UM0434 datasheet(67/390 Pages) STMICROELECTRONICS

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UM0434

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The TB is implemented to satisfy the following requirements:

●

Loading a GPR from the TB has no effect on the accuracy of the TB.

●

Storing a GPR to the TB replaces the value in the TB with the value in the GPR.

Book E does not specify a relationship between the TB update frequency and other

frequencies, such as the CPU clock or bus clock. The TB update frequency does not have to

be constant. One of the following is required to ensure that system software can keep time

of day and operate interval timers:

●

The system provides an (implementation-dependent) interrupt to software when the

update frequency of the TB changes and a way to determine the current update

frequency.

●

The update frequency of the TB is under the control of system software.

Note:

Disabling the TB or making reading the time base privileged prevents the TB from being

used to implement a covert channel in a secure system. If the operating system initializes

the TB on power-on to some reasonable value and the update frequency of the TB is

constant, the TB can be used as a source of values that increase at a constant rate, such as

for time stamps in trace entries.

Even if the update frequency is not constant, values read from the TB are monotonically

increasing (except when the TB wraps from 264 – 1 to 0). If a trace entry is recorded each

time the update frequency changes, the sequence of TB values can be post-processed to

become actual time values.

Successive readings of the TB may return identical values.

The TB is useful for timing reasonably short sequences of code (a few hundred instructions)

and for low-overhead time stamps for tracing.

4.11.4

Decrementer register

DEC, shown in

Table 45, is a decrementing counter that is enabled by setting HID0[TBEN].

The decrementer and time base update at the same frequency, which is selected in

HID0[SEL_TBCLK]. It provides way to signal a decrementer interrupt after a specified

period unless one of the following occurs:

●

Software alters DEC in the interim.

●

The TB update frequency changes.

DEC is typically used as a general-purpose software timer. The decrementer auto-reload

100 MHz

------------------------

5.90

onds

sec

(approximately 187,000 years)

Table 45.

Decrementer register (DEC)

Field

Decrementer value

Reset

Undefined on

m_por assertion, unchanged on p_reset_b assertion

R/W

SPR

SPR 22

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