On Fri, May 17, 2002 at 07:50:07AM -0700, Tal Dayan wrote:
> Can any programmer that can program 16f84 be used also to program 16f84
> in circuit ?

Doubtful. The clock causes some real issues. I'm experiencing this myself
with my Trivial LVP programmer:

http://www.finitesite.com/d3jsys

in its current state it has issues with the internal clock oscillator.

>
> In my specific case, this is the El Cheapo Programmer.

The programmer isn't really the issue, the target is. Some tips:

1) Be sure that there are resistors between the programmer's CLK/DATA pins and
the target. This ensures that if the target sets those pins to outputs that
buffers don't get blown.

2) Be careful with MCLR. Be doubly careful because often MCLR is either tied
directly to Vdd or pulled up through a resistor. If the pullup resistor is
already in place then you are probably OK. However you'll have a problem if
it's tied directly to the power supply. Be sure that the programmer has enough
juice to pull the pullup resistor to low.

3) The clock is a major issue. If it's running when you try to program it'll
often screw your programming process up. I've found that an external oscillator
through a resistor is ideal because you can then add a transistor to pull the
clock line low when programming. Does anyone know of the effect of an inline
resistor between a crystal and OSC1?

4) Lastly the programmer needs to be able to control Vdd. So the programmer
needs to be able to power the target.

Just a couple of final notes. First is that I use 16F877 parts precisely
because they are self programmable and so all of the issues go POOF! like
magic because the part is running normally. Second is the standing info
about 16F84s. They are obsolete. Check out the reasons:

http://www.finitesite.com/d3jsys/16F628.html

Hope this helps,

BAJ

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