Kevin,



-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
Of Kevin
Sent: Friday, May 04, 2007 11:17 AM
To: Microcontroller discussion list - Public.
Subject: [PIC] ICSP & Vpp-to-Vcc resistor?


Hi All,

Can someone explain why Vpp is connected to Vdd as shown in the ICSP 
diagram at http://kitsrus.com/gif/icsp.gif

>>
Sure
<<


The associated page at http://kitsrus.com/icsp.html mentions "The 27K is 
recommended to prevent VPP current from raising VCC" but I don't see how 
this works.
>>
it needs a diode to protect 5v and other purpose is on normal
operation will keep vpp 5v. Depending on how many components
you have sitting on 5 v. lets see
lets assume your 5v load is 100 ohm or 500 ohm or 1000 ohm
100 ohm load it will add 47 mv
500 ohm load it will add 230 mv
1000 ohm load it will add 464 mv

I assume your load is 500 ohm and adding 230 mv nothing
will go wrong. But it is better to add a series diode.
Here is how I calculated use simple resistor divider
top resistor is R1 and bottom is R2
Vout=R2*VDD/R1+R2 lets say 100 ohm load
100*13=1300/27100=0.047 mv

<< 



What I figure is that during ICSP when Vpp is applied (13V) shouldn't 
Vcc (5V) be (lightly) pulled up to 13V because of that 27K resistor? 
Wouldn't this then exceed the "Max voltage on Vdd with respect to Vss" 
spec and damage the PIC?

>>
pic can go up to about 7v see datasheet. so no thing will happen
<<  


I'm sure I'm missing something here (because I have used this ICSP 
circuit successfully), but I just can't see it ;-/
>>
expect to see wearied designs all the time.
<<

BTW a related question - is the MCLR pin more susceptible to ESD because 
it doesn't have an internal clamp diode to Vcc?
>>
yes but on final product they always add transorb etc.
<<



Thanks in advance,
Kevin


Andre

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