Post by thread:Driving long shift registers

I've got an application where I'm driving 90 bicolor LEDs using a 180 bit shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is driving this. Towards the far end of the shift register, I start getting errors (bit offsets). I suspect this is ringing on the clock line causing multiple shifts per clock, or perhaps propogation delay on the clock line causing the output of one shift register to be sampled after the previous bit has changed instead of before. Anyone have any suggestions on driving long shift registers like this? THANKS! Harold -- FCC Rules Online at http://www.hallikainen.com -- FCC Rules Online at http://www.hallikainen.com -- http://www.piclist.com hint: To leave the PICList spam_OUTpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Are you sure you're using transmitting on the right edge? Some PICs can keep the data steady on either rising or falling edges. You could also change the edge, and run the clock through an inverter with a schmitt trigger to clean it up a bit. --Blake. On Thu, Jul 29, 2004 at 07:59:18AM -0700, Harold Hallikainen wrote: {Quote hidden}> I've got an application where I'm driving 90 bicolor LEDs using a 180 bit > shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is > driving this. Towards the far end of the shift register, I start getting > errors (bit offsets). I suspect this is ringing on the clock line causing > multiple shifts per clock, or perhaps propogation delay on the clock line > causing the output of one shift register to be sampled after the previous > bit has changed instead of before. Anyone have any suggestions on driving > long shift registers like this? > > THANKS! > > Harold > > > -- > FCC Rules Online at http://www.hallikainen.com > > > > -- > FCC Rules Online at http://www.hallikainen.com > > -- > http://www.piclist.com hint: To leave the PICList > .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestKILLspammitvma.mit.edu

On Thu, 29 Jul 2004 07:59:18 -0700, Harold Hallikainen <.....haroldKILLspam.....hallikainen.com> wrote: > I've got an application where I'm driving 90 bicolor LEDs using a 180 bit > shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is > driving this. Towards the far end of the shift register, I start getting > errors (bit offsets). I had the same problem with a 17C756A that was driving up to 21 x 74HC595s. In the end I used 1 x 74HCT04 per 6 x 595s to sharpen the clock/data/strobe lines. Did the trick quite nicely. Dan -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspam_OUTTakeThisOuTmitvma.mit.edu

At 09:59 AM 7/29/2004, Harold Hallikainen wrote: >I've got an application where I'm driving 90 bicolor LEDs using a 180 bit >shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is >driving this. Towards the far end of the shift register, I start getting >errors (bit offsets). I suspect this is ringing on the clock line causing >multiple shifts per clock, or perhaps propogation delay on the clock line >causing the output of one shift register to be sampled after the previous >bit has changed instead of before. Anyone have any suggestions on driving >long shift registers like this? If it's physically long, you may need an AC coupled termination impedance. Maybe 100 ohms or thereabouts, with 0.1uF in series. -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

> On Thu, 29 Jul 2004 07:59:18 -0700, Harold Hallikainen > <@spam@haroldKILLspamhallikainen.com> wrote: >> I've got an application where I'm driving 90 bicolor LEDs using a 180 >> bit >> shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC >> is >> driving this. Towards the far end of the shift register, I start getting >> errors (bit offsets). > > I had the same problem with a 17C756A that was driving up to 21 x > 74HC595s. In the end I used 1 x 74HCT04 per 6 x 595s to sharpen the > clock/data/strobe lines. Did the trick quite nicely. > > Dan I'm not using an output latch, just letting the LEDs flicker a bit during updates, so I don't have a stobe line. Since the data cascades from one chip to the next, I'm not seeing how you're using the HCT04 on that line. On the clock lines, did you have the PIC drive the inputs of several HCT04s, then have each output drive the clock input of 6 shift registers (distributing the clock through the T04s)? As someone else suggested, I'll mess around with RC termination of the clock line. I tried just an R to ground, and that pulled the clock too low to drive the shift registers (this whole thing runs on 3V, so the PIC ouput current is a bit limited). THANKS! Harold -- FCC Rules Online at http://www.hallikainen.com -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

