Josh Koffman wrote:
> Hi all. I'm designing my first 3.3 volt system, and I'd like to double
> check my thinking. This is using the 18f44J11 chip which is a 3.3V
> only chip. My understanding of these chips is that they no longer
> require a high voltage on Vpp to engage programming.

Yes.  Not only is the high programming voltage not required, but it is not
allowed.  Raising MCLR of a 3.3V PIC to traditional Vpp levels can damage
the chip.  This is one purpose of my LProg programmer
(http://www.embedinc.com/products/lprog).  It doesn't produce a high Vpp
voltage and therefore can't damage a 3.3V PIC even when it can't identify
the chip.  Other programmers, like the USBProg for example, will start with
the 3.3V algorithm when they don't know what chip is out there, but will
proceed to ever higher voltages if the chip can't be identified.  You have
to remember to explicitly tell it what chip is out there or be really sure
everything is connected correctly with no glitches or noise.

> I believe they only see Vdd on MCLR

Yes, all signals, including programming signals, are limited to the Vss to
Vdd range.

> What I'm not sure about is if I should include a diode to isolate MCLR
> from Vdd, or if this is a bad idea.

There is no need.  I usually put a 20Kohm pullup between MCLR and whatever
is driving it (which could be Vdd).  That's low enough to reliably drive the
CMOS input, but high enough not to get in the way of any programmer you're
likely to encounter.


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