Post by thread:pic12c508/9

Hello ! Pics are a whole new world after designing analogue circuits for 20 years!...but I am learning fast! Can anyone help with this? I need to detect an "analogue" low signal (low battery warning!) on gpio,3 with weak pullup enabled and wake up enabled, I could use an external schmitt trigger but that would defeat the object of the pic! I have tried a pot as a potential divider but the pic doesnt seem to like the slow rate of change? Schmitt trigger in programming mode only? should I call a delay when checking gpio,3? Any help appreciated! Kev

Kev wrote: >I need to detect an "analogue" low signal (low battery warning!) on gpio,3 >with weak pullup enabled and wake up enabled, I could use an external schmitt >trigger but that would defeat the object of the pic! >I have tried a pot as a potential divider but the pic doesnt seem to like the >slow rate of change? Schmitt trigger in programming mode only? should I call >a delay when checking gpio,3? If you do it with a 12C671, you could use the AD section HŠlsningar/ Regards, Bjarne Nebelong Manager Electrical Design BINDOMATIC¨ spam_OUTbjarne.nebelongTakeThisOuTbindomatic.se +46-(0)8 709 58 46

We need more details. Are you trying to measure the voltage of the battery that you are powering the PIC with? Are you regulating the voltage to the PIC? If not this scheme won't work because the switching points of the Schmitt trigger are referenced to Vdd. As the battery voltage falls, so will the switching voltage of the Schmitt trigger. You need to compare the battery voltage to a stable reference, e.g Zener diode. Regards Mike Rigby-Jones > {Original Message removed}

<BLOCKQUOTE AUTHOR="Kev Howard"> I have tried a pot as a potential divider but the pic doesn't seem to like the slow rate of change? </BLOCKQUOTE> I'm curious as to what you mean here. Did you get false triggering? If you attach a pot directly from the batter Vpp to Gnd with the center tap leading to the PIC pin and adjust the POT until the PIC just reads a 0 with a battery that discharged to 5V plus the dropout voltage of the regulator (minimum operational voltage), it should read a 1 with a new battery (and the center tap hopefully will not produce more than 5V with the new battery!) and read 0 when the power is about to go. If you get false triggering, the problem is probably supply fluctuation or noise (especially if the pot is a very high ohmage) and you will need a capacitor from center tap to ground or will need to read the pin x times to verify that you never read a 1 before you can be confident that the power is really low. James Newton, webmaster http://get.to/techref (hint: you can add your own private info to the techref) .....jamesnewtonKILLspam@spam@geocities.com 1-619-652-0593 phone {Original Message removed}

Kev I take it you want to measure the 9V battery and give a warning before the battery gets to low to drive the PIC (below 5V)? Try what we in the R/C world call BEC (Battery Elimination Circuit). We use it to still power our reciever and servos while switching the electric motors off to save power. Have a look at this circuit: http://home.hiwaay.net/~mjn/sch.gif Look at R3, Q1, R1, R2 and U1 While the battery voltage is higher than 5V, Q1 will keep GPIO3 high, but if the battery drops to 5V and lower, Q1 will switch off and GPIO3 goes low. Hope this helps Quentin

> > >I need to detect an "analogue" low signal (low battery warning!) on > > gpio,3 The standard way is to potential divide the 9volt supply to a safe range(5v or less), this can be 1Meg to 9v and 470k to gnd. A cap is connected to ground(say 100nF) and the port pin, with maybe a series resistor to the PIC for protection (180ohm) . Make the port an output with a 0 on it, this will discharge the capacitor, make the port an input now and measure the time until the pin goes high. This time is a representation of the voltage. _____________________________ Lance Allen Technical Officer Uni of Auckland Psych Dept New Zealand _____________________________

[snip] > Have a look at this circuit: > http://home.hiwaay.net/~mjn/sch.gif > Look at R3, Q1, R1, R2 and U1 > While the battery voltage is higher than 5V, Q1 will keep GPIO3 high, > but if the battery drops to 5V and lower, Q1 will switch off and GPIO3 > goes low. > neat, if you're using a low drop out regulator. An alternative is to use a circuit similar to the brown-out detector shown in the datasheet (and similar to the above link): +Vbat | R1 +5V R1 | R1 | | e *-----b Q1 (PNP) | c | | R2 *----> to PIC (or transistor inverter) R2 R3 R2 R3 | R3 | | GND GND it works by keeping the Vbe junction reverse-biased until the battery voltage falls below a certain threshold (eg PIC sees a '0'). With R1 ~22k and R2 ~33k, Q1 starts conducting at ~7V (slightly above). The PIC then sees a '1'. reducing R1 will delay the turn-on point. Of course, using a zener as a reference would give you a more predictable switching point. or you could go way out and use a comparator :) but it doesn't sound like that's what the original poster was looking for.... only problem with Kev's particular application is that he's using GP3; I'm assuming it's configured as /MCLR? If so, i guess you could add a transistor-inverter. Or redo the ciruit to use a NPN (and active-high logic). I am still not sure what the problem was with the original voltage divider. Is it not switching at a repeatable point? Is it noise? I guess it also makes a difference if you're doing this as a hobby-type project, or for production.

Quentin, In the schematic I do not understand the purpose of Q1 I will appreciate if some one can explain it. Andre {Quote hidden}> Kev > I take it you want to measure the 9V battery and give a warning before > the battery gets to low to drive the PIC (below 5V)? > Try what we in the R/C world call BEC (Battery Elimination Circuit). We > use it to still power our reciever and servos while switching the > electric motors off to save power. > Have a look at this circuit: > http://home.hiwaay.net/~mjn/sch.gif > Look at R3, Q1, R1, R2 and U1 > While the battery voltage is higher than 5V, Q1 will keep GPIO3 high, > but if the battery drops to 5V and lower, Q1 will switch off and GPIO3 > goes low. > > Hope this helps > Quentin >

Kev Howard wrote: > >I have tried a pot as a potential divider but the pic doesnt seem to like the >slow rate of change? Schmitt trigger in programming mode only? should I call >a delay when checking gpio,3? > Kev: GP3 is a standard CMOS logic input pin when not in programming mode. The only schmitt trigger input is GP2. Jim Nagy Elm Electronics ICs for Experimenters http://www.elmelectronics.com/

Andre Abelian wrote: > > Quentin, > > In the schematic I do not understand the purpose of Q1 > I will appreciate if some one can explain it. > > Andre I should have included this in my last email as well, as it is not my design: http://home.hiwaay.net/~mjn/theory.htm The first paragraph will explain to you how it works. Quentin

Hi Kev, Speaking as a SX chips user, I'm not sure can pic12c508/9 does that: 1. set the pin as output with no pullup, and set it low 2. change the pin to input, and read it The time it takes to charge the line high again will indicate the level of your supply voltage. Check the spec. of your chip to see if diode is needed for protection. And you might need extra capacitance if the chip is not fast enough to detect the rise time of the inherent capacitance. Ling SM sm.lingKILLspamcmpmail.com http://www.i-pal.com (ready in a week time) >I need to detect an "analogue" low signal (low battery warning!) on gpio,3 with weak pullup enabled and wake up enabled, I could use an external schmitt trigger but that would defeat the object of the pic! I have tried a pot as a potential divider but the pic doesnt seem to like the slow rate of change? Schmitt trigger in programming mode only? should I call a delay when checking gpio,3?