Post by thread:[PIC]: TTL or ST ?

Can anyone clarify what's going on with RB0 INT on an F84 please Whilst writing a zero-crossing detector I measured the voltage threshholds for RB0, to find out when the interrupt would be generated. The results I've got are nothing like the data specs. If someone has documentation to back up what I've found I'd be glad to see it. I'm sure I'm not the first to run into this. If I've got it wrong I'd like to know where The assumption was that with RB0 as a Schmitt Trigger input the voltages would be 0.8V and 0.2V, but my measurements don't support this. The data sheet does say that Vhys is To Be Determined, but aside from that it seems there's a dv/dt limit for RB0, which doesn't sound like a Schmitt Trigger. With a pot in the range 500R to 100k and INTEDG set for falling voltage IRQ, it's quite easy to lower the voltage slowly (0.01V/s) on RB0 without the PIC generating an INT IRQ. If an IRQ is generated it happens at 1.6V. The ISR will be exited if btfss in the ISR is true at 1.5V. With INTEDG=1 (rising voltage IRQ), the IRQ will happen at 3.11V and exit via btfsc at 1.48V. An IRQ is always generated no matter how slowly VRB0 rises. (I've also tried this with RB3, and that will flip at 1.44V whichever way V is going) Sequence is, after Reset (using negative-going IRQ) - - LED2 on, pot at full CW, V on b0 = 5.00V - Turn pot CCW (ie lower voltage) - When VRB0=1.60V LED2 goes off, LED1 comes on. This shows that PC is in the ISR - When VRB0=1.50V LED1 goes off, LED2 comes back on, indicating completion of ISR - Repeat, back to step 1 (no h/w reset necessary) volts equ portb.0 ;pot wiper. Pot has 0V CCW terminal, 5V CW terminal led1 equ portb.1 ;LEDs are transistor-switched via diode/8k2 resistor led2 equ portb.2 org irq goto led movlw 00h ;porta all o/p, nothing connected except a4 10k p-u tris porta clrf porta movlw 01 ;portb all o/p except b0. Pot on b0, LEDs on b1 and b2 tris portb clrf portb movlw 80h ;pull-ups off, falling edge IRQ option bsf inte ;enable INT IRQ bsf gie wait bsf led2 ;LED2 on, wait for INT goto wait nop nop goto wait ;loop after ISR led bcf intf ;ISR entry point bcf led2 ;LED2 off bsf led1 ;LED1 on low btfsc volts ;test RB0 goto low ;loop until voltage drops under hysteresis pot btfss volts ;test RB0 goto pot ;loop until voltage rises above hysteresis bcf led1 ;led1 off retfie ;back to wait -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Jinx wrote: {Quote hidden}> > Can anyone clarify what's going on with RB0 INT on an F84 > please > > Whilst writing a zero-crossing detector I measured the voltage > threshholds for RB0, to find out when the interrupt would be > generated. The results I've got are nothing like the data specs. > If someone has documentation to back up what I've found I'd > be glad to see it. I'm sure I'm not the first to run into this. If I've > got it wrong I'd like to know where > > The assumption was that with RB0 as a Schmitt Trigger input > the voltages would be 0.8V and 0.2V, but my measurements > don't support this. The data sheet does say that Vhys is To > Be Determined, but aside from that it seems there's a dv/dt > limit for RB0, which doesn't sound like a Schmitt Trigger. With > a pot in the range 500R to 100k and INTEDG set for falling > voltage IRQ, it's quite easy to lower the voltage slowly (0.01V/s) > on RB0 without the PIC generating an INT IRQ. If an IRQ is > generated it happens at 1.6V. The ISR will be exited if btfss in the > ISR is true at 1.5V. With INTEDG=1 (rising voltage IRQ), the IRQ > will happen at 3.11V and exit via btfsc at 1.48V. An IRQ is > always generated no matter how slowly VRB0 rises. (I've also > tried this with RB3, and that will flip at 1.44V whichever way > V is going) The Microchip datasheets are all very specific showing: The ST input MAX VIL (input low) as 0.2v. So it must lower into the 0v-0.2v range *absolutely* to cause a low-going trigger. The ST input MIN VIH (input high) as 0.8v So it must raise into the 0.8v-5v range to cause a high-going trigger. As these are listed as the min/max figures in the datasheet I would expect that Microchip takes these figures seriously?? Annoyingly they don't list typical figures for these threshold points. :o( -Roman -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam@spam@mitvma.mit.edu

