NXP Semiconductors PCF85x3, PCF2123, PCA8565, PCA2125 user manual 0007

Page 40

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

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Contents Document information Info Content KeywordsAbstract Contact information NXP SemiconductorsRevision history Rev Date DescriptionIntroduction Features Register overview PCF8563 Address Register name BitEvent counter mode ComparisonFeatures Comparison of six real time clocksPower-on reset Power-on reset PORVoltage-low detector Voltage-low detection OscillatorOscillator-stop detection Pierce Oscillator equivalent diagram Overview of internal and external oscillator capacitorsOscillator frequency determining components UM10301 + C Typical values for crystal and surrounding capacitors Parameter Value Unit SourceUsing an external oscillator Oscillation allowanceCrystal and crystal selection Effect of temperature Modes which don’t work− f nom Capacitors and capacitor selection Accuracy Influences on time accuracy Oscillator tuning Oscillator tuning 10.1 PCF2123 Offset register Century and leap year, Daylight Saving Time Century trackingYear and leap year tracking Daylight Saving Time DSTInitialization of the RTC and setting the time Initialization and setting of alarm and timerBlock Diagram PCF8563 Alarm Setting the alarmBinary BCD Register Comments AddressAlarm function Setting the timer Setting the timerRegister Backup power supply Lithium Primary cellsBackup circuit using primary lithium cell Backup circuit using secondary cell NiCd or NiMH NiCd and NiMH secondary batteries13.3 Capacitors Charging the backup capacitor Diode selection Some suggestions for diode D11N4148 BAS716 BAS116 BAV170PCB layout guidelines PCB layout proposal for PCF8563 using leaded components Partial circuit switch down Hints to keep power consumption low Protection diodes0007 8473 ⋅ C b Rpmax as a function of bus capacitanceApplication diagram 1, I2C-bus interface Application diagram 2, SPI interfaceFirst period inaccuracy when using the timer Timer delaysTimer Source clock frequency Delay for n = General countdown timer behaviourFirst period delay for timer counter value n Timer source clock Minimum timer period Maximum timer periodTiming requirements for I2C read and write Block diagram I2C interface and Time counters I2C interfaceSequence of events example Read Troubleshooting Oscillator startup time Checking for oscillationNo communication via I2C-bus References Wrong time and date, wrong clock speedLegal information DefinitionsDisclaimers TrademarksContents
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