fmax Maximum Clock Frequency
JEDEC – The highest frequency at which a clock input of an integrated circuit can be driven, while maintaining proper operation.
TI – The highest rate at which the clock input of a bistable circuit can be driven through its required sequence, while maintaining stable transitions of logic level at the output with input conditions established that should cause changes of output logic level in accordance with the specification.
The fmax value is the value of the upper limit of the fclock specification, and is specified in the data sheet as a minimum limit. The circuit is specified to operate up to the minimum frequency value. See fclock for additional fmax testing information. Due to test-machine capability limitations, it may be necessary to test fmax or minimum recommended operating conditions (i.e., pulse duration, setup time, hold time) in accordance with the following paragraph.
The fmax parameter may be tested in either of two ways. One method is to test simultaneously the responses to the symmetrical clock-high and clock-low pulse durations that correspond to the period of the specified minimum value of fmax. The second method is to test individually the responses to the minimum clock-high and clock-low pulse durations under specified load conditions. A pulse generator is used to propagate a signal through the device to verify device operation with the minimum pulse duration. When clock-high and clock-low pulse durations are equal to or less than the corresponding fmax pulse duration, fmax testing suffices for testing clock-high and clock-low pulse durations.
The fmax and fclock parameters are two sides of the same coin. The fclock parameter tells you, the user, how fast you can reliably switch the input to the device. The fmax parameter informs TI when to reject a device that fails to function below a minimum speed. If you are a device user, you should simply disregard the fmax specification and use the fclock specification.
For products that are not clocked (e.g., buffers and transceivers) for which you would like to know the maximum operating frequency, an estimate is the fclock value from a comparable clocked part. For example, an LVC16245 maximum data frequency is conservatively similar to the LVC16374 maximum clock frequency. However, this is highly dependent upon load and is a rule-of-thumb only.
This tutorial is based on the Understanding and Interpreting Standard Logic Data Sheets application report (SZZA036). To download the full PDF version of this application report, please use the following URL: http://www-s.ti.com/sc/techlit/szza036.pdf