Nxp semiconductors, Fd a c – NXP Semiconductors UM10301 PCF2123 User Manual
Page 40
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
40 of 52
• 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
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.
F
d
A
C
r
tr
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
.