Post by thread:[PIC]: Simple PIC Frequency Counter

I currently have a circuit which uses a SN74LS628 Voltage Controlled Oscillator (VCO) as a square wave generator. I'd like to buffer the output of this and feed it back into a PIC to and calculate the frequency. Frequency range will be anywhere from about 50kHz to 500kHz. PIC clock frequency could be somewhere around 4MHz to 20MHz. Does anyone have a simple way to implement this with a PIC? Thanks -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Feed the output of your buffer to the timer/counter input of your preferred PIC, count the amount of pulses in a fixed period of time (gate time) and make the math. Take for instance your 500kHz, count the incoming pulses for 1mS and you'll have a count of 500. Using a 16-bit timer should be straight forward, for higher accuracy use longer gating times. I have measure up to 3MHz in that way without any extra counters. Check the external clock requirements for the timer/counter input of the PIC of your choice.You'll find that under the "electrical specifications" in the datasheet. Best regards VV "Mccauley, Daniel H" <.....daniel.h.mccauleyKILLspam@spam@LMCO.COM> wrote: {Quote hidden}>I currently have a circuit which uses a SN74LS628 Voltage Controlled >Oscillator (VCO) as a square wave generator. I'd like to buffer the >output of this and >feed it back into a PIC to and calculate the frequency. Frequency range >will be anywhere from about 50kHz to 500kHz. PIC clock frequency could >be >somewhere around 4MHz to 20MHz. >Does anyone have a simple way to implement this with a PIC? >Thanks --------------------------------- Do you Yahoo!? Yahoo! SiteBuilder - Free web site building tool. Try it! -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

> > I currently have a .... square wave generator. I'd like to buffer the > > output of this and feed it back into a PIC to and calculate the > > frequency. There was a clever frequency counter design around a few years ago which specs and program are probably still available for, but you could do it yourself. ?Original? was Popular Electronics, November 1994 around page 34. Ah - plot thickens. Terry J Wheeder 1993 ported to 16F84 by Peter Cousens 1998 (his last post here was on 14th January). Program is WCT5.ASM And Google says http://www.piclist.com/techref/piclist/weedfreq.htm On James's own PICLIST.COM site Always search the archives first :-) Original was from http://www.weedtech.com but there's no mention there now. _________________________________ There are 2 tricks. 1. The timer prescaler is faster than the PIC proper so 3 MHz is easy indeed. 2. Usually a prescaler outputs a half pulse for every N input pulses. At the end of a sequence you may be part way through the prescaler rolling over to the next output change and the part count adds inaccuracy to your result. So - after the timing period the input signal is gated off and the prescaler is fed with input signals BY THE PIC until it "rolls over" - you can thereby determine how many counts were in the prescaler. RM -- http://www.piclist.com hint: To leave the PICList .....piclist-unsubscribe-requestKILLspam.....mitvma.mit.edu

> There is a nifty little instrument Yes, it is. Looks pretty handy > http://members.cox.net/berniekm/super.html > The only issue is its only 4 digits, might not be enough resolution > for some apps The author says it will do 8 digits - "In the frequency counter mode, hitting button #1 switches the display to the next 4 digits of the count. For instance, the display shows '12.57' for a frequency of 12,576 hz. Holding down button #1 shows '2576' - the lowest 4 digits. If a decimal point shows, the value is in Khz, if the decimal is flashing, the value is in Mhz. Hence, a frequency of 42,345,678 hz is displayed as 42.34 with a flashing decimal. Holding down button #1 in this case will display 5678" -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Should be quite easy to change to a LCD display for nicer interface. David Jinx wrote: {Quote hidden}>>There is a nifty little instrument >> >> > >Yes, it is. Looks pretty handy > > > >>http://members.cox.net/berniekm/super.html >> >> > > > >>The only issue is its only 4 digits, might not be enough resolution >>for some apps >> >> > >The author says it will do 8 digits - > >"In the frequency counter mode, hitting button #1 switches the >display to the next 4 digits of the count. For instance, the display >shows '12.57' for a frequency of 12,576 hz. Holding down button >#1 shows '2576' - the lowest 4 digits. If a decimal point shows, >the value is in Khz, if the decimal is flashing, the value is in Mhz. >Hence, a frequency of 42,345,678 hz is displayed as 42.34 with >a flashing decimal. Holding down button #1 in this case will display >5678" > >-- >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

