Interfacing 4000 series CMOS to 74HC series TTL

[kd4pbj]

I just built an oscillator using a 4069, and wish to connect this to a divider circuit using a 74HC4060 counter and 74HC74 flip-flop (using the flip flop as a divide by 2). Is there anything I need to be aware of in interfacing the 4000 series to the 74HC series chips? Pull ups, etc? Thanks.

 

[Papabravo]

4000 series parts can run from Vcc in the range 3-18 Volts DC. The 74HC series has a smaller Vcc range of maybe 3-6 Volts DC. Read the data sheets carefully(always a good idea), use the same Vcc for all chips, use bypass capacitors, don't let the inputs float, and don't cross the streams. OK, I'm kidding about the last one.

 

[jarph]

as long as your voltage is under 5 volts everything is good. the CMOS stuff is very sensitive to static. Make sure you grounded before handling IC's

 

[audioguru]

Why not use the oscillator circuit in the 74HC4060? It is able to reach a much higher frequency than a 4069 oscillator operating on 5V.

 

[kd4pbj]

The oscillator is a 32.768 khz and I never could get the tuning fork to work directly with the 74HC4060. I might not have gotten the load caps right.

I had one other question about this. The output as seen on a scope is not a flat top square wave, but starts high and ramps down a little. What chip can I feed the output into to square the wave up a bit? Thanks.

Chris

 

[Papabravo]

I suspect the attempt to "square things up" may lead to other undesireable effects like ground bounce and ringing. You can use 74ACxx chips. They have the same Vcc range as the 74HCxx but the output drivers can source and sink more current and they have lower propagation delay. The wicked fast edges will probably not be to your liking. Such is life - eh!

BTW 74HC and TTL is kind of an oxymoron. The chips are CMOS, not TTL, but have catalog numbers that are related to the same function in the 7400 series of TTL chips.

 

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[audioguru]

Tuning fork?
Use a quartz watch crystal and the circuit for a quartz crystal oscillator. The datasheet for the quartz crystal has the circuit.

My Cmos circuits produce nearly perfect flat tops on square waves when the load resistance is high enough to avoid loss.
Maybe your 10:1 'scope probe needs adjustment or maybe your supply isn't bypassed and is sagging between pulses.

 

[kd4pbj]

Thanks guys. I will check everything out again tonight.

I have heard the 32.768 khz cylindrical crystals called tuning fork crystals.

I may have hooked up the oscillator section wrong on the 74HC4060 when I tried using the crystal with this.

 

[kd4pbj]

Working now, but with question

OK I followed audioguru's advice and tried using the 74HC4060 again with good results this time.

I had tried it several years ago with higher frequency crystals in other applications and gotten good results, just not with the crystals below 1 Mhz (I think I had left out the high resistance resistor in series with one leg of the crystal). The circuit that I am experimenting with today is with the 32.768 khz crystal.

I got this working correctly but got no output on pin 3 (the f/16386 output). I took pin 12 (reset) to ground with a 22K resistor and got 2 Hz output on pin 3, tested by soldering a LED from this pin to ground and comparing this to the clicks of radio station WWV.

However, when I fed that into the 74HC74 flip flop hooked up as a divide-by-two, I was still getting a 2 Hz output, and the LED would barely flicker a few times in between the main flashes. I removed the 74HC74 and substituted a 74LS74 and now I get a 1 Hz flash with no flickers, just steady on/off flashes every second.

Does anyone have any theories about what might have the HC part might have behaved like that? Was the HC part just too fast? It was a new part, but I guess that new parts can be bad out of the package. The 32 khz signal looked clean on my scope.


Chris

 

[audioguru]

Any Cmos output can drive an LED, but then it might not have enough voltage swing to drive a logic input.

 

[kd4pbj]

74HC4060 chip in 32.768 khz oscillator

A few months ago you guys helped me in an interfacing HC-TTL to CMOS question. I was looking for a good oscillator circuit for 32.768 khz and was going to use a 4069 and a 74HC4060 to divide it down to 1Hz.
I took you up on your advice to just use the 74HC4060 as the oscillator, and then divide the 2 hz output with a flip flop (tried a 74HC74 but that didn't work so I ended up using a 74LS74 with great results). I got a nice 1 Hz pulse.
I built up several versions of the circuit (used the circuit at
**broken link removed** but had to change the 10 pF to a 22 pF and the 22 pF to a 25 pF trimmer to get dead on frequency with the crystal that I had).

My question is this. I have built three versions now:
one with a DIP 74HC4060 and DIP 74LS74
one with a DIP 74HC4060 and a SOIC 74LS74
and now one with a SOIC 74HC4060 and a SOIC 74LS74

I can't get the one with a SOIC 74HC4060 to work correctly. I removed the 74LS74 and am now experimenting with just the oscillator in an attempt to get it to work. I have tried several times with different chips, board layouts, crystals, etc. thinking that I had a bad part and I just can't get it to oscillate correctly. It either oscillates in the several hundreds of kilohertz range, or in the 20-30 Mhz range as measured both with a scope and a frequency counter.
Any thoughts? I hate to give up on this one but I can't figure out why there was never any problem with the DIP version but the SOIC version of the chip won't work in this circuit.
Thanks!

Chris