18F4550 USB RE3 sensitive

jaka · Joined: 04 May 2014 Posts: 35 Location: Finland

This is clever solution to the problem.

The root cause of problem is that the RE3 pin doesn't have the internal ESD protection diode clamp to VDD, like all other pins have. This is because the RE3 pin must be able to swing to Vpp to enter programming mode.

If you have a positive voltage transient on RE3 pin, the pin voltage will rise to Vpp, which causes the PIC to reset. The diode you added prevents the positive transients propagating to pin.

However, if you have high voltage transients like ESD, they can 'shoot through' the 1N4148. This could be avoided by adding external clamp diode from RE3 to VDD (anode to RE3, cathode to VDD). BUT adding this clamp diode prevents you from programming the PIC in high voltage programming mode!

I have found that the best solution is to avoid using the /MCLR pin for other purposes than reset or programming. Especially when it is routed far away from the PIC or when it is possible to touch it. It also helps to put 10 nF cap on the MCLR pin, which prevents PIC from reseting if you touch the programming connector. Even though Microchip suggests to not place any caps on the line.

vtrx · Joined: 11 Oct 2017 Posts: 141

Anyway, i discovered this 'too late', as i already have 20 pcb ready without components.
I will have to order again with the modification or risk leaving it as is.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Ttelmah · Joined: 11 Mar 2010 Posts: 19215

This is why if you use the pin for something like this it is worth having
jumper enabled protection.

temtronic · Posted: Sat Nov 28, 2020 11:46 am

I decided a few years ago to have the ICSP pins DEDICATED to ICSP having been stumped for days wondering 'it should work' only to discover the 'unique' trouble I was having...grrr...
if I need extra pins, a simple 'expander' is cheap and easy to add on...