Digi Rabbit 5000, User Manual

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380 Rabbit 5000 Microprocessor User’s Manual
spreader loss, the time from clock to output for the output enable signal, the data setup
time, and a correction for the asymmetry of the original oscillator clock.
Example Output-Enable Access Time Calculation
NOTE: There is some process and temperature variation in the clock doubler settings. As
a rule of thumb, a 20% variation should be considered. When the doubler is enabled,
80% of the nominal value should be used for the memory access time calculation.
•
clock = 29.49 MHz, so T = 34 ns
•
the clock doubler has a nominal delay of 16 ns (see Table 31-8), resulting in a minimum
clock low time of 80% × 16 ns = 12.8 ns
•
clock to output enable is 5 ns
•
spectrum spreader is on in 1 ns mode, resulting in a loss of 4.5 ns worst-case (see
Table 31-9)
•
main clock asymmetry is 52% / 48%, resulting in a loss of 4% of the clock period, or
1.4 ns
The output enable access time is given by
access time = T + (min. clock low) - (clock to output enable) -
(spreader delay) - (asymmetry delay) - (data setup time)
= 34 ns + 12.8 ns - 5 ns - 4.5 ns - 1.4 ns
= 36 ns