pulses, and another to count only those in one direction. > Subtracting these two give a relative number of steps in a particular > direction. I threw together a few latches and XOR gates to convert from quadrature to tachometer and direction. The tach input drove one of my interrupt on change pins. Whenever I got that interrupt, I'd just poll the direction pin. I needed to use 16 bit maths, but that wasn't too horrible. The only problem (?) with it was that when the direction changed, my circuit produced a one cycle latency. Not too tough to fix in software. I can't remember the circuit offhand, but I seem to recall that it used two XORs, two latches, and nothing else. I'll dig it up if you'd like. Jason ( # -e^(pi*i) ) <g>

At 10:19 AM 7/2/97 -0700, you wrote: >I have a question for all the PIC experts out there: I am trying to >interface a quadrature decoder to a PIC which currently drives some >stepping motors. I can easily figure out how to build an external up/down >counter and "roll my own" absolute position decoder- but has anybody >successfully used the PIC itself to do this? Well, I did it on an 8051, but the principle would be the same. First I save the old values in bits 2 & 3 in a register. Then when the input changes I put the new values in bits 0 & 1. I use the 4 bit number as an index to a look up table. The table has a return value of either -1 or +1 depending on if the change from the old value to the new value indicates clockwise rotation (+1) or counter clockwise (-1). I simply add this value to my counter. Finally I put the new value in the old value location in preparation for the next change. Only 1/2 of the 16 byte table contains valid data. The other half should never get accessed since the encoder can't genrate those bit patterns (in theory!). The bottom line is that it is not especially memory efficient, but is pretty quick. Hope this helps. - Gary ------------------------------------------------------------------------ Gary Sutcliffe, W9XT Unified Microsystems ppvvppKILLspammixcom.com PO Box 133 Slinger, WI 53086 http://www.qth.com/w9xt 414-644-9036

Gary Sutcliffe wrote: > > At 10:19 AM 7/2/97 -0700, you wrote: > >I have a question for all the PIC experts out there: I am trying to > >interface a quadrature decoder to a PIC which currently drives some > >stepping motors. I can easily figure out how to build an external up/down > >counter and "roll my own" absolute position decoder- but has anybody > >successfully used the PIC itself to do this? > > Well, I did it on an 8051, but the principle would be the same. > > First I save the old values in bits 2 & 3 in a register. Then when the > input changes I put the new values in bits 0 & 1. I use the 4 bit number as > an index to a look up table. > > The table has a return value of either -1 or +1 depending on if the change > from the old value to the new value indicates clockwise rotation (+1) or > counter clockwise (-1). I simply add this value to my counter. Finally I > put the new value in the old value location in preparation for the next change. {Quote hidden}> > Only 1/2 of the 16 byte table contains valid data. The other half should > never get accessed since the encoder can't genrate those bit patterns (in > theory!). The bottom line is that it is not especially memory efficient, > but is pretty quick. > > Hope this helps. > > - Gary > This reply is a bit late (by about 3 months) but another method is; The encoder reads as follows: Bit 1 0 ---------- ^ 0 0 | CCW | 0 1 | CW | 1 1 v 1 0 To determine the direction and update a counter value; BEGIN . last = encoder . loop forever . . current = encoder . . if last <> current . . . direction = last.bit1 XOR current.bit0 . . . if (direction == 1) . . . . value = value + 1 . . . else . . . . value = value - 1 . . . endif . . . last = current . . endif . endloop END -- Peter Homann email: .....peterhKILLspam.....adacel.com.au Work : +61 3 9596-2991 Adacel Pty Ltd Fax : +61 3 9596-2960 250 Bay St, Brighton 3186, VIC, AUSTRALIA Mobile : 014 025-925 http://www.adacel.com.au Australian Software Engineering Excellence

