Rabbit 3000, User Manual

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User’s Manual 217
16.6 Memory Current Consumption
Since there are many different memories available, let’s look at an example using one of
the recommended flash and SRAM memories.
Flash memory—SST part SST39LF512020, 256K × 8, 45 ns access time. Standby cur-
rent: nil.
•
Static Current (chip select low): 3.5 mA @ 3.3 V
•
Dynamic Current: 7 mA at 14.7 MHz bus speed and 3.3 V
The total current is 10 mA at a clock speed of 29.49 MHz or a bus speed of 5 MHz.
The static part of the current is computed using
0.35×(chip select duty cycle).
The dynamic part is computed using
0.5×f in mA,
where f is the bus speed in MHz.
At 0.46 MHz (3.68 MHz/8), and using a short chip select, the duty cycle is about 10%,
giving a static current of about 0.35 mA. The dynamic current is 0.25 mA, for a total cur-
rent of 0.6 mA. Added to the approximately 2.5 mA operating current gives a total current
of 3.1 mA at 0.46 MHz.
In sleepy mode with a self-timed chip select of 106 ns and a clock speed of 32 kHz, the
duty cycle will be 0.106/66 = 1/600, and the static current will be 3.5/600= 6 µA. If the
clock is divided down by a factor of 2, then the static current is reduced to 3 µA. The
dynamic current will be 16 µA at 32 kHz (1000×0.5×f) and 8 µA at 16 kHz.