EM6580 datasheet(11/55 Pages) EMMICRO

11 / 55 page

EM6580

03/07, rev. J

www.emmicroelectronic.com

5. Oscillator and Prescaler

5.1 RC Oscillator or external Clock

EM6580 can use the internal RC oscillator or external clock source for its operation.

The built-in RC oscillator without external components generates the system operating clock for the CPU and

peripheral blocks. The RC oscillator is supplied by the regulated voltage.

The RC oscillator frequency can be chosen from 5 possibilities with a register with 2 basic frequencies.

These are typically 32kHz, 64kHz, 128kHz, 256kHz or 500kHz for 32kHz basic frequency or 50kHz, 100kHz,

200kHz, 400kHz or 800kHz for 50kHz basic frequency . Depending on the selected RC frequency. A special 4

stage freq. divider is available to be able to deliver to Prescaler which generates all System clock except CPU

clock, a frequency close to 32 or 50 kHz to keep the peripheral timing as close as possible to specifications.

After POR the circuit always starts with the internal RC oscillator, but it can be switched to the external clock by

setting the ExtCPUclkON bit in the register RegPresc. The external clock is input at PA[1] and must be in

range from min. 10Khz to max. 1MHz. With this external frequency input all timing for peripherals change and

the special 4 stage freq. divider must be adapted to best suit the applied external frequency to keep 32/50kHz

System clock as close as possible. The system clock must be less than 64kHz. The external clock source

must be a square wave with full amplitude from Vss to Vdd. See Table 5.2.2 for advised special divisions

depending on the external clock frequency.

Switching from internal RC oscillator to External clock or back from External clock to RC oscillator is made

without generating a glitch on the internal clock. Once the circuit is running on the external Clock one can

disable the RC oscillator by setting the RCoscOff bit in RegSCntl2 to ‘1’.

In sleep mode the oscillator is stopped. It can be stopped also by setting the RCoscOff bit. This bit can be set

only if ExtCPUClkOn was set before, indicating that the CPUck was switched from the internal RC oscillator to

the external clock which MUST be present. If the External Clock stops without going into Sleep mode first the

EM6580 can block and only POR can reset it.

Figure 8. below shows the connection of the RC oscillator and external clock and generation of CPUclk and

System clock = SysClk which is divided by the special 4 stage Freq. Divider if needed as described in 5.2 and

prescaler described in 5.3.

Figure 6. Clock source for CPU or system peripherals

MUX

RC oscillator

32kHz – 512kHz

50kHz - 800kHz

metal option selection

RCClk

External clock

10kHz – 800kHz

ExtCPUclkON

Frequency

divider

MUX

Automatic clock selection

SysClk = 32kHz or 50kHz

depending on selected

RCClk selection

CPUclk

PRESCALER

LOW 15 stages divider Ck[15:1]

MUX

foutSel[1:0]

MUX

Sout

PAout[2]

ResSys

PA[2]

To Peripherals

Ck[15:1]

SysClk

PA[1]

MUX

PAout[5]

PA[5]

Ck[1]

Ck[12]

SysClk

CpuClk

Ck[14]

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