Rabbit 3000, User Manual

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User’s Manual 209
The following factors have to be taken into account when calculating the output enable
access time required.
•
The gross output enable access time is T + minimum clock low time (it is asusmed that
the early output enable option is enabled) This is reduced by the spectrum spreader
loss, the time from clock to output for the output enable signal, the data setup time, and
a correction for the asymmetery of the original oscillator clock.
Example
•
Clock = 29.49 MHz,
•
T = 34 ns,
•
operating voltage is 3.3 V,
•
the clock doubler has a nominal delay of 16 ns, resulting in a minimum clock low time
of 12.8 ns,
•
the spectrum spreader is on in normal mode, resulting in a loss of 3 ns,
•
clock to output enable is 5 ns (assuming 20 pF load),
•
the clock asymmetery 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) - (assymmetry delay)
- (data setup time)
= 34 ns + 12.8 ns - 5 ns - 3 ns - 1.36 ns - 1 ns
= 36.5 ns