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theteaman
Joined: 04 Aug 2006
Posts: 98
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| [OT] How many 74HC ICs can I connect together before I need |
 Posted: Thu Nov 16, 2006 6:43 pm |
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Hi
I hope nobody minds me posting this off-topic question, but I'm sure someone here will know. Basically I have a number of serial ICs that I will be cascading, and I know theres a physical limit as to how many I can connect, before I need some sort of buffer to regenerate the signal in the chain. How do I calculate this limit? Just in theory
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asmallri
Joined: 12 Aug 2004
Posts: 1660
Location: Perth, Australia
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| Posted: Thu Nov 16, 2006 8:25 pm |
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Depending on the clocking frequency - thousands.
You have to look at the propagation delay through the device, multiply by the number of devices, give yourself a 100% safety margin, invert the number and this will give you your maximum clock rate (close enough).
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http://www.brushelectronics.com/software
Home of Ethernet, SD card and Encrypted Serial Bootloaders for PICs!! |
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Humberto
Joined: 08 Sep 2003
Posts: 1215
Location: Buenos Aires, La Reina del Plata
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| Posted: Fri Nov 17, 2006 8:26 am |
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| Quote: |
I know theres a physical limit as to how many I can connect, before I need some sort of buffer to regenerate the signal in the chain.
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If you are talking of cascading serial shift registers it will not be necesary to regenerate the signal because the serial output of each IC had been already buffered.
| Quote: |
How do I calculate this limit?
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The limit is a relation between your clock frequency and the total propagation
delay of the whole chain.
The propagation delay time of a gate is the time interval between the application
of an input pulse and the occurrence of a corresponding output pulse. This determines
the max speed that can operate the IC.
If you are talking of how many inputs of the 74HC family you can connect to one single
output of the same family: about of 50. Every IC family has its own fan-out capabilities.
fan-out: Is the maximum number of inputs of the same IC family series that the gate
can drive while maintaining its output levels within specified limits.
Humberto |
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theteaman
Joined: 04 Aug 2006
Posts: 98
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| Posted: Fri Nov 17, 2006 8:27 pm |
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thank you guys you are very helpful!   |
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