Nxp Semiconductors PCA2125 用户手册

 

 

UM10301_1 

© NXP B.V. 2008. All rights reserved.

SMD

CRYSTAL

SDA

CLK

V

DD

SCL

V

SS

OSCO

OSCI

INT

layer 1

ground via

GND area

bypass

capacitor

oscillator

capacitor

guard ring

 

 

: Take precautions when cleaning PCBs containing tuning fork crystals using 

ultrasound. The resonance vibration may damage the crystal. Consult the supplier of the 
crystal in case of doubt. 

 

 

The I

2

C-bus RTC circuits PCF8563, PCA8565 and PCF8593 have on the pads SDA, 

SCL and INT  a diode clamping circuit without an upper clamping diode to V

DD

, refer to 

Therefore it is possible to partially switch off V

DD

 such that the RTC is powered down or 

working at a lower supply voltage than the rest of the circuit without the risk that via such 
(upper) clamping diodes SDA, SCL and INT  would be pulled down as well. 

The other way around with only the RTC powered in order to keep time and the rest of 
the circuit switched off will be a more common situation. If during normal operation the 
complete application is powered by a certain V

DD

 of for example 3.3 V or 5 V, then during 

standby just the RTC can be operating and powered from a backup source as discussed 
in Chapter 13. See Fig 16 and Fig 18 plus Fig 19.  

PCF8583 has no protection diode from INT to V

DD

 but has protection diodes from the I

2

C-

bus pins to V

DD

 ! Battery backup will work, but with the RTC powered down the I

2

C-bus 

may get stuck. 

The SPI-bus RTC circuits PCF2123 and PCA2125 have on pins SDI and SCL only one 
clamping diode from V

SS

 to the pin. On pin SDO an additional clamping diode to V

DD

 is 

integrated. This allows for partial circuit switch down where the RTC still runs and the 
rest of the application is powered down.