Monday, September 26, 2011

Nice guide to 74LS vs 74HC vs 74HCT

I've been tinkering recently with my Launchpad, with the goal of developing a 32-bit I/O expander that I can use to simulate traffic on my CPU's buses without having to run an actual program.  This will allow me to debug individual modules without having to wire up the entire system, upload test software into the EEPROMs, and trace the signals across a half dozen boards.

Along the way I've had to start taking a hard look at signal voltages for various stuff, because the TI MSP430 is a 3.6V part, and I need to drive my 5V TTL and CMOS logic with it.  This requires some logic level conversion, and as it turns out, some deeper knowledge about the differences in the chips I'm plugging into my breadboard.

I was under the mistaken impression that 74LS (TTL) logic was effectively identical to 74HC (CMOS) logic except for maximum frequency capability.  Turns out I was way off, and in fact you cannot mix these components in a system without performing logic level conversions.  I also thought that the 74LS series was the "most preferred" for new projects; but it looks like I'm wrong there too -- the 74HC series is the "best" these days.

I found this very helpful link that describes the differences in the series and notes the interoperability of them.  It also has a really nice breakdown of the basic 74 series chips, their pinouts, and how to do basic daisy-chaining of things like counters.  An excellent page to have bookmarked so that you're not constantly digging up datasheets just to see what the pinout is for a chip...