Rev. 1.4 97
C8051F320/1
9.4. Power Management Modes
The CIP-51 core has two software programmable power management modes: Idle and Stop. Idle mode
halts the CPU while leaving the peripherals and clocks active. In Stop mode, the CPU is halted, all inter-
rupts, are inactive, and the
internal oscillator is stopped (analog peripherals remain in their selected states;
the external oscillator is not affected). Since clocks are running in Idle mode, power consumption is depen-
dent upon the system clock frequency and the number of p
eripherals left in active mode before entering
Idle. Stop mode consumes the least power. Figure 1.15 describes the Power Control Register (PCON)
used to control the CIP-51's power management modes.
Although the CIP-51 has Idle and Stop modes built in (
as with any standard 8051 architecture), power
management of the entire MCU is better accomplished through system clock and individual peripheral
management. Each analog peripheral can be disabled when not in use and placed in low power mode.
Digital peripherals, such as timers or serial buses, draw little power when they are not in use. Turning off
the oscillators lowers power consumption considerably; however a reset is required to restart the MCU.
The internal oscillator can be placed in Suspend mode (s
ee Section “13. Oscillators” on page 116). In Sus-
pend mode, the internal oscillator is s
topped until a non-idle USB event is detected, or the VBUS input sig-
nal matches the polarity selected by the VBPOL bit in register REG0CN
(Figure 8.1 on Page 70).
9.4.1. Idle Mode
Setting the Idle Mode Select bit (PCON.0) causes the CIP-51 to halt the CPU and enter Idle mode as soon
as the instruction that sets the bit completes execution. All internal registers and memory maintain their
original data. All analog and digital peripherals can remain active during Idle mode.
Idle mode is terminated when an enabled interrupt is asserted or a reset occurs. The assertion of an
enabled
interrupt will cause the Idle Mode Selection bit (PCON.0) to be cleared and the CPU to resume
operation. The pending interrupt will be serviced and the next instruction to be executed after the return
from interrupt (RETI) will be the instruction immediately following the one that set the Idle Mode Select bit.
If Idle mode is terminated by an internal or external reset, the CIP-51 performs a normal reset sequence
and begins program execution at address 0x0000.
If enabled, the Watchdog Timer (WDT) will eventually cause an
internal watchdog reset and thereby termi-
nate the Idle mode. This feature protects the system from an unintended permanent shutdown in the event
o
f an inadvertent write to the PCON register. If this behavior is not desired, the WDT may be disabled by
software prior to entering the Idle mode if the WDT was initially configured to allow this operation. This pro-
vides the opportunity for additional power savings, allow
ing the system to remain in the Idle mode indefi-
nitely, waiting for an external stimulus to wake up the system. Refer to Section “10.6. PCA Watchdog Timer
Reset” on page 102 for more information on the use and configuration of the WDT.
9.4.2. Stop Mode
Setting the Stop Mode Select bit (PCON.1) causes the CIP-51 to enter Stop mode as soon as the instruc-
tion that sets the bit completes execution. In S
top mode the internal oscillator, CPU, and all digital peripher-
als are stopped; the state of
the external oscillator circuit is not affected. Each analog peripheral (including
the external oscillator circuit) may be shut down individually prior to entering Stop Mode. Stop mode can
only be terminated by an internal or external reset. On reset, the CIP-51 performs the normal reset
sequence and begins program execution at address 0x0000.
If enabled, the Missing Clock Detector will cause
an internal reset and thereby terminate the Stop mode.
The Missing Clock Detector should be disabled if the CPU is to be put to in STOP mode for longer than the
MCD timeout of 100 µsec.