I was not aware PIC16F870 limits output/input to 25 mA. Any other device? Dmitriy. -----Original Message----- From: pic microcontroller discussion list [EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU]On Behalf Of David P Harris Sent: February 6, 2004 3:39 AM To: PICLISTspam_OUTMITVMA.MIT.EDU Subject: Re: [PIC]: Simple PIC Frequency Counter Should be quite easy to change to a LCD display for nicer interface. David Jinx wrote: {Quote hidden}>>There is a nifty little instrument >> >> > >Yes, it is. Looks pretty handy > > > >>http://members.cox.net/berniekm/super.html >> >> > > > >>The only issue is its only 4 digits, might not be enough resolution >>for some apps >> >> > >The author says it will do 8 digits - > >"In the frequency counter mode, hitting button #1 switches the >display to the next 4 digits of the count. For instance, the display >shows '12.57' for a frequency of 12,576 hz. Holding down button >#1 shows '2576' - the lowest 4 digits. If a decimal point shows, >the value is in Khz, if the decimal is flashing, the value is in Mhz. >Hence, a frequency of 42,345,678 hz is displayed as 42.34 with >a flashing decimal. Holding down button #1 in this case will display >5678" > >-- >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 -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Dmitriy Fitisov wrote : > I was not aware PIC16F870 limits output/input to 25 mA. > Any other device? > > Dmitriy. Isn't that the most common limit on the PICs ? Or did I misunderstand your question ? Jan-Erik. This communication is confidential and intended solely for the addressee(s). Any unauthorized review, use, disclosure or distribution is prohibited. If you believe this message has been sent to you in error, please notify the sender by replying to this transmission and delete the message without disclosing it. Thank you. E-mail including attachments is susceptible to data corruption, interruption, unauthorized amendment, tampering and viruses, and we only send and receive e-mails on the basis that we are not liable for any such corruption, interception, amendment, tampering or viruses or any consequences thereof. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Dmitriy Fitisov wrote : > I thought it does not have any special schematics inside to > limit the curent. > Max input/output current in my understanding is the current > which shouuld be allowed to flow by external schematic at max, > but is not limited by the chip itself to this limit. > > Dmitriy. OK, I see. That's how I understand things also. I just browsed the data sheet, and I can not find anything about some *active* current limiting from the PIC side. Jan-Erik. This communication is confidential and intended solely for the addressee(s). Any unauthorized review, use, disclosure or distribution is prohibited. If you believe this message has been sent to you in error, please notify the sender by replying to this transmission and delete the message without disclosing it. Thank you. E-mail including attachments is susceptible to data corruption, interruption, unauthorized amendment, tampering and viruses, and we only send and receive e-mails on the basis that we are not liable for any such corruption, interception, amendment, tampering or viruses or any consequences thereof. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> I thought it does not have any special schematics inside to limit the > curent. > Max input/output current in my understanding is the current which shouuld be > allowed to flow by external schematic at max, but is not limited by the chip > itself > to this limit. I ASSUME you are saying that you thought the PIC did not have any current limit per pin and that nothing internal to the PIC limits current. If so, then you need to read the PIC data sheets. ALL processors have a maximum allowable current limit per pin and ALL processors internally limit the current able to be drawn due to internal circuitry. Most PICs allow about 20 mA maximum per pin source or sink. Check the data sheet for your PIC of interest to be sure. Above this guaranteed current Microchip do not guarantee proper PIC operation AND you may damage the PIC. See the data sheet maximum allowable values for damage limits. As well as per-pin current limits there are also liable to be per port and per processor maximum allowable currents. RM -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> Dmitriy Fitisov wrote : > > > I thought it does not have any special schematics inside to > > limit the curent. > > Max input/output current in my understanding is the current > > which shouuld be allowed to flow by external schematic at max, > > but is not limited by the chip itself to this limit. > OK, I see. That's how I understand things also. > > I just browsed the data sheet, and I can not find anything > about some *active* current limiting from the PIC side. This is not completely correct. OUTPUTS: Data sheets often give values for the voltage at an output pin for several sourced currents. You will find that for eg a low output, at very low currents the voltage is approximately zero volts (as you'd expect) and as current increases the guaranteed low voltage increases. This is due to the drop across the effective resistance of the internal driver. While a PIC will drive more current than the maximum allowed into a low (or zero) impedance load a low output will then be well above zero and a high output well below Vdd. Even at short circuit the current will probably not be more than about double the maximum allowed current.ie the PIC does not limit the output current at exactly the guaranteed maximum rating but it DOES limit it to a value somewhat above that if you insist on violating its specifications. . Running the PIC in this mode may well be dangerous to its health. INPUTS PIC input currents, when the input voltages are within the allowable input range (typically Gnd-0.2v < Vin < Vdd+0.2v) will very largely be controlled by the PIC"input leakage current"s and is not a major function of external circuitry. RM -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> ALL processors internally limit the current able > to be drawn due to internal circuitry. I don't believe that is true. Could you convince me with a reference to a paricular datasheet page? And note that such an internal limit is of course only relevant (for survival) if it falls within the 'allowable maxima', or (for normal operation) if it falls within the other electrical specifications. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

