Question about 74HCT and 74HC logic ICs

[Kyle J Cardoza]

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.

 

[Lostgallifreyan]

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.

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...

 

[Peter Bennett]

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.

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.

If it matters, the EEPROM programmer I'm working on connects via PC
parallel port.

Thanks in advance.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
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Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Kyle J Cardoza]

Lostgallifreyan caught my attention on Sun, 16 Sep 2007 02:45:41 +0000 by
saying:

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...

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?

 

[Dave Platt]

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...

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?[/QUOTE]

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.