> -----Original Message----- > From: Roman Black [fastvidKILLspamEZY.NET.AU] > Subject: Re: [PIC]: TTL or ST ? > > The Microchip datasheets are all very specific showing: > > The ST input MAX VIL (input low) as 0.2v. > So it must lower into the 0v-0.2v range *absolutely* > to cause a low-going trigger. You mean 0.2 * Vdd , that is 1V on a 5V-System. > The ST input MIN VIH (input high) as 0.8v > So it must raise into the 0.8v-5v range to cause a > high-going trigger. You mean 0.8 * Vdd , that is 4V on a 5V-System. Frank. GSP Sprachtechnologie GmbH Frank Wollenberg HW-Entwicklung Tel.: +49 (0)30 769929-78 Fax: +49 (0)30 769929-12 eMail: .....f.wollenbergKILLspam.....gsp-berlin.de -- GSP Sprachtechnologie GmbH Teltowkanalstr.1, D-12247 Berlin Tel.: +49 (0)30 769929-0 Fax: +49 (0)30 769929-12 eMail: EraseMEInfospam_OUTTakeThisOuTgsp-berlin.de Web: http://www.gsp-berlin.de -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestspam_OUTmitvma.mit.edu

> The Microchip datasheets are all very specific showing: > > The ST input MAX VIL (input low) as 0.2v. > So it must lower into the 0v-0.2v range *absolutely* > to cause a low-going trigger. > > The ST input MIN VIH (input high) as 0.8v > So it must raise into the 0.8v-5v range to cause a > high-going trigger. > > As these are listed as the min/max figures in > the datasheet I would expect that Microchip > takes these figures seriously?? I didn't look, but 0.8v for a high threshold doesn't seem right. Are these numbers really 0.2 x Vdd and 0.8 x Vdd ??? Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) -- http://www.piclist.com hint: To leave the PICList @spam@piclist-unsubscribe-requestKILLspammitvma.mit.edu

Wollenberg, Frank wrote: > > > -----Original Message----- > > From: Roman Black [KILLspamfastvidKILLspamEZY.NET.AU] > > Subject: Re: [PIC]: TTL or ST ? > > > > The Microchip datasheets are all very specific showing: > > > > The ST input MAX VIL (input low) as 0.2v. > > So it must lower into the 0v-0.2v range *absolutely* > > to cause a low-going trigger. > > You mean 0.2 * Vdd , that is 1V on a 5V-System. Thanks Frank, that's a pretty serious mistake I made! Wow. -Roman -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

On Thu, 15 Feb 2001 09:58:32 +1300 Jinx <spamBeGonejoecolquittspamBeGoneCLEAR.NET.NZ> writes: > Can anyone clarify what's going on with RB0 INT on an F84 > please > > Whilst writing a zero-crossing detector I measured the voltage > threshholds for RB0, to find out when the interrupt would be > generated. The results I've got are nothing like the data specs. > If someone has documentation to back up what I've found I'd > be glad to see it. I'm sure I'm not the first to run into this. If > I've > got it wrong I'd like to know where > > The assumption was that with RB0 as a Schmitt Trigger input > the voltages would be 0.8V and 0.2V, but my measurements > don't support this. Aren't the ST threshholds proportional to Vcc? I suspect they are 0.8*Vcc and 0.2*Vcc or 4.0V and 1.0V. This seems relatively close to what you measured. Harold ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: dl.http://www.juno.com/get/tagj. -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

