Jameco Electronics Rabbit 3000 - Manual (Page 93)

84 Rabbit 3000 Microprocessor
When the clock doubler is used and there is no subsequent division of the clock, the output
clock will be asymmetric, as shown in Figure 7-2.
Figure 7-2. Effect of Clock Doubler
The doubled-clock low time is subject to wide (50%) variation since it depends on process
parameters, temperature, and voltage. The times given above are for a supply voltage of
3.3 V and a temperature of 25°C. The doubled-clock low time increases by 20% when the
voltage is reduced to 2.5 V, and increases by about 40% when the voltage is reduced fur-
ther to 2.0 V. The values increase or decrease by 1% for each 5°C increase or decrease in
temperature. The doubled clock is created by xor’ing the delayed and inverted clock with
itself. If the original clock does not have a 50-50 duty cycle, then alternate clocks will
have a slightly different length. Since the duty cycle of the built-in oscillator can be as
asymmetric as 52-48, the clock generated by the clock doubler will exhibit up to a 4%
Oscillator
Oscillator delayed
and inverted
Doubled clock
Delay
time
48% 52%
P
0.48P 0.52P 0.48P 0.52P
Data out
Example
Write
Cycle
write pulse
early write pulse
option
Example
Read
Cycle
Address / CS
Address / CS
output enb
early output enb
option
data out from mem