Digi Rabbit 5000, User Manual

Page: 381 / 414

Chapter 31 Specifications 381
31.4 Clock Speeds
31.4.1 Recommended Clock/Memory Configurations
The preferred configuration for a Rabbit-based system is to use an external crystal or reso-
nator that has a frequency one-half of the maximum internal clock frequency. The oscillator
frequency can be doubled or divided by 2, 4, 6, or 8, giving a variety of operating speeds
from the same crystal frequency. In addition, the 32.768 kHz oscillator that drives the
battery-backable clock can be used as the main processor clock and, to save the substantial
power consumed by the fast oscillator, the fast oscillator can be turned off. This scenario is
called the
sleepy mode , where the clock speed is from 2 kHz to 32 kHz.
Table 31-10 describes some recommended clock and memory configurations for both
8-bit and 16-bit memory devices. Optimal configurations for using 12 ns, 15 ns, 45–55 ns,
and 70 ns memories are shown. Note that there is always at least one wait state in the 16-
bit mode.
Table 31-10. Some Recommended Clock/Memory Configurations
Input
Frequency
(MHz)
Internal
Frequency
(MHz)
Recommended Memory Timing
Optimal Use
8-bit Devices
16-bit Devices
(1 wait state min.)
44.2368 88.4736
12 ns, 0 wait states
15 ns, 1 wait state
45 ns, 4 wait states
55 ns, 4 wait states
70 ns not possible
12 ns, 1 wait state
15 ns, 2 wait state
45 ns, 4 wait states
55 ns, 5 wait states
70 ns, 6 wait states 12 ns devices
36.8640 73.7280
12 ns, 0 wait states
15 ns, 0 wait states
45 ns, 2 wait states
55 ns, 4 wait states
70 ns, 4 wait states 12 ns, 1 wait state
15 ns, 1 wait state
45 ns, 4 wait states
55 ns, 4 wait states
70 ns, 5 wait states
15 ns devices
18.4320 36.8640
12 ns, 0 wait states
15 ns, 0 wait states
45 ns, 0 wait states
55 ns, 1 wait state
70 ns, 1 wait state
12 ns, 1 wait state
15 ns, 1 wait state
45 ns, 1 wait state
55 ns, 2 wait states
70 ns, 2 wait states
45 ns devices
14.7456 29.4912
12 ns, 0 wait states
15 ns, 0 wait states
45 ns, 0 wait states
55 ns, 0 wait states
70 ns, 1 wait state
12 ns, 1 wait state
15 ns, 1 wait state
45 ns, 1 wait state
55 ns, 1 wait state
70 ns, 2 wait states
55 ns devices