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UM10301_1 

© NXP B.V. 2008. All rights reserved.

•  Access the RTC as little as possible in order to reduce the dynamic current 

consumption by the I

2

C-bus or SPI; 

•  Disable the CLKOUT in battery backup mode. If CLKOUT needs to be enabled select 

the pull-up resistor as large as possible. However, CLKOUT enabled will  dominate 
current consumption and severely limit battery backup time; 

•  Do not connect the pull-up resistors for the serial interface to V

DD

 of the RTC but 

connect them to the supply of the rest of the circuit (V

DD1

 in Fig 16). This avoids 

unnecessary battery current drain from the battery via the pull-up resistors. If in 
“Power-Off” everything gets powered down except the RTC, the bus lines will often 
not be high impedant. In this case current could run from the battery via the pull-up 
resistors and the bus to GND which would severely reduce the possible battery 
backup time, if the pull-ups were connected to V

DD

 of the RTC; 

•  Select the I

2

C-bus pull-up resistors as large as possible. The value of the pull-up 

resistors is a compromise between current consumption and maximum clock 
frequency. Lower values result in lower RC time constants and thus faster rise time 
of the SCL and SDA lines. Using the I

2

C-bus, data transfers can be made up to 

100 kbit/s in Standard-mode and up to 400 kbit/s in Fast-mode. The corresponding 
required maximum rise times are 1 μs for Standard-mode and 300 ns for Fast-mode. 
The rise time is a product of bus capacitance and the value of the pull-up resistor. 
The bus capacitance is the total capacitance of wire, tracks, connections and pins. 
First estimate the capacities. Track capacities can be calculated with the standard 
formula for a capacitor. Depending on the PCB material used, values for ε may differ.  
For this example a track length of 3 cm is assumed, with a track width of 0.5 mm on 
a copper backed 0.7 mm strong PC-board made from FR4 glass epoxy. 
 

   

12

12

0

10

9

.

0

0007

.

0

0005

.

0

03

.

0

6

.

4

10

85

.

8

−

−

⋅

=

⋅

⋅

⋅

⋅

=

⋅

⋅

=

ε

ε

 

 

  Further capacitances are: 
    Microcontroller pin capacitance   C

i

 = 7 pF  (assumption) 

  RTC 

pin 

capacitance 

    C

i

 = 7 pF  (max value for PCF8563) 

Adding these capacitances to the 0.9 pF track capacitance results in a bus 
capacitance of 14.9 pF. 
Consider the V

DD

 related input threshold of V

IH

 = 0.7V

DD

 and V

IL

 = 0.3V

DD

 for the 

purposed of RC time constant calculation. Then V(t) = V

DD

(1 – e

-t/RC

), where t is the 

time since the charging started and RC is the time constant. 
V(t1) = 0.3 x V

DD

 = V

DD

(1 – e

-t1/RC

); then t1 = 0.3566749 x RC 

V(t2) = 0.7 x V

DD

 = V

DD

(1 – e

-t2/RC

); then t2 = 1.2039729 x RC 

T = t2 – t1 = 0.8473 x RC

 

 
The graph in Fig 17 and the equation below show maximum R

P

 as a function of bus 

capacitance for Standard-mode, Fast-mode and Fast-mode Plus. For each mode the 
R

P(max)

 is a function of the rise time maximum and the estimated bus capacitance C

b

.