Post by thread:[PIC]:16f84 Question

Steve Kasm wrote: > > Hello All, > > On a 16f84, if I bring Mclr to ground will this reset the chip? > Yes, but make sure MCLR is tied to VCC via a resistor (10K or so) to avoid a short. -- Best regards Tony mICros http://www.bubblesoftonline.com .....salesKILLspam@spam@bubblesoftonline.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Steve Kasm <.....PICLISTKILLspam.....mitvma.mit.edu> wrote: > On a 16f84, if I bring Mclr to ground will this reset the chip? Yes. -Andy === Andrew Warren -- EraseMEaiwspam_OUTTakeThisOuTcypress.com === Principal Design Engineer === Cypress Semiconductor Corporation === === Opinions expressed above do not === necessarily represent those of === Cypress Semiconductor Corporation -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> On a 16f84, if I bring Mclr to ground will this reset the chip? Yes. Holding it a ground will hold it in a reset state, when it returns to a logic 1 state the chip will start to execute instructions again. -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> On a 16f84, if I bring Mclr to ground will this reset the chip? Yes, but you risk damage to the chip if you ground it directly {Quote hidden}> > 220 ohm current limiting resistor from MCLR first of all > > From the free end of the 220R - > > 10k to Vcc > > Button + 100R to 0V > > 10nF to 0V (ie in parallel with the button and the 100R) > > The 10nF + 10k to Vcc provides some noise filtering and > > also ensures that MCLR stays low for a short time after > > Vcc goes high at power-up > > You can also add a small diode in parallel with the 10k, with > > the cathode (stripe end) to Vcc. This is a discharge path for > > the 10nF at power-down -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email KILLspamlistservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Is there a way to do this using another logic chip to control the reset or a transistor? I need to control this reset from another signal on my circuit. I posted a message about this to the [EE] board didn't think it was appropriate for this board. What I have from this point which is an inappropriate signal to work with is a rectified dc signal that as logic high is 41v dc and logic low is 14v dc. Somehow I need that 14v dc to reset the processor? I need my program to reset at this condition and I'm out of I/O pins on the processor to deal with this. Regards, Steve {Original Message removed}

> On a 16f84, if I bring Mclr to ground will this reset the chip? Yes, that's why it is called /MCLR (/ == active low). Is this a trick question? Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

> Yes, but you risk damage to the chip if you ground it directly Source for that? I know you can damage it by inrush current (when tied to a charged cap and no Vcc), but by tying it low??? Wouter -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

> > Yes, but you risk damage to the chip if you ground it directly > > Source for that? I know you can damage it by inrush current (when > tied to a charged cap and no Vcc), but by tying it low??? I think you may have misunderstood "ground it directly". Not tied to ground (although why would you want to have the PIC in permanent reset, but maybe it's my turn to misunderstand), but as you say, to reduce instantaneous currents. My sources for this are the F84 manual (yes, I'm back to trusting manuals when it suits me !!!) and work done, and advice given by, Dan Michaels -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

As for damaging the MCLR input by grounding it directly, I quote from the datasheet: "Voltage spikes below VSS at the MCLR pin, inducing currents greater than 80 mA, may cause latch-up. Thus, a series resistor of 50-100ohm should be used when applying a "low" level to the MCLR pin rather than pulling this pin directly to VSS". Okay, in practice you wouldn't want to tie MCLR directly to ground anyway. I guess they mean if you're pulling it low using a TTL signal, or with a bipolar transistor / FET. I suppose this advice would apply to any input pin in general (especially those which can be configured as outputs, of course). I guess that if you're driving the pin with a TTL signal and/or you've got some inductance in the circuit (PCB track etc) then MCLR would quite likely go below Vss. But I wonder if this is really necessary if you're, say, pulling it low with a simple FET and the track from the FET source to the PIC's Vss pin is very short? Having just brushed up on my knowledge of the MCLR input, I'm now worried that I should have implemented an RC reset in my last PIC -based product rather than connecting it straight to Vdd: there is a lot of capacitance on the 5V rail in that circuit. Am I likely to run into trouble in practice? Comments? Never stop learning... Trev > {Original Message removed}

> Having just brushed up on my knowledge of the MCLR input, > I'm now worried that I should have implemented an RC reset > in my last PIC -based product rather than connecting it straight > to Vdd: there is a lot of capacitance on the 5V rail in that circuit. > Am I likely to run into trouble in practice? Comments? If you think that there will be voltage stored in the MCLR cap after Vcc has gone from the PIC then use a signal diode in parallel with the R (a to MCLR, k to Vcc) to discharge it. Causing MCLR to rise to Vcc after the Vcc pin itself is no bad thing, but you could leave out the C and use the R as a pull-up. A link (0R) is better as an R can allow Vcc glitches and noise through and possibly cause the PIC to reset > Never stop learning... Never stop aksing -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

