NXP Semiconductors LPC24XX UM10237 - Manual (Page 660)

UM10237_4 © NXP B.V. 2009. All rights reserved.
User manual Rev. 04 — 26 August 2009 660 of 792
NXP Semiconductors
UM10237
Chapter 26: LPC24XX Real-Time Clock (RTC) and battery RAM
power consumption by using the signal from RTCX1-2 pins, and writing a 0 into the
PCRTC bit in the PCONP power control register (see
Section 4–3.4 “Power control” on
page 60 ).
Remark: Note that if the RTC is running from the 32 kHz signal and powered by VBAT, the
internal registers can be read. However, they cannot be written to unless the PCRTC bit in
the PCONP register is set to 1, see
Table 4–63 .
8. Battery RAM
The Battery RAM is a 2 kbyte static RAM residing on the APB bus. The address range is
0xE008 4000 to 0xE008 47FF.The SRAM can be accessed word-wise (32-bit) only.
The Battery RAM is powered from the VBAT pin along with the RTC, both of which exist in
a power domain that is isolated from the rest of the chip. This allows them to operate while
the main chip power has been removed.
9. RTC external 32 kHz oscillator component selection
The RTC external oscillator circuit is shown in Figure 26–136 . Since the feedback
resistance is integrated on chip, only a crystal, the capacitances C
X1
and C
X2
need to be
connected externally to the microcontroller.
Table 26–583 gives the crystal parameters that should be used. C
L
is the typical load
capacitance of the crystal and is usually specified by the crystal manufacturer. The actual
C
L
influences oscillation frequency. When using a crystal that is manufactured for a
different load capacitance, the circuit will oscillate at a slightly different frequency
(depending on the quality of the crystal) compared to the specified one. Therefore for an
accurate time reference it is advised to use the load capacitors as specified in
Table 26–583 that belong to a specific C
L
. The value of external capacitances C
X1
and
C
X2
specified in this table are calculated from the internal parasitic capacitances and the
C
L
. Parasitics from PCB and package are not taken into account.
Fig 136. RTC 32 kHz crystal oscillator circuit
RTCX1 RTCX2
LPC24xx
C
X1
C
X2
32 kHz
Xtal