Probably a stupid newbie question, but I've got a schematic for an EEPROM programmer, and it calls for 74HCT series logic ICs. Is it going to cause a problem if I use 74HC series ICs instead? Some of the 74HCT series are proving hard for me to find, but I found 74HC series of the same numbers easily, so I wondered if they could be substituted.
If it matters, the EEPROM programmer I'm working on connects via PC parallel port.
Thanks in advance.
-- Faith does not, in fact, move mountains; it can't even cure diarrhoea, or clear up acne. It certainly won't protect anyone from his own stupidity.
Kyle J Cardoza wrote in news:2P_Gi.82974$ snipped-for-privacy@newsfet05.ams:
74HCT has TTL compatibility for 74LS so you can use both in one circuit. If speed is the main need, go with HC. And get a second opinion...
The HC parts have input switching thresholds around half the supply voltage (2.5 volts for a 5 volt supply) while the HCT part have the threshold lowered somewhat to better match bipolar (LS) TTL output levels which are only guaranteed to be above 2.4 volts for a high.
If you add pull-up resistors of 5.1K or so to Vcc on all inputs to the HC devices that should ensure that the signals actually rise above the
2.5 volt threshold for a logic 1.-- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca new newsgroup users info : http://vancouver-webpages.com/nnq GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
Lostgallifreyan caught my attention on Sun, 16 Sep 2007 02:45:41 +0000 by saying:
Does it change anything to mention that all the ICs called for, with the exception of whatever EEPROM I plug in, are from the 74HCT series, with no other logic at all? Does that mean that I can just substitute 74HC chips entirely?
-- Faith does not, in fact, move mountains; it can't even cure diarrhoea, or clear up acne. It certainly won't protect anyone from his own stupidity.
The one thing I remember that you may need to watch out for, is the fact that 74HC "prefers" to have its inputs driven all the way to the rail voltages... and, unlike TTL logic, it doesn't have a built-in "weak pullup" to Vcc. If you're driving 74HC from an open-collector TTL logic (which I believe is what a PC's parallel port is) the port may not drive the CMOS input to a high-enough voltage to allow for reliable switching... it might be prone to oscillation or to excessive power dissipation.
If I recall correctly, you can get around this incompatibility to a large extent by including a 1-TTL-load pullup (a 4k7 resistor) from each of the CMOS input pins to Vcc. This is only necessary for those
74HC chips which are being driven by TTL logic... 74HC->74HC drive is fully compatible.
--
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