Architecture Overview
3-7
GMS30C7201 Data Sheet
3.4
Power management
The GMS30C7201 is designed for battery-powered portable applications and
incorporates innovative design features in the bus structure and the PMU to reduce
power consumption. The slow APB bus allows peripherals to be clocked slowly hence
reducing power consumption. The use of three buses reduces the number of nodes
that are toggled during a data access, thereby further reducing power consumption. In
addition, clocks to peripherals which are not active can also be gated.
3.4.1 Clock gating
The high performance peripherals, such as the SDRAM controller and the LCD
controller, run most of the time at high frequencies and careful design, including the use
of clock gating, has minimized their power consumption. The VGA controller can be
powered down completely when not in use (that is, when not connected to an external
monitor).
3.4.2 PMU
The Power Management Unit (PMU) is used to control the overall state the system is
in. The system can be in one of five states:
Run
The system is running normally. All clocks are running (except where gated locally),
and the SDRAM controller is performing normal refresh.
Slow
The system operates normally, except the ARM is placed into Fast Bus mode, and
hence is clocked at half its normal rate.
Idle
In this mode, the PMU becomes the bus master until there is an interrupt for the CPU,
or the peripheral DMA controller requests mastership of the bus.
Sleep
The SDRAM is placed into self-refresh mode, and internal clocks are gated off. This
mode can only be entered from Idle mode (that is, the PMU must be ASB master before
this mode can be entered). The PMU must get bus mastership to ensure that the
system is stopped in a safe state and not, for example, half-way through an SDRAM
write.
Usually this state is only to be entered briefly, on the way to entering deep sleep mode.
Deep Sleep
In deep sleep mode, the 3.6864MHz oscillator and the PLLs are disabled. This is the
lowest power state available. Only the 32kHz oscillator runs. The real time clock and
wakeup sections of the PMU are operated from this clock. Everything else is powered
down, and SDRAM is in self-refresh mode. This is the normal system
“
off
”
mode.
Sleep and Deep Sleep modes are exited either by a user wake-up event (generally
pressing the
“
On
”
key), an RTC wake-up alarm, a device reset request, or by a modem
ring indicate event. These interrupt sources go directly to the PMU. In addition, the
modem ring indicate signal also goes to the normal interrupt controller to signal an
interrupt if there is a ring indicate event in a non-sleep mode.