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5 ST7 POWER SAVING MODES
5.1 INTRODUCTION
To give a large measure of flexibility to the applica-
tion in terms of power consumption, three main
power saving modes are implemented in the De-
■ Low Power Mode (PLL OFF)
■ Wait
■ Halt
After a RESET low power mode is selected by de-
fault. This mode drives the Device (CPU and em-
bedded peripherals except USB) by means of a
master clock which is based on the main oscillator
frequency.
From this low power mode, different modes may
be selected using specific CPU instruction.
Important note: Moreover, if the USB cell is not
used, the UPO bit of the EOSCR register must be
set to avoid any USB2 PHY consumption.
Figure 9. Power Saving Mode Transitions
5.2 WAIT MODE
WAIT mode places the Device in a low power con-
sumption mode by stopping the CPU.
This power saving mode is selected by executing
the “WFI” CPU instruction.
All peripherals remain active. During WAIT mode,
the I bits in the CC register are forced to 0, ena-
bling all interrupts. All other registers and memory
remain unchanged. The Device remains in WAIT
mode until an interrupt or reset occurs. If the event
is an interrupt, the program counter immediately
branches to the starting address of the interrupt or
reset service routine. If the wake up event is a re-
set, before fetching the reset vector, there is a 512
CPU clock cycle delay to allow for stabilization.
Figure 10. WAIT Mode Flow Chart
POWER CONSUMPTION
FPWAIT
HALT
FPRUN
High
Low
LPRUN
LPWAIT
WFI INSTRUCTION
RESET
INTERRUPT
Y
N
Y
CPU CLOCK
OSCILLATOR
PERIPH. CLOCK
I1, I0 BITS
ON
CLEARED
OFF
CPU CLOCK
OSCILLATOR
PERIPH. CLOCK
I1, I0 BITS1)
ON
FETCH RESET VECTOR
OR SERVICE INTERRUPT
512 CPU CLOCK
CYCLES DELAY
IF RESET
Note: 1) Before servicing an interrupt, the CC
register is pushed on the stack. The I0 and I1
bit values for each interrupt are predefined by
the user in the ISPRx register. During the inter-
rupt routine these values are loaded into I0 and
I1 bits and cleared when the CC register is
popped.