NXP Semiconductors LPC24XX UM10237 - Manual (Page 659)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 659 of 792
NXP Semiconductors
UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
7. RTC usage notes
The RTC may be clocked by either the 32.786 kHz RTC oscillator, or by the APB
peripheral clock (PCLK) after adjustment by the reference clock divider.
If the RTC is used, VBAT must be connected to either pin V
DD(3V3)
or an independent
power supply (external battery). Otherwise, VBAT should be left floating. No provision is
made in the LPC2400 to retain RTC status upon the VBAT power loss, or to maintain time
incrementation if the clock source is lost, interrupted, or altered.
Since the RTC operates using one of two available clocks (the APB clock (PCLK) or the
32 kHz signal coming from the RTCX1-2pins), any interruption of the selected clock will
cause the time to drift away from the time value it would have provided otherwise. The
variance could be to actual clock time if the RTC was initialized to that, or simply an error
in elapsed time since the RTC was activated.
While the signal from RTCX1-2 pins can be used to supply the RTC clock at anytime,
selecting the PCLK as the RTC clock and entering the Power-down mode will cause a
lapse in the time update. Also, feeding the RTC with the PCLK and altering this timebase
during system operation (by reconfiguring the PLL, the APB divider, or the RTC prescaler)
will result in some form of accumulated time error. Accumulated time errors may occur in
case RTC clock source is switched between the PCLK to the RTCX pins, too.
Once the 32 kHz signal from RTCX1-2 pins is selected as a clock source, the RTC can
operate completely without the presence of the APB clock (PCLK). Therefore, power
sensitive applications (i.e. battery powered application) utilizing the RTC will reduce the
Table 582. Prescaler cases where the Integer Counter reload value is incremented
Fraction Counter PREFRAC Bit
14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
--- ---- ---- ---1 1 - - - - - - - - - - - - - -
--- ---- ---- --10 - 1 - - - - - - - - - - - - -
--- ---- ---- -100 - - 1 - - - - - - - - - - - -
--- ---- ---- 1000 - - - 1 - - - - - - - - - - -
--- ---- ---1 0000 - - - - 1 - - - - - - - - - -
--- ---- --10 0000 - - - - - 1 - - - - - - - - -
--- ---- -100 0000 - - - - - - 1 - - - - - - - -
--- ---- 1000 0000 - - - - - - - 1 - - - - - - -
--- ---1 0000 0000 - - - - - - - - 1 - - - - - -
--- --10 0000 0000 - - - - - - - - - 1 - - - - -
--- -100 0000 0000 - - - - - - - - - - 1 - - - -
--- 1000 0000 0000 - - - - - - - - - - - 1 - - -
--1 0000 0000 0000 - - - - - - - - - - - - 1 - -
-10 0000 0000 0000 - - - - - - - - - - - - - 1 -
100 0000 0000 0000 - - - - - - - - - - - - - - 1