ICP with MCB900 Confusion

[bruceg]

The app note here:
http://www.esacademy.com/software/flashmagic/resources/AN10258_2.pdf

I have programmed a MCB900 with the ISP to ICP bridge code and made all the connections shown in this datasheet (and verified them).

There is some confusion, however:
On page 7, it says "To control VDD, a PNP transistor is needed, because P2.3 cannot supply enough current for the LPC9xx during programming."

On the following page, a nice schematic shows a 2n2222 being used, which is of course an NPN.

So which is it? NPN or PNP?
Or put another way, when designed, was VDD intended to be applied to the application with 2.3 being driven high or low?

To add to this confusion, an older version of the appnote shows a PNP transistor in the schematic.

Thanks,
Bruce

[bruceg]

Sorry about that.  I answered my own question with a little testing.  For anybody searching:

1) Active High control is needed.  IE: when P2.3 is high, VDD should be switched to the application circuit.  

2) I found that with my Version 4 MCB900, the NPN (2222) when driven could not maintain the 3.3 volts at the emitter.  So I plopped in a seperate 3.3 volt regulator and a very fast, low drive voltage MOSFET (PowerTrench style).  During testing, I found it to be extreamly reliable so far (about 25 perfect programming cycles with no problems).

So i'm not really sure why the engineer says to use a PNP or at least he doesn't make it very clear.  I think what may be suggested is to use a PNP to drive a NPN, which may or may not work very well.

Bruce

[erikm]

LDO r5egulators u8se PNP, "regular" regulators use NPN.  Could that have something to do with it.

ANyhow I LIKE your suggestion of a MOSFET and wonder why Keil did not do that in the first place.

Erik

[bruceg]

I'm guessing Keil didn't do that in the first place because this board was designed for ISP usage and the ICP "mod" probably came after design.  

But I will say that this whole process has been unnecessarily confusing for several reasons besides just the appnote.  For example, Keil changed the names of the jumpers from the original versions of the board, yet didn't change any of the help\appnotes.  I had to trace out the jumper connections to figure out what "AV" stood for, etc.  There is a lot of conflicting information that apparently has changed over the different versions of the board, none of which has been reflected in documentation.

Oh well.