ESD problems with PIC 16F88

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.

Thanks in advance for any advice,
Bo.

On Tue, 20 Nov 2007 11:39:40 -0800 (PST), I said, "Pick a card, any
card" and lloydcam@gmail.com instead replied:

>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?

http://www.l-com.com/item.aspx?id=8721
--
Ray

Ray Haddad wrote:
> On Tue, 20 Nov 2007 11:39:40 -0800 (PST), I said, "Pick a card, any
> card" and lloydcam@gmail.com instead replied:
>
>> 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?
>
> http://www.l-com.com/item.aspx?id=8721

Now that's thinking outside the box.  ;-) 

On Tue, 20 Nov 2007 15:57:43 -0600, I said, "Pick a card, any card"
and "Anthony Fremont" <nobody@noplace.net> instead replied:

>Ray Haddad wrote:
>> On Tue, 20 Nov 2007 11:39:40 -0800 (PST), I said, "Pick a card, any
>> card" and lloydcam@gmail.com instead replied:
>>
>>> 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?
>>
>> http://www.l-com.com/item.aspx?id=8721
>
>Now that's thinking outside the box.  ;-) 

Indeed.

I believe he hasn't got an ESD problem; that would tend to be
damaging to the point of destruction of the circuit. What he most
likely has occurring is that his fingers, or his body, is picking up
stray radiation from power lines, the internal oscillator, his PC or
perhaps a nearby radio. Any of those could induce a signal into the
circuit. Thus, I suggest covering the connector so when it's not in
use, it's safe. Either that or a serial loopback connector.
--
Ray

Assuming the socket is connected to ground; shunting the rs232 input pin
to ground with 100pf, to start, might have an effect. If not, add more & 
see what happens. Maybe the same approach but close to the connecter 
rather than the ic.

Hul

lloydcam@gmail.com 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.

> Thanks in advance for any advice,
> Bo.

Ray Haddad wrote:
> On Tue, 20 Nov 2007 15:57:43 -0600, I said, "Pick a card, any card"
> and "Anthony Fremont" <nobody@noplace.net> instead replied:
>
>> Ray Haddad wrote:
>>> On Tue, 20 Nov 2007 11:39:40 -0800 (PST), I said, "Pick a card, any
>>> card" and lloydcam@gmail.com instead replied:
>>>
>>>> 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?
>>>
>>> http://www.l-com.com/item.aspx?id=8721
>>
>> Now that's thinking outside the box.  ;-)
>
> Indeed.
>
> I believe he hasn't got an ESD problem; that would tend to be
> damaging to the point of destruction of the circuit. What he most
> likely has occurring is that his fingers, or his body, is picking up
> stray radiation from power lines, the internal oscillator, his PC or
> perhaps a nearby radio. Any of those could induce a signal into the

I believe that's entirely possible.  The CMOS inputs are of such high 
impedance that touching a pin will easily trigger it.

> circuit. Thus, I suggest covering the connector so when it's not in
> use, it's safe. Either that or a serial loopback connector.

Sounds good to me, I think the OP should also consider modifying the 
software such that it can reject noise generated signals.  The unused output 
pins that have pull-up resistors should be changed to either let the outputs 
float or make them inputs, asis they are just wasting power.  I've noticed 
that the lower the power requirements of the PIC line, the more fussy they 
are about the power supply, but I don't think that has anything to do with 
the problem now.  I wonder if the OP is using a MAX232 type level convertor 
or has brought the PIC pins straight out to the external connector. 

