Nxp semiconductors – NXP Semiconductors UM10301 PCF2123 User Manual
Page 5
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
5 of 52
In BCD every digit of the decimal system is represented by a 4-bit group. For example:
157
10
= 0001 0101 0111
BCD
This is not the same as binary representation. It is clear that BCD is not the most efficient
way of coding since every 4-bit group (nibble) could represent numbers 0 through 15, but
in BCD never represents numbers bigger than 9. But for some applications it is
convenient to use BCD and real time clocks are one such application.
Each 8-bit register contains two digits each represented by one nibble. Each 4-bit nibble
can represent the value of 0 up to 9 in BCD, but for some digits the maximum value to be
represented will be lower. The minute register for example will never have to count
higher than 59. The upper most digit can here be represented by 3 bits, freeing up one
bit that can be used to indicate something else.
Not all NXP real-time clocks have exactly the same register implementation and thus the
datasheet of the particular device should be consulted. As an example the register
organization of the PCF8563 is given below. Note that this is just one example and that
register organization of other types is not necessarily exactly the same.
Table 1.
Register overview PCF8563
Bit positions labelled as x are not implemented. When setting a register, also a value must be written for the ‘x’ bit positions.
When these are read back, the read back values may differ from what was previously written.
Bit positions labelled with 0 should always be written with logic 0; if read they could be either logic 0 or logic 1.
Address
Register name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
00
HEX
control / status 1
TEST1
0
STOP
0
TESTC
0
0
0
01
HEX
control / status 2
0
0
0
TI/TP
AF
TF
AIE
TIE
02
HEX
seconds
VL
<seconds 00 to 59 coded in BCD>
03
HEX
minutes
x
<minutes 00 to 59 coded in BCD>
04
HEX
hours
x
x
<hours 00 to 23 coded in BCD>
05
HEX
days
x
x
<days 01 to 31 coded in BCD>
06
HEX
weekdays
x x x x x
<weekdays
0
to
6>
07
HEX
months / century
C
x
x
<months 01 to 12 coded in BCD>
08
HEX
years
<years 00 to 99 coded in BCD>
09
HEX
minute alarm
AE
<minute alarm 00 to 59 coded in BCD>
0A
HEX
hour alarm
AE
x
<hour alarm 00 to 23 coded in BCD>
0B
HEX
day alarm
AE
x
<day alarm 01 to 31 coded in BCD>
0C
HEX
weekday alarm
AE
x
x
x
x
<weekday alarm 0 to 6>
0D
HEX
CLKOUT
control
FE
x x x x x FD1
FD0
0E
HEX
timer
control
TE
x x x x x TD1
TD0
0F
HEX
timer
<timer countdown value>