NXP Semiconductors UM10301 PCA2125 Manuale utente

UM10301

User Manual for NXP Real Time Clocks PCF85x3, PCA8565

and PCF2123, PCA2125

Rev. 01 — 23 December 2008 User manual

Document information

Info Content

Keywords PCF8563, PCF8573, PCF8583, PCF8593, PCA8565, PCF2123,

PCA2125, PCF2120, RTC, real time clock, timekeeping, crystal,

32.768 kHz, backup.

Abstract This application note aims to assist a user of above mentioned Real Time

Clocks in achieving succesful design-in and application. It contains useful

hints with respect to electrical schematic and PCB layout as well as code

examples for the well established NXP PCF8563 and related Real Time

Clocks. Also the more recent Real Time Clocks PCF2123 and PCA2125

have been taken into account.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 2 of 52

Contact information

For additional information, please visit: http://www.nxp.com

For sales office addresses, please send an email to: [email protected]

Revision history

Rev Date Description

01 20081223 Initial version.

This application note / user manual is a complete update of a previous publication titled:

“Application note for the Philips Real Time Clocks PCF8563,73,83,93” which did not have

an official AN/UM number and is superseded by this document.

The contents were revised with lots of additional information added and errors in the

examples corrected. Additionally it includes information with respect to recently introduced

RTCs.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 3 of 52

1. Introduction

The real time clocks from NXP (previously Philips Semiconductors) have a long tradition

and are used in numerous application fields. Starting from applications like VCR, they

have been used in a wide variety or products like burglar alarm systems, water

sprinklers, (platform) timers, e-metering, time-and-attendance monitoring, building

access control, Point-of-Sale terminals, industrial applications, cars and trucks, telecom

applications such as mobile phones and in gaming machines. In those applications they

are used for functions like keeping calendar time, tariff switching, watch-dog, time

stamping or waking up a system periodically to initiate certain actions, for example

making measurements.

This application note deals with the PCF85x3 family with focus on the PCF8563, and with

the more recent additions to the NXP RTC portfolio PCF2123 and PCA2125. The

PCF2123 is an extremely low power RTC which allows fine tuning of the clock using an

offset register (electronic tuning). PCA2125 is targeted at automotive applications. Where

appropriate, comparisons to other devices are made.

PCF2120 is a low power 32.768 kHz oscillator with two integrated oscillator capacitances

and a CLKOUT pin (32.768 kHz only), but without time, date and configuration registers.

This application note is valid for the PCF2120 as well, particulary information with respect

to oscillator, crystal, crystal and capacitor selection and layout guidelines.

Chapters 2 and 3 describe the features of these RTCs and include a comparison of the

various types. Starting from chapter 4 more technical details are described that need to

be understood in order to achieve succesful application of these real time clocks.

Chapters 4 and 5 deal with the power-on reset and voltage-low detection. Chapters 6

through 10 deal with the heart of the RTC; the oscillator, the crystal, crystal and capacitor

selection, accuracy and oscillator tuning. Chapter 11 contains a description of how

century change, leap years and daylight savings time is handled or needs to be handled

in an application. This is followed by some examples in chapter 12 about how to initialize

the RTC and how to set alarm and timer. Providing backup power when the rest of the

system is not powered is covered in chapter 13. In order to make a reliable and accurate

application it is important that the PCB layout is designed carefully and guidelines to

achieve this are listed in chapter 14. This is followed by some further design tips in

chapters 15 and 16 about partial circuit switch down and low power consumption.

Sometimes a component behaves different from what one may initially expect. This does

not imply that it behaves wrongly, but in order to properly deal with it, it is important to be

aware of such behavior. Chapter 17 describes how inaccurate timer performance can be

avoided. Chapter 18 explains why the RTC will loose time if I2C and SPI read and write

operations are not finalized within one second of initiating it.

The application note is concluded with a short chapter on trouble shooting.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 4 of 52

2. Features

The NXP real-time clock portfolio includes types for low power, types for automotive and

other high temperature applications and applications that need additional RAM. A third

family of highly accurate temperature compensated real time clocks will be dealt with in a

separate application note. Designed for a range of demanding applications, these real-

time clocks/calendars are driven by a low-power 32.768 kHz quartz oscillator, use the

SPI or I2C-bus for serial data transfer, and typically consume less than 1 μW of power.

