L9908 datasheet(45/153 Pages) STMICROELECTRONICS

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5.9

Internal clock

Table 35. Internal clock electrical characteristics

Symbol

Parameter

Test condition

Min

Typ

Max

Unit

CLK_freq

System Clock Frequency

CLK_SSM_EN = 0

MHz

CLK_freq_acc

System Clock Frequency Accuracy

CLK_SSM_EN = 0

-5

5.9.1

Spread Spectrum Modulation (SSM)

L9908 clocks generator implements a frequency modulation (Spread Spectrum Modulation) feature to reduce the

main logic and Charge Pumps emissions around the main frequency by spreading the power spectrum over a

larger frequency range.

Table 36. Clock spread spectrum electrical characteristics

Symbol

Parameter

Min

Typ

Max Unit

Notes

SSM_spreading Spread Spectrum Modulation spreading range

Not subject to production test

SSM_freq

Spread Spectrum Modulating frequency

156.25

kHz Not subject to production test

Note:

All parameters are guaranteed and tested, in the voltage ranges reported above in Table 5 unless otherwise

specified. Where not specified the parametrical operating range equals the functional operating range.

The clock spread spectrum modulation function can be enabled by the dedicated SPI bit CLK_SSM_EN in the

Table 37. Clock spread spectrum enable bit

CLK_SSM_EN

Description

Clock SSM Disabled (Default)

Clock SSM Enabled

Note:

When the clock spread spectrum is enabled the internal digital timings are affected by a timing jitter equal to the

selected spread spectrum spreading range.

5.9.2

Internal Clock Monitor (ICM)

The correct operation and the precision of the internal synchronization signal are safety relevant and therefore

L9908 implements a monitoring unit to detect abnormal deviation of the clock signal frequency. The monitoring

unit is based on the use of two oscillators and two monitor chains (a Main one and an Auxiliary one) to avoid

common-cause failures.

Each monitor chain compares the other clock period, the first one samples CLK1 by means of CLK2 and the

second one samples CLK2 by means of CLK1.

If TCLK1 ≥ ICM_timeout_th: the flag INT_RST and CLK1_TIME_OUT are set.

If TCLK2 ≥ ICM_timeout_th: the flag CLK2_TIME_OUT is set.

The error flags remain set until the failure condition is removed and the flags are cleared by the SPI command.

The correct level of clock monitor is safety relevant and then a self-check procedure is implemented on its

monitor.

Table 38. Internal clock monitor electrical characteristics

Symbol

Parameter

Min

Typ

Max

Unit

Notes

ICM_err_th

ICM frequency mismatch threshold accuracy

Application information

ICM_timeout_th

ICM timeout accuracy

Application information

L9908

Internal clock

DS13546 - Rev 3

page 45/153

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