Nxp Semiconductors PCF2123 用户手册
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
• 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
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
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