Key features

•Oscillator requires 32.768 kHz external quartz crystal

•Resolution: seconds, minutes, hours, weekday, day, month, and year in 12- or 24-

hour (military) format. All time and alarm registers are in BCD format. Two types

include a 1/10th and 1/100th second resolution register

•Clock operating voltage: 1.0 V to 5.5 V or wider, see Table 2

•Low backup current: Ranging from 100 nA to 2 μA at VDD = 1 V and Tamb = 25 °C

•Three line SPI with separate I/O or I2C serial interface

•Freely programmable timer and alarm functions, each with interrupt capability

•Freely programmable Watchdog timer

•Programmable clock output for peripheral devices: 32.768 kHz, 1024 Hz, 32 Hz and

1 Hz (not all types)

•One or two integrated oscillator capacitors (connected to the output of amplifier

OSCO in case of only one integrated capacitor)

•Internal power-on reset

•Open-drain interrupt pin

•Wide variety of packages available including naked die

Addresses and data are transferred serially via an SPI bus with a maximum speed of 7.0

Mbps (PCF2123, PCA2125) or via a two-line, bidirectional I2C-bus that operates at a

maximum speed of 400 kbps (Fast-Mode, PCF8563 and PCA8565) or 100 kbps

(Standard-Mode, PCF8583 and PCF8593). The built-in word address register is

incremented automatically after each data byte is written or read.

With the PCF8583, the address pin A0 is used to program the software address, so that

two devices can be connected to the same I2C-bus without additional hardware.

Each RTC has an internal power-on reset and a programmable clock output with open

drain configuration to drive peripheral devices. A low voltage detector (not included on

the PCF8583,93 and PCA2125) warns if the integrity of all clock functions is no longer

guaranteed.

Power consumption is kept to a minimum in all the devices. The PCF2123 and PCF8563,

optimized for battery-powered applications, consume as little as 100 nA at 2V and 250

nA at 1V respectively. With careful selection of the crystal used, the PCF2123 consumes

less than 100 nA on a 1.5 V supply.

The seconds, minutes, hours, days, weekdays, months, years as well as the minute

alarm, hour alarm, day alarm and weekday alarm registers are all coded in Binary Coded

Decimal (BCD) format. This format is popular with RTCs for the reason that time and

date in BCD format can easily be displayed in human-readable style without conversion.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 5 of 52

In BCD every digit of the decimal system is represented by a 4-bit group. For example:

15710 = 0001 0101 0111BCD

This is not the same as binary representation. It is clear that BCD is not the most efficient

way of coding since every 4-bit group (nibble) could represent numbers 0 through 15, but

in BCD never represents numbers bigger than 9. But for some applications it is

convenient to use BCD and real time clocks are one such application.

Each 8-bit register contains two digits each represented by one nibble. Each 4-bit nibble

can represent the value of 0 up to 9 in BCD, but for some digits the maximum value to be

represented will be lower. The minute register for example will never have to count

higher than 59. The upper most digit can here be represented by 3 bits, freeing up one

bit that can be used to indicate something else.

Not all NXP real-time clocks have exactly the same register implementation and thus the

datasheet of the particular device should be consulted. As an example the register

organization of the PCF8563 is given below. Note that this is just one example and that

Table 1. Register overview PCF8563

Bit positions labelled as x are not implemented. When setting a register, also a value must be written for the ‘x’ bit positions.

When these are read back, the read back values may differ from what was previously written.

Bit positions labelled with 0 should always be written with logic 0; if read they could be either logic 0 or logic 1.