I know it's late, but here is what I've done to read an encoder with a 16C73. This routin runs on an interrupt, about 100 times in a second: ; ; Rutina para la lectura del encoder ; ; Se utiliza un unico byte local, ENCODER, de la forma: ; ; ENCODER: '00000000' ; |||||||+> Lectura anterior de 'Y' ; ||||||+-> Lectura anterior de 'X' ; |||||+--> '1' = Avanzar hacia adelante ; ||||+---> '1' = Avanzar hacia atras ; ++++----> Bits de avance anteriores ; ; En esta posicion los bits del ; encoder estan en PORTA: ; '00000000' <-- PORTA ; |+--> 'X' ; +---> 'Y' ; movf PORTA,W movwf AUX ; Aqui comienzan las cuentas: xorf ENCODER,F ; ENCODER <- ENCODER xor AUX rrf AUX,F ; Ahora se guardan, al reves, rlf ENCODER,F ; los dos bits del encoder en rrf AUX,F ; AUX, para la proxima cuenta. rlf ENCODER,F ; btfsc ENCODER,2 ; Por ultimo, se actualiza el incf COUNTER,F ; contador. btfsc ENCODER,3 decf COUNTER,F Sorry for the Spanish comments. I think the code is self explaining (and my english is not so good :-) First you get the encoder bits in AUX (0 and 1). Then, xor AUX with ENCODER, where are the previous encoder bits (1 and 0), but in inverse order. Finally, you get two bits (2 and 3) in ENCODER to change the position counter: if bit 2 =1 then inc(counter). If bit 3 =1 then dec(counter). Caution, it is posible to get bits 2 and 3 both set, so you inc and dec counter, but who cares?. If you care, simply add 'movlw B'00001100; xorwf ENCODER,F' before 'btfsc ENCODER,2', and you're done. -----Mensaje original----- De: Peter Homann [SMTP:EraseMEpeterhspam_OUTTakeThisOuTADACEL.COM.AU] Enviado el: miercoles 26 de noviembre de 1997 6:04 Para: PICLISTspam_OUTMITVMA.MIT.EDU Asunto: Re: UP/DOWN counter using a PIC? Gary Sutcliffe wrote: > > At 10:19 AM 7/2/97 -0700, you wrote: > >I have a question for all the PIC experts out there: I am trying to > >interface a quadrature decoder to a PIC which currently drives some > >stepping motors. I can easily figure out how to build an external up/down > >counter and "roll my own" absolute position decoder- but has anybody > >successfully used the PIC itself to do this? > > Well, I did it on an 8051, but the principle would be the same. > > First I save the old values in bits 2 & 3 in a register. Then when the > input changes I put the new values in bits 0 & 1. I use the 4 bit number as > an index to a look up table. > > The table has a return value of either -1 or +1 depending on if the change > from the old value to the new value indicates clockwise rotation (+1) or > counter clockwise (-1). I simply add this value to my counter. Finally I > put the new value in the old value location in preparation for the next change. {Quote hidden}> > Only 1/2 of the 16 byte table contains valid data. The other half should > never get accessed since the encoder can't genrate those bit patterns (in > theory!). The bottom line is that it is not especially memory efficient, > but is pretty quick. > > Hope this helps. > > - Gary > This reply is a bit late (by about 3 months) but another method is; The encoder reads as follows: Bit 1 0 ---------- ^ 0 0 | CCW | 0 1 | CW | 1 1 v 1 0 To determine the direction and update a counter value; BEGIN . last = encoder . loop forever . . current = encoder . . if last <> current . . . direction = last.bit1 XOR current.bit0 . . . if (direction == 1) . . . . value = value + 1 . . . else . . . . value = value - 1 . . . endif . . . last = current . . endif . endloop END -- Peter Homann email: @spam@peterhKILLspamadacel.com.au Work : +61 3 9596-2991 Adacel Pty Ltd Fax : +61 3 9596-2960 250 Bay St, Brighton 3186, VIC, AUSTRALIA Mobile : 014 025-925 http://www.adacel.com.au Australian Software Engineering Excellence