On 05-Feb-02 Trevor Page wrote: > As for damaging the MCLR input by grounding it directly, I quote from the > datasheet: "Voltage spikes below VSS at the MCLR pin, inducing currents > greater than 80 mA, may cause latch-up. Thus, a series resistor of 50-100ohm > should be used when applying a "low" level to the MCLR pin rather than > pulling this pin directly to VSS". I think the important thing here is to look at the figures..... I can't see now a simple switch to ground and a 10K pull up to Vcc and a small capacitor are going to create voltage spikes below ground that can pump 80mA of current into the MCLR pin. Even with a several inches of wire to the switch I doubt there would be enough inductance about to cause problems. Is the MCLR input any different in this respect to other pins ? Peter ---------------------------------- E-Mail: Peter Onion <ponionEraseME.....srd.bt.co.uk> Date: 05-Feb-02 Time: 09:36:42 This message was sent by XFMail ---------------------------------- -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

Does any of this interfere with ISCP? I have no basis for this question, just that programming this part has given me troubles. I don't know if they're due to me always trying ISCP or the programmer. I recently bought a ICD so I haven't tried any '628s lately. Thanks, Kevin Jinx wrote: {Quote hidden}> > > On a 16f84, if I bring Mclr to ground will this reset the chip? > > Yes, but you risk damage to the chip if you ground it directly > > > > 220 ohm current limiting resistor from MCLR first of all > > > > From the free end of the 220R - > > > > 10k to Vcc > > > > Button + 100R to 0V > > > > 10nF to 0V (ie in parallel with the button and the 100R) > > > > The 10nF + 10k to Vcc provides some noise filtering and > > > also ensures that MCLR stays low for a short time after > > > Vcc goes high at power-up > > > > You can also add a small diode in parallel with the 10k, with > > > the cathode (stripe end) to Vcc. This is a discharge path for > > > the 10nF at power-down > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email RemoveMElistservEraseMEEraseMEmitvma.mit.edu with SET PICList DIGEST in the body -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu

> > > Yes, but you risk damage to the chip if you ground it directly > I think you may have misunderstood "ground it directly". Not tied > to ground (although why would you want to have the PIC in > permanent reset, but maybe it's my turn to misunderstand), OK, I still can't read 'ground it directly' different from 'tied to ground' but that might be my problem. Tying MCLR either to ground or Vcc is OK, but to a current source independent from Vcc (such as a cap) is definitely bad. And why would anyone tie /MCLR to ground directly? Well, I do that with a reset switch... When I want a really form reset (of both the PIC and maybe a HD44780 which does not have a reset input) I tie Vcc to Gnd! With an 7805 that is not a big problem and it cures even a brownout-problem. Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu

> Having just brushed up on my knowledge of the MCLR input, I'm now worried > that I should have implemented an RC reset in my last PIC -based product > rather than connecting it straight to Vdd: there is a lot of capacitance on > the 5V rail in that circuit. Am I likely to run into trouble in practice? The trouble is a current source *independent* from Vcc, that would try to 'power' the PIC via a path that is not up to such a current. So tying it directly to Vcc or Gnd is no problem, but a cap *is* a problem, so in that case use a resistor between the cap and the MCLR. Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu

> I can't see now a simple switch to ground and a 10K pull up to Vcc and a small > capacitor are going to create voltage spikes below ground that can pump > 80mA of current into the MCLR pin. Even with a several inches of wire to the > switch I doubt there would be enough inductance about to cause problems. The cap can! > Is the MCLR input any different in this respect to other pins ? Yes, it does not have the normal protection diode to Vcc (because it must accept the ~14 V Vpp). I guess RA4 is similar. Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestKILLspammitvma.mit.edu

> Does any of this interfere with ISCP? I have no basis for this question, just > that programming this part has given me troubles. I don't know if they're due > to me always trying ISCP or the programmer. sources of ICSP trouble: - load on /MCLR (esp. caps), so the rise from vcc to vpp is too slow - load on any of the involved pins (esp. caps) - not tying LVP enable low When you want to use an RC-reset with ICSP split the R: Vcc - 10k - C - 22k - /MCLR with a diode over the 10k. Wouter van Ooijen Van Ooijen Technische Informatica: http://www.voti.nl Jal compiler for PIC uC's: http://www.voti.nl/jal PICs kopen? http://www.voti.nl/shop -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestSTOPspamspam_OUTmitvma.mit.edu

