So, you’re building a system that will use several logic gates. The system will be running at 80 MHz. You plan to fan-out the clock signal through several buffers and a flip-flop. The flip-flop you use has a maximum frequency (Fmax) of 200 MHz so you know it’s okay, however the rest of the logic buffers do not have an Fmax spec listed. How do you know if the devices can run that fast? This is a typical problem when designing with logic. Only the devices that have clocks will have an Fmax specified, so you’re left figuring out (on you own) whether or not your devices can meet the need for speed.
Most buffers and gates will not have a maximum frequency specified in the datasheet. Some may think you can calculate the max frequency from the propagation values. However, propagation delay time (Tpd) is not really related to Fmax. The proper way to calculate Fmax is to use output rise and fall time values – Fmax = 1/(Trise + Tfall). Rise and fall times will, of course, depend on load conditions. But, an idea of the max frequency into datasheet loads will help when designing for specific frequencies.
The best way to get the rise and fall time values is to use an IBIS model. Then, you can see TR/TF values with your actual load conditions across voltage, temperature and process variations.
Unfortunately IBIS models are not always available for legacy logic devices. So when one isn’t available… a simple way to determine the maximum frequency is to look at a clocked part in the same family. There will always be a flip-flop in every family. A 74 or 374 function can usually be found. The clocked parts will always have Fmax values in the datasheet and these values can be used for all parts in that same family.
If you are searching for the Fmax for a little logic device it is better to use a little logic flip-flop, because they are newer and usually use a newer process. They will usually run at higher speeds than the standard logic in the same family. For example: You need the maximum frequency of an SN74LVC1G34. Since this is little logic, look at a little logic part in that same family that has a clock, such as the SN74LVC1G74. Here are the max frequencies:
Now, you have a good Idea of how fast all the little logic part in the LVC family can run.
This should help next time you need to know the maximum frequency of a logic device.
Do you encounter any other issues when selecting logic devices?
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