> As someone else suggested, I'll mess around with RC termination of >the clock line. I tried just an R to ground, and that pulled the >clock too low to drive the shift registers (this whole thing runs on >3V, so the PIC ouput current is a bit limited). That's why I suggested it. No real current requirement, just damps the reflections. The 100 ohms is a guess, you could literally stick a pot on it and tune for best results. -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

>-----Original Message----- >From: pic microcontroller discussion list >[spamBeGonePICLISTspamBeGoneMITVMA.MIT.EDU] On Behalf Of David VanHorn >Sent: 29 July 2004 17:35 >To: TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU >Subject: Re: Driving long shift registers > > >> As someone else suggested, I'll mess around with RC >termination of >>the clock line. I tried just an R to ground, and that pulled >the clock >>too low to drive the shift registers (this whole thing runs on 3V, so >>the PIC ouput current is a bit limited). > >That's why I suggested it. >No real current requirement, just damps the reflections. >The 100 ohms is a guess, you could literally stick a pot on it >and tune for best results. A low value series R in the clock line at the PIC end may also help to match the line impedance. Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

> >A low value series R in the clock line at the PIC end may also help to match >the line impedance. Another good idea. Just enough to slope the edges. So many people try to fix this stuff by hanging caps on it, or driving it harder. :) -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestEraseME.....mitvma.mit.edu

At 07:59 AM 7/29/2004 -0700, you wrote: >I've got an application where I'm driving 90 bicolor LEDs using a 180 bit >shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is >driving this. Towards the far end of the shift register, I start getting >errors (bit offsets). I suspect this is ringing on the clock line causing >multiple shifts per clock, or perhaps propogation delay on the clock line >causing the output of one shift register to be sampled after the previous >bit has changed instead of before. Anyone have any suggestions on driving >long shift registers like this? > >THANKS! Another possible problem is your grounding. If you're driving several amperes with nanosecond edges you may be getting a lot of ground bounce at the (physically) far end of your SR which is fouling up your clock line. You could try feeding the clock in at the far end to see if that's the problem. Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" EraseMEspeffinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

Perhaps it's already clear from other comments, but you ought to lower the signal fan out of the clock. One pin on the PIC should not be driving 23 devices, even if the current capability is in spec. Use a buffer or inverter (7404) with all the inputs connected to the clock line and each of the 6 outputs connected to 4 chips. If the wires are particularily long then you may try placing resisters to ground at the ends of the clock lines, or driving fewer chips per buffer/inverter output. -Adam Harold Hallikainen wrote: {Quote hidden}>I've got an application where I'm driving 90 bicolor LEDs using a 180 bit >shift register (23 TC74VHC164FT) running at 3V. The SPI output of a PIC is >driving this. Towards the far end of the shift register, I start getting >errors (bit offsets). I suspect this is ringing on the clock line causing >multiple shifts per clock, or perhaps propogation delay on the clock line >causing the output of one shift register to be sampled after the previous >bit has changed instead of before. Anyone have any suggestions on driving >long shift registers like this? > >THANKS! > >Harold > > >-- >FCC Rules Online at http://www.hallikainen.com > > > >-- >FCC Rules Online at http://www.hallikainen.com > >-- >http://www.piclist.com hint: To leave the PICList >RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu > > > > > -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

> I'm not using an output latch, just letting the LEDs flicker a bit > during updates, so I don't have a stobe line. Since the data cascades > from one chip to the next, I'm not seeing how you're using the HCT04 > on that line. On the clock lines, did you have the PIC drive the > inputs of several HCT04s, then have each output drive the clock input > of 6 shift registers (distributing the clock through the T04s)? Sorry, I should have been a bit clearer. I used a couple of the inverters on the HCT04 back to back to form a buffer. This way I could buffer the clock/data/strobe lines with one HCT04. Each line came out of the PIC, into an HCT04, daisychained through 6 x HC595s, then through another HCT04, daisychained through another 6 x HC595s ad nauseum. Dan -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu