Philips P89LPC907 用户手册

2003 Dec 8     

47

The P89LPC906/907/908 has a simple Real-time clock/system timer that allows a user to continue running an accurate timer 
while the rest of the device is powered down. The Real-time clock can be an interrupt or a wake-up source (see Figure 6-2). The 
Real-time clock is a 23-bit down counter. 

On the P89LPC906 the clock source for this counter can be either CCLK or the XTAL1-2 oscillator (XCLK) . On the  P89LPC907  
and P89LPC908 devicesthe clock source for this counter is CCLK. Please refer to Figure 2-3 "Block Diagram of Oscillator Control 
- P89LPC906" in  section "Clocks" on page 25. CCLK can have either the XTAL1-2 oscillator, the internal RC oscillator, or the 
Watchdog oscillator as its clock source.  If the XTAL1-2 oscillator is used for producing CCLK, the RTC will use either  the XTAL1-
2 oscillator’s output or CCLK as its clock source. The possible clocking combinations are shown in Table , below.

There are three SFRs used for the RTC:

• RTCCON - Real-time clock control.
• RTCH - Real-time clock counter reload high (bits 22-15).
• RTCL - Real-time clock counter reload low (bits 14-7).
The Real-time clock/system timer can be enabled by setting the RTCEN (RTCCON.0) bit. The Real-time clock is a 23-bit down 
counter (initialized to all 0’s when RTCEN = 0) that is comprised of a 7-bit prescaler and a 16-bit loadable down counter. When 
RTCEN is written with ’1’, the counter is first loaded with (RTCH,RTCL,’1111111’) and will count down. When it reaches all 0’s, 
the counter will be reloaded again with (RTCH,RTCL,’1111111’) and a flag - RTCF (RTCCON.7) - will be set.

Any write to RTCH and RTCL in-between the Real-time clock reloading will not cause reloading of the counter. When the current 
count terminates, the contents of RTCH and RTCL will be loaded into the counter and the new count will begin. An immediate 
reload of the counter can be forced by clearing the RTCEN bit to ’0’ and then setting it back to ’1’ .