NXP Semiconductors PCA8565, PCF85x3, PCF2123, PCA2125 user manual Partial circuit switch down

NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

layer 1	guard ring	oscillator	bypass
		capacitor	capacitor

SMD

CRYSTAL

OSCI		VDD
OSCO	PCF8563TS	CLK
INT		SCL
VSS		SDA

ground via GND area 001aai849

Fig 15. PCB layout proposal for PCF8563 using SMD components and guard ring

Remark: 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.

15. Partial circuit switch down

The I2C-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 VDD, refer to Fig 16.

Therefore it is possible to partially switch off VDD 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 VDD 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 VDD but has protection diodes from the I2C- bus pins to VDD ! Battery backup will work, but with the RTC powered down the I2C-bus may get stuck.

The SPI-bus RTC circuits PCF2123 and PCA2125 have on pins SDI and SCL only one clamping diode from VSS to the pin. On pin SDO an additional clamping diode to VDD is integrated. This allows for partial circuit switch down where the RTC still runs and the rest of the application is powered down.

UM10301_1		© NXP B.V. 2008. All rights reserved.
User manual	Rev. 01 — 23 December 2008	38 of 52