
Power Management
1-5
Power-Management Control (PMC) pins can be used to control power to peripheral
devices on a per-mode basis. Software can restore power to any peripherals whose
power (controlled by its associated PMC pin state) was removed by a previous transition
to a lower-power mode.
1.1.1.2
Low-Speed PLL Mode
Low-Speed PLL mode is the first level of power conservation. It is entered after a speci-
fied elapsed time with no activity, programmed using the High-Speed to Low-Speed Mode
Timer register at Index 83h. In this PMU mode, the CPU, DMA, and other internal system
clocks run at reduced rates. The low-speed PLL clock, whose rate is always fixed at
18.432 MHz, is sent through a programmable divider. The minimum divisor is 2. This
yields a maximum programmable CLK2 rate of 9.216 MHz, which results in a maximum
internal operation speed (CPUCLK) of 4.608 MHz. PLL divisors of 2, 4, 8, and 16 can be
selected for dividing the low-speed PLL clock in Low-Speed PLL mode. No dynamic
switching of CLK2 to the high-speed PLL rate is done in this mode. All other clocks and
peripherals run at full speed.
Power-management software may optionally shut off the high-speed PLL. Depending on
the frequency of this PLL, up to 750
μ
A may be saved by this action. If this is done, a PLL
start-up delay of 256 ms (programmed in the Clock Control register at Index 8Fh) must
elapse before High-Speed PLL mode can be re-entered.
PMC pins may be used to control power to peripheral devices on a per-mode basis. Soft-
ware may restore power to any peripherals whose power (controlled by their associated
PMC pin states) was removed by a previous transition to a lower power mode.
1.1.1.3
Doze Mode
Doze mode is the second level of power conservation. The CPU, system, and DMA
clocks, and the high-speed PLL are stopped. This mode is entered after a programmed
time without activity has elapsed. For details, see “Low-Speed to Doze Timer Register
(Index 84h)” on page 4-44.
By default, the CPU clock is stopped in Doze mode, along with the DMA clock and inter-
nal system clock (see Table 1-1 on page 1-6). The video, UART, and 8254 timer clocks
are driven by the low-speed PLL, which is enabled by default. By allowing these clocks to
run, it is possible for timer and keyboard interrupts to be generated.