EM6812F2TP24B datasheet(25/81 Pages) EMMICRO

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EM6812

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8.2.1.1

RC switch off

The RC oscillator may be switched off to save power consumption by setting bit EnRC = '0'. In this state the system clock

must come from either the low frequency clock domain or the external high-speed clock input. While it is sufficient to switch

on the RC Oscillator by setting bit EnRC = '1', it is not sufficient to clear this bit for switching off the RC. The RC oscillator

will only stop if no peripheral circuitry (prescaler or CPU) has the RC clock as its input selection.

A special case occurs during CPU Halt mode. In this state, the RC will be switched off automatically until the CPU returns

from Halt, either by IRQ or Reset. This automatic switch off will not take place if one of the prescalers have an RC based

clock as an active input clock.

RC switch off procedure (running on XTAL):

st:

Enable the Crystal oscillator, if not yet enabled, by writing EnXtal = '1' in register RegSys2.

nd:

Switch the prescalers on the 32kHz Clock; Pr1CkSel[2:0]='000', AutoSel='1', Pr2CkSel='0' in register

RegPrCkSel

rd:

Switch the CPU to 32kHz operation; Sel32k ='1' in register RegSys2

th:

Once Crystal is ready, Flag FlagXtal='1', disable the RC oscillator with EnRC='0' in register RegSys1

The RC oscillator may also be switched off when running on the high speed external clock (SelExtHFck='1' and

ENRC=’0’).

8.2.2

High frequency external clock

It is possible to use an external clock instead of the RC oscillator. There are two pads from the Port A which usable as an

input clock. External clock selection is performed with bit SelExtHFck = '1' and the clock source is chosen with bit

SelHFckSource

, both in register RegSys2.

• PA[4] if SelHFckSource = ‘0’ in RegSys2

• PA[5] if SelHFckSource = ‘1’ in RegSys2

If using one of these pads as clock source, it must be configured as input. Pull resistor selection remains available.

Using

the

loop

function,

external

oscillator

can

build

using

PA[4]

clock

input.

The clock switching is based on the scheme as shown in Figure 17. Synchronous Clock switching .

8.2.2.1

Switching from RC to external clock

A glitch free clock-switching scheme is implemented. Switch over procedure:

st:

CPU writes clock selection change bit SelExtHFck

nd:

on the next falling edge of the current clock the clock signal is forced '0'.

rd:

the newly selected input will become the clock source at its following falling edge.

Figure 17. Synchronous Clock switching

The clock selection output will switchover to the new clock on its next falling clock edge when the initial selected clock has

been disabled.

After switchover to external clock, the RC oscillator can be stopped to save current consumption.

SelExtClk1

RC Osc

Ext clock

Clock

RC_osc

Force 0

Ext clock

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