mmm@cup.portal.com (Mark Robert Thorson) (01/19/91)
Quoting from the data sheet for the Intel 82311: "Note that the above names/numbers are frequency independent; i.e. they refer to a generic functional VLSI device. To actually implement for example, a 20 MHz system, however, requires an 82311-20 Chip Set as opposed to an 82311-16 Chip Set. The 25 MHz version of the 82308 (dubbed the 82308HS-25) cannot be used at 16 MHz or 20 MHz." I speculate that this frequency-dependence is caused by counting off clock ticks to pulse external signals for a certain duration. If you change the clock frequency, the pulse duration changes. Normally this isn't a problem, but the 82311 implements an interface to a standard bus (Micro Channel), so changing the clock frequency can put your bus timing out of spec. It's like using the execution time of instructions to construct a software timing loop, with the same result (i.e. the design breaks when run faster or slower).