Direct interfacing of CD4000 logic powered at 3.3v to the Propeller?
LoopyByteloose
Posts: 12,537
Hi,
The working voltage range of the CD4000 series logic is 3-18 volts.
So I am wondering if this is a a logic for direct interface to the Propeller at 3.3 volts. I can't purchase 74lvxxx locally. Most of the documentation indicates it can go at low as 3.0 volts, but the examples and schedules just discuss 5 volt and above.
The working voltage range of the CD4000 series logic is 3-18 volts.
So I am wondering if this is a a logic for direct interface to the Propeller at 3.3 volts. I can't purchase 74lvxxx locally. Most of the documentation indicates it can go at low as 3.0 volts, but the examples and schedules just discuss 5 volt and above.
Comments
Just off the top of my head I don't think there is a problem with that voltage, it's just slow (the Prop could execute 4 instructions in the time it takes a signal to propagate through the gate). I normally use 74HCT parts with the Prop so I can use 5V or less and still have TTL thresholds. However if you are running the logic at 3.3V then there is no problem with 74HC as it will work down to 2V.
I am sure I can get 74HCxxx locally, but not so sure about 74HCTxxx.
It will work, and the NXP HEF4000 series is slightly better at lower voltages than other brands.
Just don't expect super speed from any of the gates, but it will be low power.
My main criteria are essentially to be able to buy these locally, over-the-counter. Ordering from Mouser and Digikey add shipping expenses and a time delay.
And, I am pondering alternatives to bridging the jump from 5.0 volts to 3.3 volts with these older chips. The 74LVxxx series is ideal, but seems to be most available is SMDs.. yet another challenge to deal with.
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I suspect there are a lot of other people in the backwaters of the world that have similar situations. The USA and EU seem to sell all and everything. But elsewhere, electronics components may have been stocked decades ago with a full inventory of DIPs and then the retailer decided that adding new generations were carrying too much inventory that turns over to slowly.
I am sure, in the manufacturing zones in Taiwan -- all the latest logic chips are used in vast quantity. But that is all for export and never reaches the retail electronic shops.
That is what I was hoping for...
Here in Kaohsiung, we have plenty of CD4049 an CD4050, but none of the 74lvxxxx series. And I suspect I might wait a decade or so before the retailers stock them.
Speed is not always the most critical item. But having parts for prototypes, can be a great boost. The transistion times are indeed slow (up to 120ns). I suppose I could insert a 74hC schmitt trigger on the 3.3 volt side if that is causing trouble. Some of the CDxxxx have schmitt trigger included.
The slow propigation delay seems moot to me.
It may not be that bad, NXP give these formula (at 5V)
Tphl = 3 ns + (0.35 ns/pF)CL
Tplh = 10 ns + (1.00 ns/pF)CL
One combination to be wary of would be 5V power on HEF4049 and 3v3 drive in, as they guarantee < 3.5V (typ 2.5v)
Power from 3v3 and 5V in is fine.