UPSD3422 datasheet(64/293 Pages) STMICROELECTRONICS

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Power saving modes

uPSD34xx

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15.2

Power-down Mode

Power-down Mode will halt the 8032 core and all MCU peripherals (Power-down Mode

blocks MCU_CLK, USB_CLK, and PERIPH_CLK). This is the lowest power state for the

MCU Module. When the PSD Module is also placed in Power-down mode, the lowest total

current consumption for the combined die is achieved for the uPSD34xx. See

Section 28.1.16: Power management on page 191 in the PSD Module section for details on

how to also place the PSD Module in Power-down mode. The sequence of 8032 instructions

is important when placing both modules into Power-down Mode.

The instruction that sets the PD Bit in the SFR named PCON (Table 26 on page 66) is the

last instruction executed prior to the MCU Module going into Power-down Mode. Once in

Power-down Mode, the on-chip oscillator circuitry and all clocks are stopped. The SFRs,

DATA, IDATA, and XDATA are preserved.

Power-down Mode is terminated only by a reset from the supervisor, originating from the

RESET_IN_ pin, the Low-Voltage Detect circuit (LVD), or a JTAG Debug reset command.

Since the clock to the WTD is not active during Power-down mode, it is not possible for the

supervisor to generate a WDT reset.

Table 24 on page 65 summarizes the status of I/O pins and peripherals during Idle and

Power-down Modes on the MCU Module. Table 25 on page 65 shows the state of 8032

MCU address, data, and control signals during these modes.

15.3

Reduced Frequency Mode

The 8032 MCU consumes less current when operating at a lower clock frequency. The MCU

can reduce its own clock frequency at run-time by writing to three bits, CPUPS[2:0], in the

SFR named CCON0 described in Table 22 on page 62. These bits effectively divide the

clock frequency (fOSC) coming in from the external crystal or oscillator device. The clock

division range is from 1/2 to 1/2048, and the resulting frequency is fMCU.

This MCU clock division does not affect any of the peripherals, except for the WTD. The

clock driving the WTD is the same clock driving the 8032 MCU core as shown in Figure 14

on page 61.

MCU firmware may reduce the MCU clock frequency at run-time to consume less current

when performing tasks that are not time critical, and then restore full clock frequency as

required to perform urgent tasks.

Returning to full clock frequency is done automatically upon an MCU interrupt, if the CPUAR

Bit in the SFR named CCON0 is set (the interrupt will force CPUPS[2:0] = 000). This is an

excellent way to conserve power using a low frequency clock until an event occurs that

requires full performance. See Table 22 on page 62 for details on CPUAR.

See the DC Specifications at the end of this document to estimate current consumption

based on the MCU clock frequency.

Note:

Some of the bits in the PCON SFR shown in Table 26 on page 66 are not related to power

control.

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