User`s manual

74 rabbit.com Low-Power Design and Support
9.1 Details of the Rabbit 4000 Low-Power Features
This section goes into more detail about the Rabbit 4000 low-power features.
9.1.1 Special Chip Select Features
Unlike competitive processors, the Rabbit 4000 has two special chip select features designed to minimize
power consumption by external memories. This is significant because, if not handled well, external memo-
ries can easily become the dominant power consumers at low clock frequencies. Primarily because most
memory chips draw substantial current at zero frequency. (When the chip select and output enable are held
enabled and all other signals are held at fixed levels.)
In situations where the microprocessor is operating at slow frequencies, such as 2.048 kHz, the memory
cycle is about 488 µs and the memory chip spends most of its time with the chip enable and the output
enable on. The current draw during a long read cycle is not specified in most data sheets. The Hynix
HY62KF08401C SRAM, according to the data sheet, typically draws 5mA/MHz when it is operating.
When performing reads at 2.048 kHz, we’ve found that this SRAM consumes about 14 mA. At the same
frequency, with the short chip select enabled, the SRAM consumes about 23 µA—a substantial reduction
in power consumption.
As shown, both special chip select modes (i.e. short chip select and self-timed chip select) reduce memory
current consumption since the processor spends most of its time performing reads (i.e., instruction
fetches).
The self-timed chip select feature is available in sleepy and ultra sleepy mode; i.e., when the processor is
running off the 32 kHz oscillator, or when the oscillator is divided by 2, 4, 8 or 16.
The short chip select feature may be used when the main oscillator is divided by 4, 6, or 8. This division
can be done regardless of whether the clock doubler is on or off. Currently, interrupts must be disabled
when both the short chip select feature is enabled and an I/O instruction is used. Interrupts can be disabled
for a single I/O instruction by using code such as:
push ip ; save interrupt state
ipset 3 ; interrupts off
ioe ld a,(hl) ; typical I/O instruction
pop ip ; reenable interrupts
NOTE: Short chip selects and self-timed chip selects only take place during memory reads.
During writes the chip selects behave normally.
For a detailed description of the chip select features, please see the Rabbit 4000 Microprocessor User’s
Manual.