Address Register name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0

00HEX control / status 1 TEST1 0 STOP 0 TESTC 0 0 0

01HEX control / status 2 0 0 0 TI/TP AF TF AIE TIE

02HEX seconds VL <seconds 00 to 59 coded in BCD>

03HEX minutes x <minutes 00 to 59 coded in BCD>

04HEX hours x x <hours 00 to 23 coded in BCD>

05HEX days x x <days 01 to 31 coded in BCD>

06HEX weekdays x x x x x <weekdays 0 to 6>

07HEX months / century C x x <months 01 to 12 coded in BCD>

08HEX years <years 00 to 99 coded in BCD>

09HEX minute alarm AE <minute alarm 00 to 59 coded in BCD>

0AHEX hour alarm AE x <hour alarm 00 to 23 coded in BCD>

0BHEX day alarm AE x <day alarm 01 to 31 coded in BCD>

0CHEX weekday alarm AE x x x x <weekday alarm 0 to 6>

0DHEX CLKOUT control FE x x x x x FD1 FD0

0EHEX timer control TE x x x x x TD1 TD0

0FHEX timer <timer countdown value>

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 6 of 52

The PCA8565 and PCA2125 oscillators operate over a wider temperature range (up to

125 ºC) and are suitable for use in the harsh environments found within automobiles.

Power consumption remains low — only 700 nA at 2 V. Serial interface is I2C or SPI.

All the RTCs have ESD protection that exceeds 2000 V HBM per JESD22-A114, 200 V

MM per JESD22-A115. Charge Device Model values vary from 500 V to 2000 V CDM

per JESD22-C101. Refer to the datasheet of the respective device. Latch-up testing,

performed in accordance with JEDEC Standard JESD78, exceeds 100 mA.

3. Comparison

Table 2 on the next page gives a quick overview of the features, specifications and

differences between the RTCs dealt with in this User Manual. The PCF8573 which

belongs to the PCF85x3 family is no longer in production and has thus not been included

in the table. However, this user manual is useful for this type as well.

Further there are some derived types from the main types listed in the table with small

differences in for example delivery form or the number of integrated oscillator capacitors.

Consult NXP for more details.

3.1 Event counter mode

Two real time clocks, PCF8583 and PCF8593, have an extraordinary feature. It is the

event counter mode which can be selected by setting the appropriate bits in the control

high impedance state. This mode can be used to count pulses applied to the oscillator

input OSCI. There is no crystal in the circuit and OSCO is left open circuit. The event

counter stores up to 6 digits of data. Events are stored in BCD format. The 6 digits use

three 8 bit registers (hundredth of a second, seconds, and minutes). D5 is the most

significant and D0 the least significant digit. Every digit can contain values ranging from 0

to 9 and thus up to 999 999 events can be stored.

It is also possible to set an event counter alarm. When this function is enabled, the alarm

occurs when the event counter registers match the programmed value. In this event the

alarm flag is set. The inverted value of this flag can be transferred to the interrupt pin by

setting the alarm interrupt enable in the alarm control register. In this mode the timer

increments once for every one, one hundred, ten thousand or 1 million events,

depending on the programmed value of the alarm control register. In all other events, the

timer functions are as in clock mode.

Note that immediately following power-on, all internal registers are undefined and must

be defined by software. It is also possible that upon power-on the device is initially in

event-counter mode in which event the oscillator will not operate until the correct settings

are written into the control registers.

The count value will increment on the falling edge. However, after a new count value has

been programmed at least one rising edge must have occurred before events will be

detected on the falling edge.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 7 of 52

Table 2. Comparison of six real time clocks

PCx85x3 family PCx212x familyFeatures

PCF8563 PCA8565 PCF8583 PCF8593 PCF2123 PCA2125

Unique features Very low

power

consumption

AEC-Q100

automotive

qualification

High

resolution,

RAM, event

counter

High

resolution,

event

counter

Extremely

low power

consumption,

electronic

tuning

AEC-Q100

automotive

qualification

Type of interface I2C I2C I2C I2C SPI SPI

Interface bus speed 400 kHz 400 kHz 100 kHz 100 kHz 7 MHz 7 MHz

Scratch pad RAM no no 240 bytes no no no

Year / leap year tracking yes / yes yes / yes yes / yes yes / yes yes / yes yes / yes

Year counter

2 digit +

1 century bit

2 digit +

1 century bit

2 bit

(4 years)

2 bit

(4 years)

2 digit

(99 years)