> Aren't the ST threshholds proportional to Vcc? I suspect > they are 0.8*Vcc and 0.2*Vcc or 4.0V and 1.0V. This seems > relatively close to what you measured. My mistake also for saying ST limits are 0.2V and 0.8V, of course it should be 0.2Vcc and 0.8Vcc, but I don't want that to detract from what is puzzling me The figures I measured are nowhere near 4V and 1V. With a negative-going voltage the top figure is ~1.60V and the lower is ~1.50V = only ~0.1V of hysteresis (plus there's the ability to sneak past the 1.60V IRQ point without the PIC noticing). With a positive-going voltage they are 3.11V and 1.48V respectively = 1.63V of hysteresis, which is more reasonable although 50% away from the datasheet max/min. Why the big difference between -ve and +ve going voltages with INTEDG set appropriately ? If someone has 10 minutes to put a pot on b0 and confirm/refute this that would be appreciated. I've tried it on 3 new F84As with the same result. As far as I can tell the method I'm using is OK -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

At 10:56 AM 2/16/01 +1300, you wrote: >> Aren't the ST threshholds proportional to Vcc? I suspect >> they are 0.8*Vcc and 0.2*Vcc or 4.0V and 1.0V. This seems >> relatively close to what you measured. > >My mistake also for saying ST limits are 0.2V and 0.8V, of >course it should be 0.2Vcc and 0.8Vcc, but I don't want that >to detract from what is puzzling me > >The figures I measured are nowhere near 4V and 1V. With a >negative-going voltage the top figure is ~1.60V and the lower >is ~1.50V = only ~0.1V of hysteresis (plus there's the ability to >sneak past the 1.60V IRQ point without the PIC noticing). With >a positive-going voltage they are 3.11V and 1.48V respectively >= 1.63V of hysteresis, which is more reasonable although 50% >away from the datasheet max/min. Why the big difference between >-ve and +ve going voltages with INTEDG set appropriately ? The minimum and maximum figures on the data sheet are the values that Microchip guarantees will be recongnized as a 1 or 0. Your measurements are within the range, so it is doing what it is supposed to be doing. The amount of hysteresis Vhys is listed as 0.1V typical on the current data sheet (no limits given). Note also from the schematic (Page 15) that the RB0 input buffer will generally have *different* thresholds from the ST interrupt input, they are really two different circuits connected to the same input pin. There is *no* guarantees on how they will overlap in their thresholds. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" speffEraseME.....interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

> The minimum and maximum figures on the data sheet are the > values that Microchip guarantees will be recongnized as a 1 or 0. > Your measurements are within the range, so it is doing what it > is supposed to be doing. > The amount of hysteresis Vhys is listed as 0.1V typical on the > current data sheet (no limits given). Yes, you're absolutely right, and I'm pleased that you've been able to show that in the documentation. I was sure I'd read the latest pdf correctly as TBD and got distracted by the 1V - 4V thing, but on re-reading it does say 0.1Vhys. And I'm not likely to forget now ;-) > > there's the ability to sneak past the 1.60V IRQ point without the > > PIC noticing Now that I know I'm measuring a good PIC correctly, then this dv/dt limitation is also a fact of life, even though I expected an ST to detect slowly decreasing voltages and generate an IRQ (slowly increasing ones ARE detected reliably) ? -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

At 12:22 PM 2/16/01 +1300, you wrote: >Now that I know I'm measuring a good PIC correctly, then this >dv/dt limitation is also a fact of life, even though I expected an >ST to detect slowly decreasing voltages and generate an IRQ >(slowly increasing ones ARE detected reliably) ? Yes, for the interrupt you should have no problem with clean slow-changing voltages. Note though that Microchip does not guarantee the minimum amount of hysteresis. It would be imprudent to assume it must be larger than about 30 millivolts, IMO, or less than maybe 0.3 volt. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" RemoveMEspeffspam_OUTKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

> Yes, for the interrupt you should have no problem with > clean slow-changing voltages. I know that I "should" have no problem, but I do. If I apply a decreasing V at about 0.01V/s (not hard to do with a 10-t pot) to RB0 with INTEDG=0 then no IRQ happens. I can take V right down to 0V without the ISR being entered. Note that 0.01V/s is a guesstimate, just going by what I see on a multimeter, and it is repeatable -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu