EM6620WS27 datasheet(11/56 Pages) EMMICRO

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EM6620

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5. Oscillator and Prescaler

5.1 Oscillator

A built-in crystal oscillator generates the system operating clock for the CPU and peripheral blocks, from an

externally connected crystal (typically 32.768kHz). The oscillator circuit is supplied by the regulated voltage,

Vreg. In sleep mode the oscillator is stopped.

EM’s special design techniques guarantee the low current consumption of this oscillator. The external

impedance between the oscillator pads must be greater than 10 MOhm. Connection of any other components

to the two oscillator pads must be confirmed by EM Microelectronic-Marin SA.

5.2 Prescaler

The prescaler consists of fifteen elements divider chain which delivers clock signals for the peripheral circuits

such as timer/counter, buzzer, LCD voltage multiplier, debouncer and edge detectors, as well as generating

prescaler interrupts. The input to the prescaler is the system clock signal. Power on initializes the prescaler to

Hex(0001).

Table 5.2.1 Prescaler Clock Name Definition

Function

Name

32 KHz Xtal

Function

Name

32 KHz Xtal

System clock

Ck[16]

32768 Hz

System clock / 256

Ck[8]

128 Hz

System clock / 2

Ck[15]

16384 Hz

System clock / 512

Ck[7]

64 Hz

System clock / 4

Ck[14]

8192 Hz

System clock / 1024

Ck[6]

32 Hz

System clock / 8

Ck[13]

4096 Hz

System clock / 2048

Ck[5]

16 Hz

System clock/ 16

Ck[12]

2048 Hz

System clock / 4096

Ck[4]

8 Hz

System clock / 32

Ck[11]

1024 Hz

System clock / 8192

Ck[3]

4 Hz

System clock / 64

Ck[10]

512 Hz

System clock / 16384

Ck[2]

2 Hz

System clock / 128

ck [9]

256 Hz

System clock / 32768

Ck[1]

1 Hz

Table 5.2.2 Control of Prescaler Register RegPresc

Bit

Name

Reset

R/W

Description

PWMOn

R/W

see 10 bit counter

ResPresc

R/W

Write Reset prescaler

1 -> Resets the divider chain

from Ck[14] down to

Ck[2], sets Ck[1].

0 -> No action.

The Read value is always '0'

PrIntSel

R/W

Interrupt select.

0 -> Interrupt from Ck[4]

1 -> Interrupt from Ck[6]

DebSel

R/W

Debouncer clock select.

0 -> Debouncer with Ck[8]

1 -> Debouncer with Ck[11] or

Ck[14]

With DebSel = 1 one may choose either the Ck[11] or Ck[14] debouncer frequency by selecting the

corresponding metal mask option. Relative to 32kHz the corresponding max. debouncer times are then 2 ms or

0.25 ms. For the metal mask selection refer to chapter 16.1.5.

Switching the PrIntSel may generate an interrupt request. Avoid it with MaskIRQ32/8 = 0 selection during the

switching operation.

Figure 8. Prescaler Frequency Timing

System Clock

Ck[16]

Ck[15]

Ck[14]

Horizontal Scale Change

Ck[2]

Ck[1]

First positive edge of 1 Hz clock is 1s after

the falling reset edge

Prescaler Reset

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