ESD damage to MCLR pin of PIC18F26K20?

Hello,
We have some offline, linear regulator based LED drivers which are dimmable with their PIC18F26K20. (we are using ICSP with Pickit3 to program it)
Some of the PCBs work, most don’t, and some work the first time they are powered up but never thereafter. By “non-working”, I mean they light up but don’t respond to DALI dimming commands sent to the PIC18F26K20.
We have had 1000 of the PCBs made and assembled, and then realised that we forgot to add a 10k pullup resistor from the MCLR pin to Vdd. (MCLR reset is disabled, so its an input pin).
We then had an external modification applied to the PCBs which involves wiring a 10k resistor from MCLR pin to Vdd, and a 10n capacitor from MCLR pin to Vss. I undid this modification on one of the few working boards and then found that stopped it working. –But when I re-did this modification the PCB still did not work.
I am assuming that we have violated the dreaded MCLR pin here… The MCLR pin has no ESD protection diodes due to its use in ICSP, and so am I right in saying that the MCLR pin is supersensitive to ESD, and dies very easily?
(To make matters worse our supply capacitor next to the micro is just a 4n7, 0402 capacitor. However, there are two 10u, 0805 ceramic capacitors about 8mm away from the PIC.)
Do you think we are wasting our time trying to modify these boards? Is the MCLR pin so ultra-sensitive to ESD that everywhere from the PCB assembly house to our factory it is going to get its MCLR pin killed because we forgot to add the 10k resistor from MCLR to Vdd?

PIC18F26K20 datasheet...
**broken link removed**

MCLR isn't super sensitive to ESD. It still has a diode clamp to VSS to prevent reverse polarity and any voltage higher than VPP will activate the programming circuitry which can consume as much as 10mA and therefore presents a lowish impedance. True, it doesn't have the usual clamp diode to VDD but if one was present it would be impossible to program the IC.

In thousands of products using PICs, I have never seen one die through ESD damage. I have seen internal oscillators suddenly jump off frequency and I've seen UARTs produce garbage out but never anything I can attribute to electrical damage from the outside World .

The capacitors you are using should be more than adequate to decouple the supply provided they are wired directly across VSS and VDD without too much inductance in the tracks. I think you are looking in the wrong place for your fault but without seeing your design, software and PCB I can't say where it really lies.

If you want me to cast an eye over one, I'm willing to do so without charge as long as you pay postage - and don't hold me responsible if I can't fix it!. I'm retired and have no interest in getting back into manufacturing so have no fears I will pirate the board.

Brian.

You seem to be overly fixated on the problem being ESD. From your description above I really doubt it's due to ESD on that specific pin. That pin is used as the programming voltage and has NO requirement for a pullup resistor if you disable the MCLR function (which you stated was done in your post).

Diode, pull resistors, capacitor to ground, and a series resistor are recommended if the Vdd slope is too slow. There is also a need for a diode between the MCLR pin and the Vpp if using MCLR functionality. But as you aren't using MCLR (are you sure you've set this correctly) if the pin was programmed with MCLR functionality I could easily see you having problems with boards working then not working etc as some of them may never come out of reset.

What about the programming station? is the PIC board being powered from a common outlet with no difference in the ground potential between the two? I've seen stuff damaged when things were plugged into different outlets that were not at the same ground potential and there was 10V+ differential between the grounds (lab was badly wired).

This can also be a problem if you are using an isolated bench supply and don't short the common and the chassis ground of the bench supply. As a PC doesn't have a floating ground if you attach some programming pod from the PC to the target you may end up having to replace the target or the programmer. I saw one bench setup where the chassis ground wasn't connected to the common and there was more than a few volts difference between the two. Fortunately it didn't cause a problem before I noticed it and strapped the common to the chassis ground. Couldn't believe that the board designer didn't set the bench supplies correctly.

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Oops, it seems betwixt has also posted about it not being ESD too.

have never seen a Pic processor damaged by ESD. About the only way you can damage a PIC is to apply a to high input voltage or negative voltage. If the MCLR pin is programmed as a input then why should it make any difference to the operation of the circuit if the 10k and 10nf is present or not? (although it is a good idea to tie high unused inputs). Why use a 4n7 capacitor as a supply decoupling capacitor? it is doing nothing, change it to 0.1uF ceramic this may account for the unreliable operation of your circuit.

treez said:

Some of the PCBs work, most don’t, and some work the first time they are powered up but never thereafter. By “non-working”, I mean they light up but don’t respond to DALI dimming commands sent to the PIC18F26K20.

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Something is clearly fishy but I doubt it is ESD. I suggest the following steps:

1. see the osc is running
2. see all the I/O pins are connected (not hanging)
3. disable mclr in software

What about the programming station?

Click to expand...

Thank, we power the PIC18F26K20 from the Pickit3 during programming.

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What about the programming station?

Click to expand...

Thank, we power the PIC18F26K20 from the Pickit3 during programming.

If you want me to cast an eye over one, I'm willing to do so without charge as long as you pay postage - and don't hold me responsible if I can't fix it!. I'm retired and have no interest in getting back into manufacturing so have no fears I will pirate the board.

Click to expand...

Thankyou Brian this is a really kind offer. I will mention to the management if its OK. I am sure this board and its software is so far within your capability that we wouldnt worry as its nothing that you dont already know.

Thankyou Brian this is a really kind offer. I will mention to the management if its OK. I am sure this board and its software is so far within your capability that we wouldnt worry as its nothing that you dont already know.

Click to expand...

:-D

Actually, I was thinking more about your company design rights and confidentiality than my own education!
The first lighting controller using a PIC I worked on used a PIC1650 back in 1989/1990. That's many years before white LEDs were even available and it's purpose was to sequence the start up of halide lighting in large factories and warehouses. Just switching that kind of lighting on and off wrongly could cost a fortune in exploded lamps!

Brian.