2 digit

(99 years)

100 ms, 10 ms time register no no yes yes no no

Electronic tuning register no no no no yes no

Programmable alarm and timer

functions

yes yes yes yes yes yes

Low voltage detector yes yes no no yes no

Event counter mode no no yes yes no no

Option to select between two I2C

addresses

no no yes no no no

Integrated oscillator capacitor 1 at OSCO 1 at OSCO 1 at OSCO 1 at OSCO 2 1 at OSCO

Supply voltage range 1.8 V – 5.5 V 1.8 V – 5.5 V 2.5 V – 6.0 V 2.5 V – 6.0 V 1.6 V – 5.5 V 1.6 V – 5.5 V

Clock operating voltage 1.0 V – 5.5 V 1.8 V – 5.5 V 1.0 V – 6.0 V 1.0 V – 6.0 V 1.1 V – 5.5 V 1.3 V – 5.5 V

Typical current consumption 250 nA at

VDD = 1 V

650 nA at

VDD = 3 V

2 μA at

VDD = 1 V

1 μA at

VDD = 2 V

100 nA at

VDD = 2 V

550 nA at

VDD = 3 V

Operating temperature range -40 °C to

+85 °C

-40 °C to

+125 °C

-40 °C to

+85 °C

-40 °C to

+85 °C

-40 °C to

+85 °C

-40 °C to

+125 °C

AEC-Q100 qualified no Yes

(TSSOP8)

no no no yes

Packages U

[1], DIP8,

SO8,

TSSOP8,

HVSON10

TSSOP8,

HVSON10

U [1], DIP8,

SO8,

HVQFN20

DIP8, SO8 U [1],

HVQFN16,

TSSOP14

[1] Naked die

Some derived versions are available such as PCF8563A and PCA8565A which include

two integrated oscillator capacitors and are also available as naked die.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 8 of 52

4. Power-on reset (POR)

Traditionally a power-on reset circuit is a circuit that generates a reset pulse once the

supply voltage has reached a certain value upon power-up. The purpose is to ensure a

defined behavior at start-up. This type of power-on reset is not present in these RTCs.

The power-on reset circuit (POR) for these RTCs does not look at the supply voltage, but

instead it is based on an internal reset circuit which is active whenever the oscillator is

stopped, refer to Fig 1. When power is applied to the device it will take some time for the

oscillator to start and during this time the circuit will generate a reset. Also when during

operation the OSCI- or OSCO-pin is pulled to ground, causing oscillation to stop, the

POR will generate a reset pulse. In the reset state the serial bus logic is initialized and all

registers are reset according to the register reset values. Not all registers will be reset.

The only registers that are reset are the ones that control a function i.e. decide on clock

mode, enable an alarm etc. Refer to the datasheet of the respective device for details.

The power on reset duration is thus directly related to the crystal oscillator start-up time.

Due to the long start-up times experienced by these types of circuits on-board testing of

the device would take longer too. In order to speed up this, a mechanism has been built

in to disable the POR (not for PCF8583, PCF8593 and PCF2123). This is called Power-

on reset override. Again, refer to the respective datasheet for details. Once the override

mode has been entered, the device stops immediately being reset and set-up operation

e.g. entry into the external clock test mode, may commence via the serial interface.

001aaf897

chip in reset chip not in reset

oscillation

internal

reset

Fig 1. Power-on reset

5. Voltage-low detector

PCF8563, PCA8565 and PCF2123 have an on-chip voltage-low detector, see Fig 2 and

Fig 3. When VDD drops below a certain limit defined as Vlow, bit VL in the seconds register

of PCF8563 and PCA8565 is set. Generally the VL-bit is intended to indicate that the

time might be wrong, not that it necessarely is wrong. It will be set if one of the following

four conditions occur:

•The power has just been applied;

•The power has dipped down and then recovered;

•The power has gone away and then come back again;

•When the oscillator stops running.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 9 of 52

The implementation in the PCF2123 is slightly different. There a bit OS (Oscillator

Stopped) is present instead of VL. The OS flag is set whenever the oscillator is stopped,

and therefore also when this is due to the supply voltage dropping too low. The flag can

only be cleared by software and only if the oscillator is running again.

