|
Question & Answer
Classification:
Date: Feb 20, 2000
Using the parallel port control register to strobe the XS40 Board
Q:
Can the parallel port control register could be used to give more PC outputs to an XS40 board (v1.4) after programming, provided that the lower two bits (_PROGRAM and CCLK) are held high and low (port set to 0b0000xx00)? Specifically, I would like to write an 8-bit value to the board through the data port and use the two extra pins for a write signal and a mode select.
A:
The potential problem is that the control pins you intend to use are not buffered with Schmitt trigger inverters. That makes it possible you will get multiple triggers on your write signal because of noise superimposed on a slowly rising signal passing through the parallel port cable. The Schmitt trigger reduces false triggers by moving the logic threshold as soon as the signal crosses it.