MC908AB32, ISP programmer - will it work?

bartek_zet — Tue, 25 Sep 2007 04:53:45 GMT

Hello.
I've build programmer shown on the picture. My question is: Will it work? Microcontroller will be supplied by 5V and Vtst will be about 7,6V

Re: MC908AB32, ISP programmer - will it work?

bigmac — Tue, 25 Sep 2007 09:55:19 GMT

Hello,

Welcome to the forum.

I can see a couple of potential problems with your schematic -

For the standard MAX232 device, the capacitors C1, C2, C3 and C5 should have values of at
least 1u0. If you wish to retain the 100n values, you would need to use the MAX202 device
instead.
The source for Vtst should really be from the V+ pin, rather than T2out. However, the use of
the LM317 regulator is problematic. Due to its significant drop-out voltage, and when
supplied from the MAX232, it is unlikely that there would be suffient input voltage for the
output to properly regulate to 8.0 volts.

Typically, there should be approximately 9 volts at the V+ pin, but this could vary, depending on the
serial port load. Vtst should lie within the limits 7.5 to 8.0 volts (not a large margin for this particular
device). Therefore, the regulator would need to be a low drop-out type.

As an alternative to using a regulator, you might connect two small signal diodes in series between
V+ and Vtst pins.

Regards,

Mac

Message Edited by bigmac on 2007-09-25 01:02 PM

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MC908AB32, ISP programmer - will it work?

Re: MC908AB32, ISP programmer - will it work?