I think my basic approach is somewhat different from the others, so here is a schematic to illustrate the concept:



The idea is to count the number of clocks in a certain time interval. Here is a little pseudocode to use it:

Initial conditions:
LOAD = 0
EN_CLK = 0

1 Strobe LOAD
2 Let Start_Count = BIT[27:0]
3 Assert EN_CLK
4 Wait 2 seconds
5 Deassert EN_CLK
6 Strobe LOAD
7 Let End_Count = BIT[27:0]
8 If End_Count < Start_Count then Let End_Count = End_Count + 2^28
9 Frequency = End_Count - Start_Count

The answer (Frequency) is the frequency in Hertz.

Of course the designer would add on a fancier output to the 8051, perhaps a shift register or memory map it to 4 bytes.

Is this a good approach?

- Lee