{Quote hidden}> Dmitriy Fitisov wrote : > > > I thought it does not have any special schematics inside to > > limit the curent. > > Max input/output current in my understanding is the current > > which shouuld be allowed to flow by external schematic at max, > > but is not limited by the chip itself to this limit. > OK, I see. That's how I understand things also. > > I just browsed the data sheet, and I can not find anything > about some *active* current limiting from the PIC side. >This is not completely correct. >OUTPUTS: Data sheets often give values for the voltage at an output pin >for several sourced currents. You will find that for eg a low output, at >very low currents the voltage is approximately zero volts (as you'd expect) >and as current increases the guaranteed low voltage increases. This is due >to the drop across the effective resistance of the internal driver. While a >PIC will drive more current than the maximum allowed into a low (or zero) >impedance load a low output will then be well above zero and a high output >well below Vdd. Even at short circuit the current will probably not be more >than about double the maximum allowed current.ie the PIC does not limit the >output current at exactly the guaranteed maximum rating but it DOES limit it >to a value somewhat above that if you insist on violating its >specifications. . >Running the PIC in this mode may well be dangerous to its health. >INPUTS PIC input currents, when the input voltages are within the >allowable input range (typically Gnd-0.2v < Vin < Vdd+0.2v) will very >largely be controlled by the PIC"input leakage current"s and is not a major >function of external circuitry. 1. That's understandable that eventually there is a limit on current per pin, the point was it does not much max rating. If you see the link above in this thread - there is no limit resistors, "because current may be only 25 ma" in the schema. 1. Sorry my mistake - I did not mean INPUT/OUTPUT mode, I meant IN/OUT current in OUTPUT mode RM -- 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

I think we have a mixup here as i understand it. there is no "current limiting" circutry in a pic. there is a "limit" to the ammount of current that can be drawn safely, and that is 20ma or so. If there were current limiting circutry you could tie the outputs to ground and drive them high all day long. doing that is however a good way to splodificate ur pic. when the OP said limit he meant that the pic has no current limiting circutry within itself and it was the job of the outside stuff to make sure it dosent excede the spec. > {Original Message removed}

> > ALL processors internally limit the current able > > to be drawn due to internal circuitry. > I don't believe that is true. Could you convince me with a reference to > a paricular datasheet page? Yes :-) * (results given at end appear very surprising) > And note that such an internal limit is of course only relevant (for > survival) if it falls within the 'allowable maxima', or (for normal > operation) if it falls within the other electrical specifications. I largely agree and wasn't meaning to suggest otherwise. The comments were aimed not at experts, but at taking a step towards educating people who seemed to believe that the PIC was a semi infinite current source rather than a strictly limited one. * As I know you know, all microprocessor output drivers are non ideal switches and have an effective resistance. This necessarily limits maximum available current. In practice this is far from linear and they often pretty much refuse to supply more than a few times more than the maximum rated operating current. For chapter and verse on this (not for Wouter's edification but for other people's) (Although, the results I have just found are surprising and suggest that the PIC may in fact self limit its output current for Vout high with pin shorted to ground to below its guaranteed max safe (non operating) value ! 16F87X: BACKGROUND: Absolute maximum ratings 25 mA source sink abs max allowed 300 mA Vss 250 mA Vdd Max source or sink all 3 ports A B E 200 mA Max C D combined 200 mA PORT PINS Vout-low Vdd=4.5v, Iout = 8.5 mA = 0.6V SUGGESTS Reffective = 0.6/.0085 = 70 ohms. Or Isc = 5/70 =~~~ 70 mA with 5v connected to a low output pin. In practice it will be less. For a high output Vout = Vdd-0.7v at 3 mA at Vdd = 4.5v THIS SHOWS CURRENT LIMITING IN ACTION. The Reffective here is unlikely to DECREASE as current increases. Reffective = 0.7/.003 =~~ 230 ohms Giving Isc = ~~~ 21 mA !!!!!!!!!!!! This is truly a pathetic drive level, if true. The above is for 16F873/4/6/7 -04 & -20 commercial industrial over relevant temperature ranges. A quick glance doesn't reveal guaranteed max OPERATING Iout and the pathetic 3 mA / Vdd-0.7 V for Vout high is so far away from the 25 mA max damage limit that it may indeed be possible to short the high output pin to ground and STILL not exceed the damage spec. There is no apparent guaranteed operating output current spec apart from the wimpy one above. Someone by all means please read data sheet in more detail and prove me wrong. (PDF skimming is not the ideal way to gather good data but I don't want to print this datasheet at present. RM -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> I think we have a mixup here > as i understand it. > there is no "current limiting" circutry in a pic. > there is a "limit" to the ammount of current that can be drawn safely, and > that is 20ma or so. > > If there were current limiting circutry you could tie the outputs to ground > and drive them high all day long. > doing that is however a good way to splodificate ur pic. I think we're all on about the same wavelength. However, see my post just before this one. There is SOME current limiting - the question is, is it enough to keep you within abs max limits. I would have said no. However, it APPEARS that with a 16F87x you may be able to short a high output pin to ground and not exceed spec. It would be interesting to know how low (while not below ground) in fact you have to pull a 16F87X high output before you get 25 mA (if ever) RM -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> A quick glance doesn't reveal guaranteed max OPERATING Iout > and the pathetic > 3 mA / Vdd-0.7 V for Vout high is so far away from the 25 mA > max damage > limit that it may indeed be possible to short the high output > pin to ground > and STILL not exceed the damage spec. There is no apparent guaranteed > operating output current spec apart from the wimpy one above. But there is also no guaranteed upper limit on the current, so without external current limit you will be outside the specs. This of course does not guarantee that there will be any trouble, it is just that: outside the specs. Just like overclocking, current through the protection diodes, and numerous other abuses which you must avoid for a guaranteed working design, or might apply as a design optimisation at the expense of some (unspecified amount of) uncertainty. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

