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STC90C516RD+ Microcontoller Vcc

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PhelarNord:
Hi!

I´m tryin to repair a disco laser that is controlled by a STC90C516RD+ microprocessor. I´ve started to check the voltage around the circuit.(No schematic over the laser)
There is one thing i dont really understand, maybe a noob thing.... I´ve checked the data sheet and this chip wants 3-5v on vcc pin 40 and its ground on pin 20.
In this particular circuit its grounded on pin 40 and with reference to that it has -5v on pin 20...the ground pin. There is a 7905 up stream that feeds pin 20.

Can someone explain this?

http://www.stcmcu.com/datasheet/stc/STC-AD-PDF/STC90C58RD+-english.pdf

amyk:
Voltage is relative... all the IC needs is for Vcc to be above ground by its supply voltage, which is what that circuit provides. They probably did it that way so that the outputs would be at the right levels to interface with the rest of the circuit.

PhelarNord:
Thanks a lot!! Thats clear as chrystal!

Cheers

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