NXP Semiconductors | UM10301 |
| User Manual PCF85x3, PCA8565 and PCF2123, PCA2125 |
•Access the RTC as little as possible in order to reduce the dynamic current consumption by the I2C-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 VDD of the RTC but connect them to the supply of the rest of the circuit (VDD1 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 VDD of the RTC;
•Select the I2C-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 I2C-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.
C tr | = | ε 0 ⋅ ε r ⋅ A | = | 8.85 | ⋅10 | −12 ⋅ 4.6 ⋅ 0.03 ⋅ 0.0005 | = 0.9 | ⋅10 | − | 12 F |
d | | | 0.0007 | | |
| | | | | | | | | |
Further capacitances are: | | | | | | | | |
Microcontroller pin capacitance | Ci = 7 pF | (assumption) | | | |
RTC pin capacitance | | | Ci = 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 VDD related input threshold of VIH = 0.7VDD and VIL = 0.3VDD for the purposed of RC time constant calculation. Then V(t) = VDD(1 – e-t/RC), where t is the time since the charging started and RC is the time constant.
V(t1) = 0.3 x VDD = VDD(1 – e-t1/RC); then t1 = 0.3566749 x RC V(t2) = 0.7 x VDD = VDD(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 RP as a function of bus capacitance for Standard-mode, Fast-mode and Fast-mode Plus. For each mode the RP(max) is a function of the rise time maximum and the estimated bus capacitance Cb.
UM10301_1 | | © NXP B.V. 2008. All rights reserved. |
User manual | Rev. 01 — 23 December 2008 | 40 of 52 |
