Re: ESD problems with PIC 16F88
- From: karthikbalaguru <karthikbalaguru79@xxxxxxxxx>
- Date: Thu, 22 Nov 2007 00:48:54 -0800 (PST)
On Nov 21, 12:39 am, lloyd...@xxxxxxxxx wrote:
I have built a number of battery operated circuits using PIC16F88s.
They are housed in a plastic case with a socket to plug in a serial
connection for controlling LED outputs by RS232 comms.
Sometimes when you touch the socket with your finger, the PIC responds
in a manner suggesting that it has received some random data on the Rx
pin of the USART. The problem occurs much more frequently in cold dry
weather which I know is worse for static generally.
Presumably this suggests that electrostatic discharge is being
interpreted by the PIC as a signal.
Any ideas on how to prevent this?
(Ideally with modifications that I can do to existing circuits as I
have made quite a few already.)
Here is what I have already tried, in an attempt to fix it:
- Now there is a 1K resistor in series between the USART Rx pin and
the socket pin corresponding to serial data.
- I have also tried a 100K pull-up from the Rx pin on the PIC to Batt
+ve.
- I have attached 100k pull-down resistors to the other unused pins of
the PIC that were previously not connected. (They are all set as
outputs anyway)
- The PIC's Vdd has a 100nF capacitor connected directly between it
and Vss (within 20mm of pins)
- There is also a 10uF capacitor between Vdd and Vss (but located a
little further away on the board - about 150mm from PIC)
The PICs I am using are SOIC package. I made a very similar batch of
circuits a couple of years ago using DIP package (also PIC16F88)
running the same program and didn't have any such problems.
Normally ICs worried about ESD come up with some buffer technique
internally or you can
also implement externally (just like a fuse) such that the buffer will
get
affected or will cut the path to the core part.
I am not sure if PIC has such a technique internally . Maybe , you can
try one
of those methods in your external circuit to avoid problem to your
circuit.
One more trick can be to prevent the user from placing the finger over
the circuit
using some covers/shields.
If you think, that you have to uncover for connecting something
frequently, then, I think, you
can device an external circuitry for auto-ON/OFF based on the ESD
impact level.
Karthik Balaguru
Karthik Balaguru
.
- References:
- ESD problems with PIC 16F88
- From: lloydcam
- ESD problems with PIC 16F88
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