AN3133 datasheet(7/43 Pages) STMICROELECTRONICS

7 / 43 page

AN3133

Real-time clock overview

Doc ID 16956 Rev 5

7/43

The STM8L/STM8AL RTC calendar is provided in BCD format: this saves from having to

perform binary to BCD software conversion routines, which use significant program memory

space and CPU-time that may be critical in certain real-time applications.

Figure 2.

Example of calendar display on an LCD

1.1.1

Sub-seconds

Sub-second values are read from RTC registers RTC_SSRH and RTC_SSRL.

The sub-seconds field is adjustable and can be up to 0xFFFF, or 65535 in decimal,

depending on the value set on RTC_SSRH and RTC_SSRL.

SSSS[15:0] (included in RTC_SSRH/RTC_SSRL) is the value in the synchronous

prescaler’s counter. Given that this counter continually counts down to zero and then

reloads the value from RTC_SPRE[14:0], following is the formula for calculating the fraction

of a second:

Second fraction = (PREDIV_S – SS) / (PREDIV_S + 1)

For example: If RTC_SPRE[14:0] = 0x7FFF, then calendar sub-seconds SS starts

downcounting from 0x7FFF to 0. which means that the sub-second resolution is equal to

1/(PREDIV_S + 1) = 30.517578125 µs.

Note:

The sub-seconds field can be up to 0xFFFF when using the “shift control” feature, by adding

0x7FFF sub-second fractions.

SS can be larger than RTC_SPRE only after a shift operation. In this case, the “second

fraction” is negative which (intuitively) indicates that the correct time/date is at least a

second less than indicated by RTC_TRx/RTC_DRx.

1.1.2

Fine RTC calendar adjustments

For accurate RTC adjustments, a “shift control” feature enables the user to add/subtract a

number of sub-seconds to/from the current calendar.

The shift is used to synchronize the RTC to a master clock: SS[15:0] (included in

RTC_SSRH/RTC_SSRL) can be read with RTCCLK/PREDIV_A resolution, and a correction

can be applied with RTCCLK/(PREDIV_A+1).

The number of sub-seconds that can be added is “1s– n” and the number of sub-seconds

that can be subtracted is “–n” (where n can be up to 32767 (0x7FFF) sub-seconds).

RTC calendar adjustment examples

If RTC_SPRE[14:0] = 1023, RTC_APRE = 31 and RTC current calendar time is 3h, 25mn,

32s and SS = 511, the calendar time is read as 3h, 25mn, 32s and 500ms (03h25’32’’500)

since (1023–511)*32/32768 = 500 ms.

11:15:28 PM

WED NOV 19 2010

AN3133 数据表(PDF) 7 Page - STMicroelectronics

AN3133 数据表(HTML) 7 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

数据表下载