Suchil,

You are right. We can set the programming voltages to exactly 5 and 12 for both versions of AT89C51.

On page 9 of datasheet, there are min and max values for Vpp. For 12V part, these are 11.5 and 12.5V.

I think the designer, Mr. Asim Khan, took into account the Vce(sat) drop of the transistor T2 so he advised that U2 be calibrated at 12.5 volt.

This makes sure that the device under programming receives at least the minimum programming voltage (i.e. 11.5V)


Also he advised to set U5 at 12.75V(using P1). This might be the requirement of some other device in the ATMEL family.

BTW: Why can't you use a little high voltage wall adapter so that the requirement is satisfied? You should note the full wave bridge at the power input also.

Regards,

IA