UM10139

Philips SemiconductorsVolume 1UM10139Chapter 19: RTC19.5 RTC usage notesTable 275: Alarm registersName Size Description Access AddressALSEC 6 Alarm value for Seconds R/W 0xE002 4060ALMIN 6 Alarm value for Minutes R/W 0xE002 4064ALHOUR 5 Alarm value for Hours R/W 0xE002 4068ALDOM 5 Alarm value for Day of Month R/W 0xE002 406CALDOW 3 Alarm value for Day of Week R/W 0xE002 4070ALDOY 9 Alarm value for Day of Year R/W 0xE002 4074ALMON 4 Alarm value for Months R/W 0xE002 4078ALYEAR 12 Alarm value for Years R/W 0xE002 407CIf the RTC is used, V BAT must be connected to either pin V 3 or an independent powersupply (external battery). Otherwise, V BAT should be tied to the ground (V SS ). No provisionis made in the LPC2141/2/4/6/8 to retain RTC status upon the V BAT power loss, or tomaintain time incrementation if the clock source is lost, interrupted, or altered.Since the RTC operates using one of two available clocks (the VPB clock (PCLK) or the32 kHz signal coming from the RTCX1-2pins), any interruption of the selected clock willcause the time to drift away from the time value it would have provided otherwise. Thevariance could be to actual clock time if the RTC was initialized to that, or simply an errorin 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 alapse in the time update. Also, feeding the RTC with the PCLK and altering this timebaseduring system operation (by reconfiguring the PLL, the VPB divider, or the RTC prescaler)will result in some form of accumulated time error. Accumulated time errors may occur incase 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 canoperate completely without the presence of the VPB clock (PCLK). Therefore, powersensitive applications (i.e. battery powered application) utilizing the RTC will reduce thepower consumption by using the signal from RTCX1-2 pins, and writing a 0 into thePCRTC bit in the PCONP power control register (see Section 3.9 “Power control” on page35).19.6 Reference clock divider (prescaler)The reference clock divider (hereafter referred to as the prescaler) allows generation of a32.768 kHz reference clock from any peripheral clock frequency greater than or equal to65.536 kHz (2 × 32.768 kHz). This permits the RTC to always run at the proper rateregardless of the peripheral clock rate. Basically, the Prescaler divides the peripheralclock (PCLK) by a value which contains both an integer portion and a fractional portion.The result is not a continuous output at a constant frequency, some clock periods will beone PCLK longer than others. However, the overall result can always be 32,768 countsper second.© Koninklijke Philips Electronics N.V. 2005. All rights reserved.User manual Rev. 01 — 15 August 2005 282