EM6812F2TP24B datasheet(26/81 Pages) EMMICRO

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8.3

Low frequency clock source

Before switching on a low frequency clock source, make sure that the pre-devised high frequency clock is at least 8 times

higher than the expected low frequency clock. Once the low frequency clock is up and running the high frequency clock

source may be stopped if not needed anymore. The clock switching is based on the scheme as shown in Figure 17.

Synchronous Clock switching.

There are several conditions which all activate the 2 possible low frequency clock sources, Crystal oscillator or external

clock.

For Crystal oscillator:

EnXTAL = ’1’

: Forces the XTAL on (must never be set if no XTAL present)

Sel32k = ‘1’

: CPU on low frequency clock source (F1)

Pr1CkSel[2:0] = ‘000’ : Prescaler 1 running on low frequency clock source (F1)

Pr2CkSel = ‘0’

: Prescaler 2 running on low frequency clock source (F1)

For external clock:

EnXtal = ‘0’ AND SelExtLFCk = ‘1’:

To be able to run on external low frequency clock, above condition must be true before any of the crystal oscillator selection

conditions is true.

Do not select a low frequency clock source if this source is not present or does not vehicule a clock.

8.3.1

Crystal oscillator

The Xtal oscillator is the main clock generator of the low frequency domain. It is off by default.

Writing EnXtal = ‘1’ in the register RegSys2 enables the Xtal oscillator. Now the Xtal management system waits for a

defined number of oscillation periods before it allows using this source as a CPU clock. This phase is called Xtal cold-start.

It is possible to set this wait time using OPTCldStart[1:0] in RegSys1 between typically 1s and ¼ second. The peripheral

Crystal derived clocks are not blocked during cold-start (Prescaler inputs). This cold start time is also active after every

crystal oscillator re-start.

Whenever a peripheral block or the CPU get a low frequency clock selection – but not external low frequency clock

(SelExtLFCk= ‘0’) – then the crystal oscillator gets switched on also even if EnXtal is ‘0’. Again every start-up of the crystal

oscillator is followed with a cold-start period. To avoid frequent cold start delays, one may permanantly switch on the Xtal

with EnXtal=’1’.

Table 19. Table of Xtal cold-start duration and selection.

Number of cold-start pulses

Typical wait time

OPTCldStart[1]

OPTCldStart[0]

32768

24576

3/4s

16384

1/2s

8192

1/4s

During the startup phase, the CPU can check if the Xtal cold start is done or not reading FlagXtal in RegSys1. The Xtal is

not available as a CPU clock source while FlagXtal = ‘0’. After the cold-start time, this flag becomes ‘1’ and thus allows to

switchover.

Note:

Avoid high frequency operation and fast transitions on PA7 while the 32kHz Crystal Oscillator is running. PA7 induced

crosstalk on OscOut terminal (PCB, Package) can influence the good Crystal operation.

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