PCF2123TS datasheet(34/55 Pages) NXP

34 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

34 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

In Figure 26, the Months and Years registers are read. In this example, pins SDI and SDO

are not connected together. For this conﬁguration, it is important that pin SDI is never left

ﬂoating. It must always be driven either HIGH or LOW. If pin SDI is left open, high IDD

currents may result. Short transition periods in the order of 200 ns will not cause any

problems.

9.14.1 Interface watchdog timer

During read/write operations, the time counting circuits are frozen. To prevent a situation

where the accessing device becomes locked and does not clear the interface by setting

pin CE LOW, the PCF2123 has a built in watchdog timer. Should the interface be active for

more than 1 s from the time a valid subaddress is transmitted, then the PCF2123 will

automatically clear the interface and allow the time counting circuits to continue counting.

CE must return LOW once more before a new data transfer can be executed.

Fig 25. Serial bus write example

001aaf915

address

counter

SDI

SCL

seconds data 45BCD

minutes data 10BCD

R/W

addr 02HEX

Fig 26. Serial bus read example

001aaf916

address

counter

SDO

SDI

SCL

months data 11BCD

years data 06BCD

R/W

addr 07HEX

PCF2123TS Datasheet(PDF) 34 Page - NXP Semiconductors