STR720 datasheet(152/401 Pages) STMICROELECTRONICS

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STR720 - REAL TIME CLOCK (RTC)

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14.3.2

Reset procedure

All system registers are asynchronously reset by either an external Reset event (RSTIN

device input pin, forced low) or an internal Reset event (WDG or Software Reset), except

RTCA, RTCP, RTCCNT registers. To reset them, the RTCRST device input pin has to be

forced low: RTCA, RTCP and RTCCNT registers are reset to FFFF_FFFF_FFFFh, 0_8000h

and 0000_0000_0000h respectively.

14.3.3

Free-running mode

After reset the peripheral will enter in free-running mode. In this operating mode the RTC

programmable counter starts counting. Interrupt flags are activated too but, since interrupt

signals are masked, there is no interrupt generation. Interrupt signals must be enabled by

setting the appropriate bits in RTCCR register. In order to avoid spurious interrupt generation

it is recommended to clear old interrupt requests before enabling them.

14.3.4

Configuration mode

To write to the RTCCNT, RTCP and RTCA registers, the peripheral must enter Configuration

mode. This is obtained writing CNF bit at ‘1’.

In addition, the writing operation is only enabled if the previous writing operation is finished. To

enable software to detect this situation the status bit RTOFF is provided in the RTCCR control

written to the RTC counters until the status bit value is ‘0’ since as long as RTOFF bit is ‘0’ all

attempts to write any RTC register will fail.

A special case of configuration mode is entered also as soon as RTCCR register is written

with CNF at ‘0’. In this case RTOFF becomes ‘0’ immediately and it remains as such for 1 or 2

CLK_AF cycles, preventing any subsequent modification until its normal state is restored.

During this special configuration mode, only RTCCR contents are updated, while RTCCNT,

RTCP and RTCA registers remain unaffected.

14.4 Register description

The RTC registers can only be accessed by word. The reserved bits can not be written and

they are always read at ‘0’, unless otherwise specified.

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