The rise time specification for strobe input signals to chips are necessary for many reasons. Here are a few reasons why a fast rise time is necessary.

1) Many times the strobe signal goes on board and must operate a number of separate internal circuits in parallel (i.e. the signal goes to multiple internal gates and flipflops). If the input signal has slow rise time it will cause the internal circuits to trigger at different times depending on the logic threshold voltages of the various circuits. Some times this can make logic that is supposed to operate "synchronous" to operate in an asynchronous manner. This can lead to flipflop state machines going to the wrong or invalid states.

2) With CMOS circuits each stage of logic on the chip is composed of P and N channel FETs stacked up in totem pole push pull inverter/gate stages. Power dissapation on these circuits can increase dramatically if both the N and P channel devices are on at the same time. A logic stage such as this with a slow rise time input will spend a lot more time in the no-mans land between V_ih and V_il logic thresholds where both the P and N devices are partly on.

3) To equip the input receiver circuits on an IC with Schmidt trigger inputs has a tendancy to slow down the propagation delay of the input circuit. Such capability would make inputs much more tolerant to slow rise times but at the expense of performance. So this leads to a design tradeoff on the part of the chip designer to eliminate the input hysterisis im favor of attaining the faster circuit operation.

4) Rise times are often specified for strobe signals so that the edges of these signals provide a measurement reference for other specified timing characteristics of the device. For example the data input setup time to a chip may be specified relative to the rising edge of a WR/ signal. The specified setup time in the data sheet can only be guarenteed of the write strope has a maximum rise time of say 20 nanoseconds. If the signal rise time were to be 1 microsecond there is no guarentee that the input setup time can be effectively specified, or measured, or even expected to work correctly to the data sheet timing.

Michael Karas