VL set

normal power

operation

period of battery

operation

VDD

Vlow

mgr887

<OS>

Battery operation

Main supply

OSC(MIN)

(1) Valid for PCF8563 and PCA8565 (2) Valid for PCF2123

Fig 2. Voltage-low detection Fig 3. Oscillator-stop detection

In the case of PCF8563/PCA8565 bit VL set indicates that the integrity of the clock

information is no longer guaranteed. If the oscillator hasn’t stopped, the clock information

will still be ok, but with VDD having dropped below Vlow there is no guarantee that this still

is the case because there is no way to be sure that the oscillator kept running. The VL

flag can only be cleared by software.

Both VL and OS are intended to detect the situation when VDD is decreasing slowly, for

example under battery operation. Should VDD reach the limit where the flag is set before

power is re-asserted, then the flag VL or OS will indicate that time may be (VL) or is (OS)

corrupted. VDD dropping below Vlow or Vosc(min) in itself does not cause any register to be

reset. Once the oscillator stops some registers will be reset.

6. Oscillator

A crystal oscillator as used in a real-time clock, see Fig 4, is built on the principle of

Pierce and uses an inverting amplifier with a crystal in the feedback path and load

capacitors CIN and COUT to provide the necessary additional phase shift. Some phase

shift is contributed as a result of the amplifier’s non-zero output impedance in

combination with COUT. The oscillator operates at the frequency for which the crystal is

anti-resonant (i.e. parallel resonant) with the total capacitive load of the oscillating circuit

as seen from the pins of the crystal. This total capacitance is called the load capacitance.

The load capacitance is defined as the capacitance seen from the pins of the crystal and

is formed by CIN, COUT and CSTRAY indicated in Fig 4. Electrically the crystal’s C0is also a

load capacitance which affects oscillator characteristics. However, it is not part of the

defined ‘load capacitance’. During manufacturing the crystal is tuned to the specified

frequency with a specified load capacitance connected to the crystal. Since C0is part of

the crystal, it is automatically taken into account during the adjustment procedure.

NXP Semiconductors UM10301

User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

User manual Rev. 01 — 23 December 2008 10 of 52

CSTRAY is a result of parasitic capacitances due to PCB traces, IC pins etc. and is directly

in parallel with C0of the crystal. In a practical situation care needs to be taken to keep

these parasitic capacitances as low as possible since it will add to the load capacitance

and this load capacitance must meet the specified value for the crystal that is being used.

If the load capacitance presented to a crystal is smaller than what the crystal was

designed for, the oscillation frequency will be too high and thus if used with an RTC, the

clock will run too fast.

001aah846

Cstray

Cin Cout

crystal

RTC-IC

OSCOOSCI

Fig 4. Pierce Oscillator equivalent diagram

The inverting amplifier (with feedback resistor, and drive resistor which are not included

in Fig 4) is incorporated within the integrated circuit device. On the other hand, the quartz

crystal is a discrete device external to the integrated circuit. In the PCF85x3, PCA8565

and PCF2123, PCA2125 the output capacitor COUT is integrated on the integrated circuit.

PCF8563A, PCA8565A and PCF2123 also include CIN, see Table 3 for overview.

Table 3. Overview of internal and external oscillator capacitors

PCx85x3 family PCx212x familyFeatures

PCF8563 PCA8565 PCF8583 PCF8593 PCF2123 PCA2125

Integrated oscillator capacitor 1 at OSCO 1 at OSCO 1 at OSCO 1 at OSCO 2 1 at OSCO

Targeted crystal load capacitance 12.5 pF 12. 5 pF 12.5 pF 12. 5 pF 7 pF [1] 12.5 pF

Value of integrated CIN, typ. - - 14 pF -

Value of integrated COUT, typ. 25 pF 25 pF 40 pF 25 pF 14 pF 25 pF

Theoretically required at pin OSCI 25 pF 25 pF 18 pF 25 pF 0 pF 25 pF

[1] Can be used with 9 pF and 12.5 pF as well if external capacitance is added

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