Re: PS/2 mouse as 1351?

From: john/lori (
Date: 2003-09-17 19:54:32

Hársfalvi Levente wrote:

> Neither did I fully understand the situation, I just noticed that it
> received no interrupts after I changed the microcontroller ;-). I suspect,
> the Schmitt-trigger input will notice the level drop (since the signal level
> on the POTX/Y lines is always close to 0v -- remember, the input voltage
> threshold value of the SID inputs is 0.8v and not Vcc/2), but probably won't
> notice the rise.

According to the datasheet, the POTX, POTY trigger voltage is Vcc/2

>  A simple TTL input will switch both forth and back at
> around 0.8~1V, but the Schmitt-trigger won't (without a larger voltage
> swing) as it has hysteresis.

The AVR data sheet gives a hysterisis of about .13V at 5V Vcc

The AVR datasheet also gives the minimum gauranteed hi as .6 Vcc
so if the SID discharges the cap as soon as the cap hits a trigger
voltage of .5 Vcc it may not get high enough for the AVR to recognize.

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