245
27.4 Clock Speeds
27.4.1 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, and the operating
system current consumption of 10 to 120 A depends on frequency and voltage.
Table 27-10 describes some recommended clock and memory configurations for 8-bit
memory devices. Optimal configurations for using 15 ns, 45–55 ns, and 70 ns memories
are shown.
The Rabbit 3000 is rated for a minimum clock period of 16 ns for both commercial and
industrial specifications (preliminary). The commercial rating calls for a ±5% voltage
variation from 3.3 V, and a temperature range from -40 to + 70°C. The industrial ratings
stretch the voltage variation to ±10% over a temperature range from -55 to + 85°C. This
corresponds to maximum clock frequencies of about 58.8 MHz (industrial). If the clock
doubler or spectrum spreader is used, these maximum ratings must be reduced as shown in
Table 27-10. Recommended Clock/Memory Configurations
Input
Frequency
(MHz)
Internal
Frequency
(MHz)
Recommended Memory Setup
Use
SRAM
Flash
29.4912
58.9824
8 bits, 15 ns,
0 wait states
8 bits, 45–55 ns,
2 wait states
Fastest 8-bit configuration
without wait states
(run code from SRAM)
22.1184
44.2368
8 bits, 15 ns,
0 wait states
8 bits, 45–55 ns,
1 wait state
Fastest 8-bit, 55 ns configuration
with 1 wait state
(run code in SRAM)
14.7456
29.4912
8 bits, 45–55 ns,
0 wait states
8 bits, 45–55 ns,
0 wait states
Fastest 8-bit, 55 ns configuration
without wait states
11.0592
22.1184
8 bits, 70 ns,
0 wait states
8 bits, 70 ns,
0 wait states
Fastest 8-bit, 70 ns configuration
without wait states