ST10F271B datasheet(148/180 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F271B/ST10F271E

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24.8.4

Prescaler operation

When pins P0.15-13 (P0H.7-5) equal ’001’ during reset, the CPU clock is derived from the

internal oscillator (input clock signal) by a 2:1 prescaler.

The frequency of fCPU is half the frequency of fXTAL and the high and low time of fCPU (i.e.

the duration of an individual TCL) is defined by the period of the input clock fXTAL.

The timings listed in the AC Characteristics that refer to TCL therefore can be calculated

using the period of fXTAL for any TCL.

Note that if the bit OWDDIS in SYSCON register is cleared, the PLL runs on its free-running

frequency and delivers the clock signal for the Oscillator Watchdog. If bit OWDDIS is set,

then the PLL is switched off.

24.8.5

Direct drive

When pins P0.15-13 (P0H.7-5) equal ’011’ during reset the on-chip phase locked loop is

disabled, the on-chip oscillator amplifier is bypassed and the CPU clock is directly driven by

the input clock signal on XTAL1 pin.

The frequency of CPU clock (fCPU) directly follows the frequency of fXTAL so the high and

low time of fCPU (i.e. the duration of an individual TCL) is defined by the duty cycle of the

input clock fXTAL.

Therefore, the timings given in this chapter refer to the minimum TCL. This minimum value

can be calculated by the following formula:

For two consecutive TCLs, the deviation caused by the duty cycle of fXTAL is compensated,

so the duration of 2TCL is always 1/fXTAL.

The minimum value TCLmin has to be used only once for timings that require an odd number

of TCLs (1,3,...). Timings that require an even number of TCLs (2,4,...) may use the formula:

The address float timings in Multiplexed bus mode (t11 and t45) use the maximum duration of

TCL (TCLmax = 1/fXTAL x DCmax) instead of TCLmin.

Similarly to what happen for Prescaler Operation, if the bit OWDDIS in SYSCON register is

cleared, the PLL runs on its free-running frequency and delivers the clock signal for the

Oscillator Watchdog. If bit OWDDIS is set, then the PLL is switched off.

24.8.6

Oscillator watchdog (OWD)

An on-chip watchdog oscillator is implemented in the ST10F271. This feature is used for

safety operation with external crystal oscillator (available only when using direct drive mode

with or without prescaler, so the PLL is not used to generate the CPU clock multiplying the

frequency of the external crystal oscillator). This watchdog oscillator operates as following.

The reset default configuration enables the watchdog oscillator. It can be disabled by setting

the OWDDIS (bit 4) of SYSCON register.

When the OWD is enabled, the PLL runs at its free-running frequency, and it increments the

watchdog counter. On each transition of external clock, the watchdog counter is cleared. If

TCL

min

⁄

XTALl

xlDC

min

duty cycle

2TCL

XTAL

⁄

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