Jameco Electronics 3000 ユーザーズマニュアル

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 
current: 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

3.5 × (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.