EM6819 datasheet(55/154 Pages) EMMICRO

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EM6819

07 /09 – rev.D

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10.

PRESCALER1

The prescaler1 is a 15 stage clock divider. It is typically used to deliver the input clocks to the digital peripherals (timers,

SPI, etc..). Its last stage output is on 1Hz (at 32768Hz input clock) and therefore most often used to construct a RTC

(Real Time Clock) system.

It can also be used as a free running counter by reading the current status of Pr1Ck0(MSB) to Pr1Ck7(LSB) in register

RegPresc1Val.

10.1 PRESCALER1 CLOCK SELECTION

The Prescaler1 input clock Ck_Pr1 is derived from divided or undivided Ck_Hi or Ck_Lo input clock. Below table is an

overview of the different prescaler1 clocking possibilities. The prescaler clock divider selection is done in register

RegClockCfg3 bits SelCkPr1.

SelCkPr1

Prescaler1 Clock

000

Ck_Hi (divided by 1)

001

Ck_Hi divided by 2

010

Ck_Hi divided by 4

011

Ck_Hi divided by 8 (default)

100

Ck_Lo (divided by 1)

Others

Ck_Hi divided by 8

The default Prescaler1 clock source after system reset (ResSys) shall be Ck_Hi divided by 8 (selection 0x3).

It is possible to run the 15 stage precaler1 on 13 stages only. This is typically used when connecting the RC_8K

oscillator as the prescaler1 clock source and allow to keep the nominal prescaler output frequencies as if there would be

an 32kHz Xtal oscillator connected (prescaler at 15 stages). The prescaler1 length selection is done in register

RegPrescCfg bit Presc1Len (‘0’= 15 stages, ‘1’=13 stages).

The Signals Pr1Ck14 and Pr1Ck13 are thus not influenced by the shortening.

Assuming a Prescaler1 with N stages, then the signal Pr1Ck[N] is the input of the first stage, Pr1Ck[N-1] is the output of

the first stage (input divided by 2) and Pr1Ck0 is the output of the last stage (the lowest frequency). This leads to

following clock source name definitions.

Presc1Len = ‘0’

Presc1Len = ‘1’

Prescaler1

stage

Clock Name

Division by

Fout

Division by

Fout

Prescaler source:

Pr1Ck15

2^0

32K

2^0

Stage 1

Pr1Ck14

2^1

16K

2^1

Stage 2

Pr1Ck13

2^2

Stage 3

Pr1Ck12

2^3

2^1

Stage 4

Pr1Ck11

2^4

2^2

Stage 5

Pr1Ck10

2^5

2^3

Stage 6

Pr1Ck9

2^6

512

2^4

512

Stage 7

Pr1Ck8

128

2^7

256

2^5

256

Stage 8

Pr1Ck7

256

2^8

128

2^6

128

Stage 9

Pr1Ck6

512

2^9

128

2^7

Stage 10

Pr1Ck5

2^10

256

2^8

Stage 11

Pr1Ck4

2^11

512

2^9

Stage 12

Pr1Ck3

2^12

2^10

Stage 13

Pr1Ck2

2^13

2^11

Stage 14

Pr1Ck1

16K

2^14

2^12

Stage 15

Pr1Ck0

32K

2^15

2^13

The frequencies Fout given in this table are based on 32 KHz clock selection as a prescaler1 input source.

EM6819 Datasheet(PDF) 55 Page - EM Microelectronic - MARIN SA

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