Post by thread:[PIC] Diodes on MCLR

I have seen some schematics with a PIC included which have had two diodes on the MCLR pin connected like my mighty fine ;-) ASCII schema below. VCC | | | | 10K | | | ------------ | | Diod V V Diod | | | | _____________________ MCLR What function does the diodes have? /Ake -- --- Ake Hedman (YAP - Yet Another Programmer) eurosource, Brattbergavägen 17, 820 50 LOS, Sweden Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 Company home: http://www.eurosource.se Kryddor/Te/Kaffe: http://www.brattberg.com Personal homepage: http://www.eurosource.se/akhe Automated home: http://www.vscp.org

I'm not sure why there are two, but one diode can be useful when in circuit programming. It prevents the high voltage that you need to apply to MCLR from feeding back into the +5V line, possibly damaging the PIC, other components, and the programmer itself. Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams On 4/19/05, Ake Hedman <spam_OUTakheTakeThisOuTeurosource.se> wrote: {Quote hidden}> I have seen some schematics with a PIC included which have had two > diodes on the MCLR pin connected > like my mighty fine ;-) ASCII schema below. > > VCC > | > | > | | 10K > | | > | > ------------ > | | > Diod V V Diod > | | > | | > _____________________ MCLR > > What function does the diodes have?

Looks like it's there for isolating the Vcc line from the MCLR line during ICSP. Not sure why two though, but might guess that it has something to do with the reverse current limit? There is another arrangement when the diode goes the other way (pointing towards Vcc), and there is a capacitor between the MCLR line and ground as well -- it's an RC startup delay, with the diode helping the capacitor discharge quickly after power off. Cheers, -Neil. On Tuesday 19 April 2005 03:09 pm, Ake Hedman scribbled: {Quote hidden}> I have seen some schematics with a PIC included which have had two > diodes on the MCLR pin connected > like my mighty fine ;-) ASCII schema below. > > VCC > > | | 10K > > ------------ > > Diod V V Diod > > > _____________________ MCLR > > What function does the diodes have? > > /Ake

In this schematic, not only they have nonsens, but also they can do an inexpected behaviour of the PIC, because the current flowing into the MCLR could be too small if VCC is 3.3V and/or R is greater than 10K. There is no need of any diode connected like this when programming a PIC, using HVP and large enough pull up resitor to Vcc from MCLR. Vasile http://surducan.netfirms.com On 4/19/05, Ake Hedman <.....akheKILLspam@spam@eurosource.se> wrote: {Quote hidden}> I have seen some schematics with a PIC included which have had two > diodes on the MCLR pin connected > like my mighty fine ;-) ASCII schema below. > > VCC > | > | > | | 10K > | | > | > ------------ > | | > Diod V V Diod > | | > | | > _____________________ MCLR > > What function does the diodes have? > > /Ake > > -- > --- > Ake Hedman (YAP - Yet Another Programmer) > eurosource, Brattbergavägen 17, 820 50 LOS, Sweden > Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 > Company home: http://www.eurosource.se > Kryddor/Te/Kaffe: http://www.brattberg.com > Personal homepage: http://www.eurosource.se/akhe > Automated home: http://www.vscp.org > >

This is also the recommendation form Microchip. No diodes, no capacitors just a 10K resistor to VDD. I have also seen a transistor used instead of the diodes. And NPN with the base connected directly connected to the emitter which goes to MCLR and the collector to a resistor which is connected to VDD. But I guess this is a diode. So two diodes is just double nonsense then...? ;-) /Ake vasile surducan wrote: {Quote hidden}>In this schematic, not only they have nonsens, but also they can do an >inexpected behaviour of the PIC, because the current flowing into the >MCLR could be too small if VCC is 3.3V and/or R is greater than 10K. >There is no need of any diode connected like this when programming a >PIC, using HVP and large enough pull up resitor to Vcc from MCLR. > >Vasile >http://surducan.netfirms.com > > >On 4/19/05, Ake Hedman <akheKILLspameurosource.se> wrote: > > >>I have seen some schematics with a PIC included which have had two >>diodes on the MCLR pin connected >>like my mighty fine ;-) ASCII schema below. >> >> VCC >> | >> | >> | | 10K >> | | >> | >> ------------ >> | | >> Diod V V Diod >> | | >> | | >> _____________________ MCLR >> >>What function does the diodes have? >> >>/Ake >> >>-- >>--- >>Ake Hedman (YAP - Yet Another Programmer) >>eurosource, Brattbergavägen 17, 820 50 LOS, Sweden >>Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 >>Company home: http://www.eurosource.se >>Kryddor/Te/Kaffe: http://www.brattberg.com >>Personal homepage: http://www.eurosource.se/akhe >>Automated home: http://www.vscp.org >> >>

So to round up. The designs I have seen is done by someone that just have a lot of diodes he/she want to use somewhere. Thanks guys! /Ake Bob Ammerman wrote: {Quote hidden}>> This is also the recommendation form Microchip. No diodes, no >> capacitors just a 10K resistor to VDD. > > > The 10K to Vdd should work just fine, unless the Vdd power supply > isn't 'stiff' enough to avoid rising in voltage with the injected > (Vpp-Vdd)/10k current. This is highly unlikely to be an issue except > in the very lowest power designs. > > Bob Ammerman > RAm Systems > -- --- Ake Hedman (YAP - Yet Another Programmer) eurosource, Brattbergavägen 17, 820 50 LOS, Sweden Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 Company home: http://www.eurosource.se Kryddor/Te/Kaffe: http://www.brattberg.com Personal homepage: http://www.eurosource.se/akhe Automated home: http://www.vscp.org

On 4/19/05, Ake Hedman <.....akheKILLspam.....eurosource.se> wrote: > I have seen some schematics with a PIC included which have had two > diodes on the MCLR pin connected > like my mighty fine ;-) ASCII schema below. > > VCC > | > | > | | 10K > | | > | > ------------ > | | > Diod V V Diod > | | > | | > _____________________ MCLR > > What function does the diodes have? While others have commented on the function of having a diode in that position, there may be two diodes on the schematic because they don't know which diode they are going to use. They will only populate a single diode position when the device is built. Seems unusual with a diode, but perhaps the board design is made for two different assembly houses with two cheapest diodes whose land patterns don't overlap very well. If you have the board layout to look at, you might even find that one is a diode and the other is actually a transistor used in a diode configuration. Bradley

And the winner is.... Bradley Ferguson!! You are absolutely right. There is actually a transistor mounted on the board. Mystery solved. Thanks all for enlighten me! Cheers /Ake Bradley Ferguson wrote: {Quote hidden}>On 4/19/05, Ake Hedman <EraseMEakhespam_OUTTakeThisOuTeurosource.se> wrote: > > >>I have seen some schematics with a PIC included which have had two >>diodes on the MCLR pin connected >>like my mighty fine ;-) ASCII schema below. >> >> VCC >> | >> | >> | | 10K >> | | >> | >> ------------ >> | | >> Diod V V Diod >> | | >> | | >> _____________________ MCLR >> >>What function does the diodes have? >> >> > >While others have commented on the function of having a diode in that >position, there may be two diodes on the schematic because they don't >know which diode they are going to use. They will only populate a >single diode position when the device is built. Seems unusual with a >diode, but perhaps the board design is made for two different assembly >houses with two cheapest diodes whose land patterns don't overlap very >well. If you have the board layout to look at, you might even find >that one is a diode and the other is actually a transistor used in a >diode configuration. > >Bradley > > > -- --- Ake Hedman (YAP - Yet Another Programmer) eurosource, Brattbergavägen 17, 820 50 LOS, Sweden Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 Company home: http://www.eurosource.se Kryddor/Te/Kaffe: http://www.brattberg.com Personal homepage: http://www.eurosource.se/akhe Automated home: http://www.vscp.org

At 21.08 2005.04.20 +0200, you wrote: >And the winner is.... Bradley Ferguson!! > >You are absolutely right. There is actually a transistor mounted on the board. Mystery solved. BTW: exactly in what regard does a transistor connected that way behave better than a diode? {Quote hidden}>Thanks all for enlighten me! > >Cheers >/Ake > >Bradley Ferguson wrote: > >>On 4/19/05, Ake Hedman <akhespam_OUTeurosource.se> wrote: >> >> >>>I have seen some schematics with a PIC included which have had two >>>diodes on the MCLR pin connected >>>like my mighty fine ;-) ASCII schema below. >>> >>> VCC >>> | >>> | >>> | | 10K >>> | | >>> | >>> ------------ >>> | | >>> Diod V V Diod >>> | | >>> | | >>> _____________________ MCLR >>> >>>What function does the diodes have? >>> >> >>While others have commented on the function of having a diode in that >>position, there may be two diodes on the schematic because they don't >>know which diode they are going to use. They will only populate a >>single diode position when the device is built. Seems unusual with a >>diode, but perhaps the board design is made for two different assembly >>houses with two cheapest diodes whose land patterns don't overlap very >>well. If you have the board layout to look at, you might even find >>that one is a diode and the other is actually a transistor used in a >>diode configuration. >> >>Bradley >> >> > > >-- >--- >Ake Hedman (YAP - Yet Another Programmer) >eurosource, Brattbergavägen 17, 820 50 LOS, Sweden >Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 >Company home: http://www.eurosource.se >Kryddor/Te/Kaffe: http://www.brattberg.com >Personal homepage: http://www.eurosource.se/akhe >Automated home: http://www.vscp.org > > >

