
1-16
Power Management
1.3
CLOCK-SWITCHING LOGIC
The élanSC310 microcontroller’s clock-switching logic handles the task of switching clock
speeds as directed by the PMU or other input, and sequencing the shutdown and startup
of the clocks and PLLs.
1.3.1
CPU/Memory Clock Switching
The CPU clock-switching circuit delivers a signal that switches cleanly between the high-
speed PLL clock and a low-speed clock source in High-Speed PLL mode. The low-speed
clock source switches cleanly between the low-speed PLL clock (9.2 MHz) and a select-
able slow clock on a PMU state transition (see Table 1-1 on page 1-6).
The high-speed PLL clock is used only under narrowly defined conditions. The high-
speed clock must be enabled by setting bit 6 of the I/O Wait State register at Index 61h.
Doing this enables the use of the high-speed clock after the next refresh cycle if the high-
speed PLL is already started.
In addition, the PMU must be in High-Speed PLL mode and the current bus cycle must be
one of the following types of cycles:
I
CPU idle
I
Local DRAM
I
Fast ROM
I
Local bus
Use of the high-speed clock is disallowed when all of the following conditions are true:
I
BL1 is Low
I
Bit 5 of the PMU Control 2 register at Index AFh has been set to enable this feature
I
ACIN is Low
If the auto low-speed logic is enabled, use of the high-speed clock is disallowed periodi-
cally to conserve power. In cases where the use of the high-speed PLL clock is disal-
lowed, the low-speed PLLs 9.2-MHz CPU clock is used.
When the PMU is in Low-Speed PLL mode, the CPU clock is generated from the low-
speed clock source. This is a programmable divider chain controlled by bits 1–0 of the
PMU Control 3 register at Index ADh which provides a clock frequency of 9.216 MHz that
is divided by 2, 4, 8, or 16. These bits should be changed only when the PMU is not in
Low-Speed PLL mode.
When a temporary-on condition occurs while the PMU is in Doze, Sleep, Suspend, or Off
mode—and the CPU/memory clock is enabled to run—the 9.2-MHz CPU clock is used.
The high-speed PLL clock frequency may be selected by writing to bits 4–3 of the Func-
tion Enable 2 register at Index B1h. These bits should not be changed when the high-
speed clock is enabled.
1.3.2
Clock Startup and Shutdown Logic
The clock startup and shutdown logic provides a mechanism for properly coordinating the
activation of the élanSC310 microcontroller’s PLLs with the CPU’s clock-enabling logic.
The flowchart in Figure 1-2 on page 1-18 illustrates the logic flow. For the high-speed
clock, there is a choice of turning off the PLL in either Low-Speed PLL mode or Doze