NXP Semiconductors UM10301 PCF2123 - Manual (Page 47)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 47 of 52
Fig 22. Sequence of events (example READ)
From this follows:
• A I 2
C read must be terminated within one second of initiation;
• The RTC will automatically terminate the read if it remains active for longer than one
second;
• Each time auto termination occurs, the RTC looses one second;
• The signal BLOCK is also active during a write. A write must also last less than one
second;
• BLOCK is necessary for a write since the registers must not update whilst new data
is being written. That is impossible anyway, since the clock is switched from the 1 Hz
clock to the internal I
2
C clock.
Remark : The RTCs allow to set the word address in order to address a particular
register, for example the seconds register. If the application software is written such that
separate bus accesses are used to read the individual time/date registers it is possible
that after reading/writing those registers the time as read or written by the system
controller is not correct. This is because time counter freeze – implemented to avoid such
errors - is active only during one access, starting from the START condition and ending
when the STOP condition occurs. Now still the errors mentioned in the introduction of this