BigEd wrote:
BTW, with your diode annotation [...]
Sorry, Ed, if I threw you off the scent. In normal circumstances the diodes don't conduct and might as well not exist. So, I intended the diode symbols to be simply a reminder that, "These particular figures don't pertain to the discussion." I only want to examine the simple case of a gate that drives a CPU input --
what is the voltage threshold?Clearly there are lots of scenarios where the threshold matters, but the one that triggered my interest today is the "ghost interrupt" threat that's present when the CPU's /IRQ or /NMI input is driven by laggardly, open-drain (aka open-collector) logic. Garth discusses this on his web site, about 20% of the way down
this page.
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As shown, the waveform for
Actual Virq is, unfortunately, an RC curve. What we really want is to pass the logic-low to logic-high threshold sooner rather than later. There are two ways to cross the threshold sooner:
- change the RC (use a lower-value resistor, or reduce capacitance), or
- change the threshold
Two implications come to mind.
- A person could get bitten (haunted?) by the ghost interrupt merely by replacing a non-WDC CPU with a WDC CPU, and
- in the general case (not just CPU/IRQ), open-collector logic operates more slowly when the input of the device being driven uses CMOS rather TTL levels. Speed can be improved by interposing a 74HCT-series buffer between the open-collector node and the device being driven. This can be considered an additional precaution regarding the ghost-interrupt hazard. (Garth explains how proper coding can allow much more time for the open-collector logic to react.)
BTW, here's an excerpt from WDC's datasheet for their 65c22 VIA's. The "S" version wants 20%-80% of VDD but the "N" version accepts TTL input levels (and also has the traditional open-drain /IRQ output).
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BigDumbDinosaur wrote:
I had brought up the fact that the ISSI static RAM I am using in POC V1.1 generates what are essentially TTL levels
Yes, there are several threads relating to this, including the following (not necessarily a complete list). If we manage to shed any light on the matter then it would make sense if these threads linked got back to here.
WDC 65c02 signal levelsWDC MPU TTL CompatibilityWDC 65c02 signal levelsUM6502, MOS, TTL/CMOS difference -- Jeff
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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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