Steve Kasm wrote: > > Is there a way to do this using another logic chip to control the reset > or a transistor? I need to control this reset from another signal on my > circuit. I posted a message about this to the [EE] board didn't think it > was appropriate for this board. What I have from this point which is an > inappropriate signal to work with is a rectified dc signal that as logic > high is 41v dc and logic low is 14v dc. Somehow I need that 14v dc to > reset the processor? I need my program to reset at this condition and > I'm out of I/O pins on the processor to deal with this. Sure, use two voltage dividers (4 resistors total) and a logic chip, quad nand gate etc. Each voltage divider is different, and each goes to a input pin. At 30v+ dc both inputs are high, at 14v dc one one logic gate is high. Configure the gates with whatever chip you prefer so that it generates a low output when the input is 14v. :o) -Roman -- http://www.piclist.com hint: To leave the PICList spamBeGonepiclist-unsubscribe-requestSTOPspamEraseMEmitvma.mit.edu

> OK, I still can't read 'ground it directly' different from 'tied to ground' Think we're talking about two completely different things. I interpret "ground it directly" as using a low-ohms device (switch) to connect MCLR to ground so as to reset the PIC. After the switch is released, MCLR goes back to being held high via a pull-up. I interpret "tied to ground" as a permanent ohmic connection, eg a wire link or PCB trace, that holds MCLR at 0V forever and ever amen (the opposite of the more usual "tying" MCLR to Vcc via a trace when no reset facility other than power-down-power-up is required). Permanently holding the PIC in reset by "tying" MCLR down didn't make sense, but it seems we were both victims of semantics -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

On 05-Feb-02 wouter van ooijen & floortje hanneman wrote: >> I can't see now a simple switch to ground and a 10K pull up to Vcc and a > small >> capacitor are going to create voltage spikes below ground that can pump >> 80mA of current into the MCLR pin. Even with a several inches of wire to > the >> switch I doubt there would be enough inductance about to cause problems. > > The cap can! Wouter, Please explain in more detail how you think this can happen. Do you mean that the capacitor (acting as a capacitor) can create the negative voltage, OR that the capacitor (acting as an inductor) can create the negative voltage. > >> Is the MCLR input any different in this respect to other pins ? > > Yes, it does not have the normal protection diode to Vcc (because it must > accept the ~14 V Vpp). I guess RA4 is similar. OK, but we are talking about problems caused by negative spikes going below Vss not posititive spikes going above Vcc. Peter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On 25-Jun-07 jim wrote: > Wouldn't a cap be a voltage source, not current? The charge stored within a capacitor gives rise to the voltage across it. If you put an external load on a charged capacitor the voltage will cause a current to flow in the load. The loss of stored charge causes the voltage to drop. Maybe you are thinking of constant current source rather than constant voltage source ? The capacitor is neiter of these of course ! Peter -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Peter, I am just saying it would supply a voltage, not a current as capacitors are voltage devices. And as an FYI, I didn't say it would be a constant anything source. Just that it is more a voltage source than a current source. I guess it really doesn't matter. It was just an observation. Thanks and Regards, Jim {Quote hidden}> On 25-Jun-07 jim wrote: >> Wouldn't a cap be a voltage source, not current? > > The charge stored within a capacitor gives rise to the voltage across > it. If you put an external load on a charged capacitor the voltage will > cause a current to flow in the load. The loss of stored charge causes > the voltage to drop. > > Maybe you are thinking of constant current source rather than constant > voltage source ? The capacitor is neiter of these of course ! > > Peter > > -- > http://www.piclist.com hint: PICList Posts must start with ONE topic: > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On 06-Feb-02 James Paul wrote: > > Peter, > > I am just saying it would supply a voltage, not a current as capacitors > are voltage devices. I don't think you can classify capacitors as either a voltage OR a current device, atleast not in the same way you might call a bipolar transisor a current device but a FET a voltage device (actually its a transconductance device if I remember correctly). > And as an FYI, I didn't say it would be a constant anything source. I know you didn't, and no where did I say you did. Mind you, rereading my post I didn't make my point very clearly. > Just that it is more a voltage source than a current source. > I guess it really doesn't matter. It was just an observation. Whether you consider it to be acting as a current or voltage source depends on how you chose to model it and the circuit it is connected to. Look up Norton & Thevenin equivalent circuits and you will see that voltage and current sources are interchangeable. Peter. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

James Paul wrote: > > Peter, > > I am just saying it would supply a voltage, not a current as capacitors > are voltage devices. And as an FYI, I didn't say it would be a constant > anything source. Just that it is more a voltage source than a current > source. I guess it really doesn't matter. It was just an observation. > Having stuck screwdrivers across the terminals of thousands of capacitors, they sure seem like a current source to me! ;o) -Roman -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads