• AVR Freaks

AnsweredHot!Any need for external pullup on MCLR anymore?

Author
upand_at_them
Super Member
  • Total Posts : 649
  • Reward points : 0
  • Joined: 2005/05/16 07:02:38
  • Location: Pennsylvania
  • Status: online
2020/08/08 08:31:16 (permalink)
0

Any need for external pullup on MCLR anymore?

Is there a need anymore for an external pullup resistor on MCLR for modern PIC's such as 16F1825/9?  They have an automatic weak pullup when MCLR is enabled in config.  And even if I want to disable the MCLR reset function in config I could just enable the weak pullup as part of my init code, right?
 
#1
NorthGuy
Super Member
  • Total Posts : 6291
  • Reward points : 0
  • Joined: 2014/02/23 14:23:23
  • Location: Northern Canada
  • Status: online
Re: Any need for external pullup on MCLR anymore? 2020/08/08 10:40:01 (permalink) ☼ Best Answerby upand_at_them 2020/08/08 10:57:03
0
upand_at_them
Is there a need anymore for an external pullup resistor on MCLR for modern PIC's such as 16F1825/9?  They have an automatic weak pullup when MCLR is enabled in config.  And even if I want to disable the MCLR reset function in config I could just enable the weak pullup as part of my init code, right?



No need for external pullup unless you want to make the pullup stronger to prevent spurious resets.
 
If you disable MCLRE and LVP then you can use the MCLR pin as input. It won't reset anyway. In theory, if it's floating then there's still a danger that the noise may pull MCLR above VDD and move the PIC into the programming mode. You may consider an external pull-up to prevent this. However, if you use MCLR pin as an input then something is probably already driving it.
#2
isoar
Starting Member
  • Total Posts : 10
  • Reward points : 0
  • Joined: 2015/11/23 23:43:41
  • Location: Ottawa, Canada
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/08 16:00:28 (permalink) ☄ Helpfulby upand_at_them 2020/08/08 17:23:17
+2 (2)
Do be aware that PIC16F1825/9 are susceptible to Vpp overshoot.
 
The datasheet recommends a TL431 shunt-regulator limiting the pin to 8.5 Volts, when using Vpp programming (always required when nMCLR function is disabled).
 
The ICD 2 is specifically mentioned as an example of a bad programmer, but the PICkit4 is also known to overshoot Vpp, but that can easily be fixed with a small 100/220/470 Ohm resistor in series with the ICSP connector and nMCLR to slow the rise.  Or the Pickit4 can be permanently modified (see ETN-37 MPLAB® PICkit 4 VPP Overshoot Modification).
#3
upand_at_them
Super Member
  • Total Posts : 649
  • Reward points : 0
  • Joined: 2005/05/16 07:02:38
  • Location: Pennsylvania
  • Status: online
Re: Any need for external pullup on MCLR anymore? 2020/08/08 16:15:07 (permalink)
0
isoarDo be aware that PIC16F1825/9 are susceptible to Vpp overshoot.
 
The datasheet recommends a TL431 shunt-regulator limiting the pin to 8.5 Volts, when using Vpp programming (always required when nMCLR function is disabled).
 
The ICD 2 is specifically mentioned as an example of a bad programmer, but the PICkit4 is also known to overshoot Vpp, but that can easily be fixed with a small 100/220/470 Ohm resistor in series with the ICSP connector and nMCLR to slow the rise.  Or the Pickit4 can be permanently modified (see ETN-37 MPLAB® PICkit 4 VPP Overshoot Modification).

 
Thank you!  I forgot that I've been putting a 10k resistor and diode on MCLR for Vpp programming.
#4
Hen
Senior Member
  • Total Posts : 92
  • Reward points : 0
  • Joined: 2018/10/24 04:01:44
  • Location: 0
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/08 21:10:59 (permalink)
0
isoar
The ICD 2 is specifically mentioned as an example of a bad programmer, but the PICkit4 is also known to overshoot Vpp, but that can easily be fixed with a small 100/220/470 Ohm resistor in series with the ICSP connector and nMCLR to slow the rise.  Or the Pickit4 can be permanently modified (see ETN-37 MPLAB® PICkit 4 VPP Overshoot Modification).

Thank you for the heads-up.
I did not know about this issue until now.
#5
Hen
Senior Member
  • Total Posts : 92
  • Reward points : 0
  • Joined: 2018/10/24 04:01:44
  • Location: 0
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/08 22:16:55 (permalink)
0
I inserted the 470R and it still works, hopefully better.
 
But why on earth would you lift (unsolder) Q3'1 and draw a patch towards Q4'3 from there?
Would that not undo the *pull-down* done by the R54, maybe I miss something...
 
BTW, I lifted Q4'3 instead and adding the new R142 back-to-back on R54 then drew a patch.
#6
isoar
Starting Member
  • Total Posts : 10
  • Reward points : 0
  • Joined: 2015/11/23 23:43:41
  • Location: Ottawa, Canada
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/09 00:09:41 (permalink)
+1 (1)
Just to be clear, there are a few issues here:
  1.  Protecting the Rxx/nMCLR/Vpp pin from excessive applied programming voltage (my point above).    
  2.  Protecting the circuitry driving the Rxx/nMCLR/Vpp pin from Vpp programming voltage.    
  3.  Doing #1 and #2 without interfering with in-circuit programming.    
Using an external pull-up resistor to Vdd, with a Schottky diode to prevent Vpp feeding into Vdd, doesn't address any of the above.
 
It also isn't really necessary:  at max 9 Volts applied to Vpp and Vdd at 5 Volts, there is 4 Volts across the external pull-up resistor.  At 10 kOhms, that means 400 uA flows from Vpp into Vdd.  The PIC's own operating current consumption along with anything else connected to Vdd likely draws more than 400 uA, and is effectively a pull-down that can counter and keep Vdd from rising above 5 Volts.  At 47 kOhms for the pull-up, it is reduced to a paltry 85 uA into Vdd.
#7
isoar
Starting Member
  • Total Posts : 10
  • Reward points : 0
  • Joined: 2015/11/23 23:43:41
  • Location: Ottawa, Canada
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/09 00:43:05 (permalink)
0
Hen
But why on earth would you lift (unsolder) Q3'1 and draw a patch towards Q4'3 from there?
Would that not undo the *pull-down* done by the R54, maybe I miss something...

I think the intention was to reduce the slew rate, avoiding overshoot, by using the gate capacitance of Q3 and the resistor as an RC network.
#8
Hen
Senior Member
  • Total Posts : 92
  • Reward points : 0
  • Joined: 2018/10/24 04:01:44
  • Location: 0
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/15 21:44:19 (permalink)
0
isoar
 
I think the intention was to reduce the slew rate, avoiding overshoot, by using the gate capacitance of Q3 and the resistor as an RC network.

I got that allright but not the removal of R54.
 
BTW, when opening the enclosure I realized you can press the lightspreader but have not invested time to understand what that button achieve. Anybody less lazy out there?
#9
Jerry Messina
Super Member
  • Total Posts : 550
  • Reward points : 0
  • Joined: 2003/11/07 12:35:12
  • Status: offline
Re: Any need for external pullup on MCLR anymore? 2020/08/16 04:48:04 (permalink)
0
That's the "bootloader mode" button (same as the black button on the PK3)
 
#10
Jump to:
© 2020 APG vNext Commercial Version 4.5