Russell McMahon wrote: > ALL processors have a maximum allowable current limit per pin and ALL > processors internally limit the current able to be drawn due to > internal circuitry. Right, but I think the point was that PICs don't contain deliberate current limit circuitry that keeps the pin current within the absolute maximum ratings of the data sheet. The current is only limited as an artifact of the circuitry, and is never supposed to get that high. In other words, it is up to the external circuitry to make sure the maximum pin current is not exceeded. If you tie a PIC pin to Vdd then drive it low, the current will be finite due to the limitations of the FET, but will most likely exceed the maximum rating and cause damage. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>Right, but I think the point was that PICs don't contain deliberate current >limit circuitry that keeps the pin current within the absolute maximum >ratings of the data sheet. The current is only limited as an artifact of >the circuitry, and is never supposed to get that high. In other words, it >is up to the external circuitry to make sure the maximum pin current is not >exceeded. A few months ago, I measured the E/I characteristics for a 16F877 pin in output mode. The manual does not say that current limiting is applied, but the E versus I plot looks like it has current limiting when sourcing current. Into a short circuit, I measured a maximum of 28mA with a distinct break point the E/I slope around 18mA. In the range 0...18mA, the pin looks like 80 ohms R in series with an ideal 5V supply. When you get to 28mAm, the equivalent series R looks like about 700 ohms. I certainly agree that it's not good practice to source current into a short circuit. >If you tie a PIC pin to Vdd then drive it low, the current will be finite >due to the limitations of the FET, but will most likely exceed the maximum >rating and cause damage. The E/I plot for a PIC pin driven low looks exactly like a 30 ohm resistor, up to 30mA. At that point, I finished taking data points. But the plot is exceedingly linear over the range 0...30mA. If you applied 5V to it, you would see 167mA, assuming the equivalent series R stays at 30 ohms, way over the maximum safe current. Jack Smith -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> I thought it does not have any special schematics inside to limit the > curent. > Max input/output current in my understanding is the current which > shouuld be > allowed to flow by external schematic at max, but is not limited > by the chip > itself > to this limit. > > Dmitriy. The max allowable current per PIC pin is 25mA, but you are correct, there is nothing in the PIC that limits it to that current it is very easy to sink/source more current,, but since you are then operating out of spec the results are not guaranteed. Where did you here that PICs limit the current to what is "safe"? TTYL ---------------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/ -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

>> I thought it does not have any special schematics inside to limit the >> curent. >> Max input/output current in my understanding is the current which >> shouuld be >> allowed to flow by external schematic at max, but is not limited by >> the chip >> itself >> to this limit. >> >> Dmitriy. > > The max allowable current per PIC pin is 25mA, but you are > correct, there > is nothing in the PIC that limits it to that current it is very easy to > sink/source more current,, but since you are then operating out of spec > the results are not guaranteed. > > Where did you here that PICs limit the current to what is > "safe"? TTYL "Usually, separate resistors need to be used in series with each segment drive in order to evenly drive the display. The PIC chip, however, limits the current flow to about 25 ma per line." http://members.cox.net/berniekm/super.html > ---------------------------------- > Herbert's PIC Stuff: > http://repatch.dyndns.org:8383/pic_stuff/ > > -- > 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