Many thanks for the replies,

re. Ray Haddad's suggestion of covering the sockets:
Good idea BUT the problem has happened a few times when I was going to
plug a wire into the socket... So it has to be uncovered at that
moment anyway.
In fact, I should have mentioned this in the original post - sometimes
the strange behavior occurs just as a plug first touches the socket -
i.e. before the pins inside the plug and socket have properly mated.
So in this case, it is not my fingers but a wire connected to another
PCB which make it happen. But several other times, it happens just
when touching with fingers even if no other circuits or wires are
nearby. It has happened at times when there is no other electronic
apparatus nearby at all. So my guess is that the source of the
discharge (if that's what it is) is body static or as you say possibly
something else within the circuit... the circuit is fairly simple - a
bunch of LEDs connected to mosfets, an Xtal connected to the PIC and a
little battery charging chip.
The socket is not actually DB9, but a mini-USB B. However USB protocol
is not used, RS232 is used as mentioned previously. (That socket was
chosen just for its size and convenience getting leads etc).

to Anthony Fremont:
The unused Output pins are pulled down to GND (not up) and they are
set as output zero so I don't think I'm wasting any power there. The
only pull-up is on the Rx pin which is an input.
I'm not sure how to modify the software to reject noise-generated
signals as I'm using the PIC's built-in USART.
I'm not using a MAX232 within the circuit. When the device is plugged
in, the device that communicates with it has a FTDI232 chip that
converts to TTL levels. But when the device is not plugged in, there
is nothing in the circuit to intentionally affect the RS232 RX input.

to Hul:
I will try a 100pF cap between Rx and GND either close to the socket
or close to the PIC and let you know the result.

Thanks again to all of you.

One other thought - rs232 data lines are normally negative, in a formal 
case, 10-12 v below ground or greater that 3 v negative. If the ic expects 
that, ie not designed for an inverter "pre-amp", then you may well be 
looking at behaviour caused by logic, not transients. If so, pull down, 
not up. 

Hul


lloydcam@gmail.com wrote:
> Many thanks for the replies,

> re. Ray Haddad's suggestion of covering the sockets:
> Good idea BUT the problem has happened a few times when I was going to
> plug a wire into the socket... So it has to be uncovered at that
> moment anyway.
> In fact, I should have mentioned this in the original post - sometimes
> the strange behavior occurs just as a plug first touches the socket -
> i.e. before the pins inside the plug and socket have properly mated.
> So in this case, it is not my fingers but a wire connected to another
> PCB which make it happen. But several other times, it happens just
> when touching with fingers even if no other circuits or wires are
> nearby. It has happened at times when there is no other electronic
> apparatus nearby at all. So my guess is that the source of the
> discharge (if that's what it is) is body static or as you say possibly
> something else within the circuit... the circuit is fairly simple - a
> bunch of LEDs connected to mosfets, an Xtal connected to the PIC and a
> little battery charging chip.
> The socket is not actually DB9, but a mini-USB B. However USB protocol
> is not used, RS232 is used as mentioned previously. (That socket was
> chosen just for its size and convenience getting leads etc).

> to Anthony Fremont:
> The unused Output pins are pulled down to GND (not up) and they are
> set as output zero so I don't think I'm wasting any power there. The
> only pull-up is on the Rx pin which is an input.
> I'm not sure how to modify the software to reject noise-generated
> signals as I'm using the PIC's built-in USART.
> I'm not using a MAX232 within the circuit. When the device is plugged
> in, the device that communicates with it has a FTDI232 chip that
> converts to TTL levels. But when the device is not plugged in, there
> is nothing in the circuit to intentionally affect the RS232 RX input.

> to Hul:
> I will try a 100pF cap between Rx and GND either close to the socket
> or close to the PIC and let you know the result.

> Thanks again to all of you.

dbr@kbrx.com wrote:
> 
> One other thought - rs232 data lines are normally negative, in a
> formal case, 10-12 v below ground or greater that 3 v negative.
> If the ic expects that, ie not designed for an inverter "pre-amp",
> then you may well be looking at behaviour caused by logic, not
> transients. If so, pull down, not up.

There is something wrong here.  The hardware RS232 spec. is for
logic levels below zero for one sense, and above zero for the
other.  IIRC the minimum level is 3V in either direction.  Many PCs
effectively use TTL levels, though, and most receivers have the
ability to resolve this.

Please do not top-post.  Your answer belongs after (or intermixed
with) the quoted material to which you reply, after snipping all
irrelevant material.  See the following links:

-- 
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-- 
Posted via a free Usenet account from http://www.teranews.com

On Nov 21, 12:39 am, lloyd...@gmail.com 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