ST20-GP6 datasheet(67/123 Pages) STMICROELECTRONICS

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ST20-GP6

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Low power timer

The timer keeps track of real time, even when the internal clocks are stopped. The timer is a 64-bit

counter which runs off an external clock (

LowPowerClockIn). This clock rate must not be more

than one eighth of the system clock rate.

Low power alarm

There is also a 40-bit low power alarm counter. A write to the

LPAlarmStart register starts the low

power alarm counter and the ST20-GP6 enters low power mode. When the counter has counted

down to zero, assuming no other valid wake-up sources occur ﬁrst, the ST20-GP6 exits low power

mode and the global clocks are turned back on. Whilst the clocks are turned off the

LowPowerSta-

tus pin is high, otherwise it is low.

11.2 Low power conﬁguration registers

The low power controller is allocated a 4k block of memory in the internal peripheral address

space. Information on low power mode is stored in registers as detailed in the following section.

The registers can be examined and set by the

devlw (device load word) and devsw (device store

word) instructions, see Table 7.19 on page 49. Note, they can not be accessed using memory

instructions.

LPTimerLS and LPTimerMS

The

LPTimerLS and LPTimerMS registers are the least signiﬁcant word and most signiﬁcant word

of the

LPTimer register. This enables the least signiﬁcant or most signiﬁcant word to be written

independently without affecting the other word.

Table 11.1

LPTimerLS register format

Table 11.2

LPTimerMS register format

When the

LPTimer register is written, the low power timer is stopped and the new value is avail-

able to be written to the low power timer.

LPTimerStart

A write to the

LPTimerStart register starts the low power timer counter. The counter is stopped

and the

LPTimerStart register reset if either counter word (LPTimerLS and LPTimerMS)iswrit-

ten.

LPTimerLS

LPC base address + #400

Read/Write

Bit

Bit ﬁeld

Function

31:0

LPTimerLS

Least signiﬁcant word of the low power timer.

LPTimerMS

LPC base address + #404

Read/Write

Bit

Bit ﬁeld

Function

31:0

LPTimerMS

Most signiﬁcant word of the low power timer.

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