It doesn't 'limit' so much as it 'overheats' with an increase in resistance due to the overheating, which does indeed limit the current. This is not a 'feature' but a side effect of using CMOS. Not recommended for production work. In this case, the pulses to the LED are so short that the overcurrent probably doesn't overheat the PIC, and I'm sure that much more than 25mA is being delivered to the displays. Makes for a bright display. Will probably last forever. Still wouldn't trust it for more than a few one offs. -Adam Dmitriy Fitisov wrote: >I was not aware PIC16F870 limits output/input to 25 mA. >Any other device? > >Dmitriy. > >{Original Message removed}

> > The max allowable current per PIC pin is 25mA, but you are > > correct, there > > is nothing in the PIC that limits it to that current it is very easy to > > sink/source more current,, but since you are then operating out of spec > > the results are not guaranteed. > > > > Where did you here that PICs limit the current to what is > > "safe"? TTYL > > "Usually, separate resistors need to be used in series with each segment > drive in order to evenly drive the display. The PIC chip, however, > limits the current flow to about 25 ma per line." > > http://members.cox.net/berniekm/super.html Wow, well, that's another example of why you can't trust everything you see on the web. While, in that particular case, with that particular PIC, from that particular batch, at that particular temperature, it may appear to be limited to 25mA, this is NOT something you can trust. TTYL ---------------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/ -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Friday, Feb 6, 2004, at 05:42 US/Pacific, Jack Smith wrote: > A few months ago, I measured the E/I characteristics for a 16F877 pin > in > output mode. Real data. Cool. {Quote hidden}> The manual does not say that current limiting is applied, but the E > versus > I plot looks like it has current limiting when sourcing > current. Into a short circuit, I measured a maximum of 28mA with a > distinct break point the E/I slope around 18mA. In the range 0...18mA, > the pin looks like 80 ohms R in series with an ideal 5V supply. When > you > get to 28mAm, the equivalent series R looks like about 700 ohms. > > I certainly agree that it's not good practice to source current into a > short circuit. > The E/I plot for a PIC pin driven low looks exactly like a 30 ohm > resistor, > up to 30mA. At that point, I finished taking data points. But the plot > is > exceedingly linear over the range 0...30mA. If you applied 5V to it, > you > would see 167mA, assuming the equivalent series R stays at 30 ohms, > way over the maximum safe current. > MOS transistors are inherently current limitting, aren't they? (Although as everyone else has pointed out, not necessarily to current levels that aren't damaging overall.) In the case of this particular circuit, there is an LED between a PIC pin sourcing current, and a pin sinking current. Using your data, that's "about" 120ohms equivalent series resistance, and about 3V, for ... 25mA. Pretty neat. And it's multiplexed 32:1 so "average current" is much lower. I don't think PICs have a separate spec for pulsed current, but it makes me feel more comfortable... BillW -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

On Friday, Feb 6, 2004, at 11:09 US/Pacific, Herbert Graf wrote: >> [quoting from the "superprobe" project description] >> "Usually, separate resistors need to be used in series with each >> segment >> drive in order to evenly drive the display. The PIC chip, however, >> limits the current flow to about 25 ma per line." >> Ouch. OTOH, it's a often-recurring questions just how "ok" it is to drive an LED directly from a PIC pin (without a current limiting resistor.) It's not a good idea according to the data sheets, but it's pretty common, even in the ap-notes that microchip publishes. In this case, the PIC is both the source and sink for the current, the LED imposes a substantial voltage drop, and the pins are multiplexed, so it's probably safer all around than many circuits. For non-power circuits, the absolute max pin current ratings might be the point at which the pin voltage no longer meets THAT spec, rather than a power dissipation/damage sort of thing. I'm not sure whether the PIC qualifies as a "non-power" circuit here, though; relatively high current drivers ARE a touted and valuable feature of PICs. It IS a near project, though. Four resistors more and it would have been entirely unobjectionable. In this area, anyway... BillW -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

William Chops Westfield wrote: > In the case of this particular circuit, there is an LED between a > PIC pin sourcing current, and a pin sinking current. Using your > data, that's "about" 120ohms equivalent series resistance, and about > 3V, for ... 25mA. For that PIC at that temperature. This is not something specified by Microchip and could vary widely from PIC to PIC. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads