NXP Semiconductors UM10301 PCF2123 - 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
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.
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
.