power back

sanjeevk · ‎2004-12-05

Posted on December 06, 2004 at 04:41

power back

sanjeevk · ‎2004-12-03

Posted on December 03, 2004 at 11:11

Hi All,

I have a strange problem. I have interfaced a Parallel port printer to an ST7 controller(through ports D and F and 74HCT244). It's working OK. But the problem is when I switch off the power to the MCU and if the printer is still ON, the MCU continues to work(it keeps an Icon based LCD display fairly ON :-(). This, as i could make out,is because of strong pull-ups provided within the Printer. Now I want to know whether the MCU is behaving properly or is it supposed to provide enough isolation. Otherwise, I think, I need to provide optocouplers for isolation. Also whether providing pull-down resistors would be a good idea? Your views and comments please.

thanks for the attention,

Sanjeev

alexis23 · ‎2004-12-05

Posted on December 06, 2004 at 04:41

Hi,

You are right, you are probably driving the power through input protection diodes that connect input wires to VCC of the chip.

I think it would be safe to put serial resistors in your connections (10K for instance) to limit the current through these diodes (I believe they are specified for 2mA max).

Alexei