ThePicMan wrote: >At 21.08 2005.04.20 +0200, you wrote: > > >>And the winner is.... Bradley Ferguson!! >> >>You are absolutely right. There is actually a transistor mounted on the board. Mystery solved. >> >> > >BTW: exactly in what regard does a transistor connected that way behave better than a diode? > > > > Isn't it just that they got the transistor at a lower cost then the diode? /Ake {Quote hidden}> > > >>Thanks all for enlighten me! >> >>Cheers >>/Ake >> >>Bradley Ferguson wrote: >> >> >> >>>On 4/19/05, Ake Hedman <@spam@akheKILLspameurosource.se> wrote: >>> >>> >>> >>> >>>>I have seen some schematics with a PIC included which have had two >>>>diodes on the MCLR pin connected >>>>like my mighty fine ;-) ASCII schema below. >>>> >>>> VCC >>>> | >>>> | >>>> | | 10K >>>> | | >>>> | >>>> ------------ >>>> | | >>>>Diod V V Diod >>>> | | >>>> | | >>>> _____________________ MCLR >>>> >>>>What function does the diodes have? >>>> >>>> >>>> >>>While others have commented on the function of having a diode in that >>>position, there may be two diodes on the schematic because they don't >>>know which diode they are going to use. They will only populate a >>>single diode position when the device is built. Seems unusual with a >>>diode, but perhaps the board design is made for two different assembly >>>houses with two cheapest diodes whose land patterns don't overlap very >>>well. If you have the board layout to look at, you might even find >>>that one is a diode and the other is actually a transistor used in a >>>diode configuration. >>> >>>Bradley >>> >>> >>> >>> >>-- >>--- >>Ake Hedman (YAP - Yet Another Programmer) >>eurosource, Brattbergavägen 17, 820 50 LOS, Sweden >>Phone: (46) 657 413430 Cellular: (46) 73 84 84 102 >>Company home: http://www.eurosource.se >>Kryddor/Te/Kaffe: http://www.brattberg.com >>Personal homepage: http://www.eurosource.se/akhe >>Automated home: http://www.vscp.org >> >> >>-

On 4/21/05, ThePicMan <KILLspamthepicmanKILLspaminfinito.it> wrote: > At 21.08 2005.04.20 +0200, you wrote: > >And the winner is.... Bradley Ferguson!! > > > >You are absolutely right. There is actually a transistor mounted on the board. Mystery solved. > > BTW: exactly in what regard does a transistor connected that way behave better than a diode? I believe other than cheaper cost, fewer parts/board, higher part-volume, if you use the C-B junction as the diode you have the entire reverse breakdown voltage of the transistor available to you. This breakdown voltage might be higher than the standard "jelly bean" diode used by the assembler. Bradley

On Thu, 21 Apr 2005, ThePicMan wrote: > At 21.08 2005.04.20 +0200, you wrote: >> And the winner is.... Bradley Ferguson!! >> >> You are absolutely right. There is actually a transistor mounted on >> the board. Mystery solved. > > BTW: exactly in what regard does a transistor connected that way > behave better than a diode? In the way that it can be pushed into (undesirable) reverse breakdown of the BE junction under certain conditions. This occurs at -4..-9V Vbe. Peter

On Thu, 21 Apr 2005, Bradley Ferguson wrote: > I believe other than cheaper cost, fewer parts/board, higher > part-volume, if you use the C-B junction as the diode you have the > entire reverse breakdown voltage of the transistor available to you. > This breakdown voltage might be higher than the standard "jelly bean" > diode used by the assembler. I do not agree. A jellybean diode will have Vr = 80V (1N4148). Transistors with such specs are not 'jellybean'. The only time it makes sense to use a transistor as a diode is when you already use transistors in the circuit, and no diodes. Then the stocking problems are reduced and the equivalent price of the transistor is much lower since it is bought in bulk. Peter

At 20.05 2005.04.21 +0300, you wrote: >On Thu, 21 Apr 2005, ThePicMan wrote: > >>At 21.08 2005.04.20 +0200, you wrote: >>>And the winner is.... Bradley Ferguson!! >>> >>>You are absolutely right. There is actually a transistor mounted on the board. Mystery solved. >> >>BTW: exactly in what regard does a transistor connected that way behave better than a diode? > >In the way that it can be pushed into (undesirable) reverse breakdown of the BE junction under certain conditions. This occurs at -4..-9V Vbe. Why not use a zener instead?