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Salvatore Damino


In the previous issue (E.F. n° 199 - November 2000) we mounted the K51-AVR card and, as a final action, we executed the alarm clock program. As You have seen the program runs autonomously and it requires only a connection to a power supply source. The alarm clock program drives the on board buzzer to manage the striking bell and, above all, it has many comments that clarify the reason why of the performed choices, even for the unexperienced programmers.
As for all the micro electronics applications the complete knowledge of the software (or more properly in this conditions: the firmware) is only one of the presupposed requirements to advantageously operate in this area. Another aspect, of the same importance, is the complete knowledge of the used hardware. For these reasons it is really important to get the data sheets of all the components installed in the system. Thanks to internet, today is really easy to find these information from the web network but, for the lazy people or to simplify the research, we inform You that they are available in
grifo® site.


Real Time Clock - PCF 8583

This integrated circuit has 8 pins and it is a diffused clock manufactured by Philips that is driven through the comfortable I2C-BUS.
To connect the component to the others devices that use this protocol, only 2 interconnection wires are sufficient. The firmware can properly communicate with all the devices with no requiements of complicated external addressing circuits.
To correctly operate, the PCF 8583 needs only a 32 KHz crystal, a battery and few passive components. The diagram block shows that the component includes also an internal RAM read and write directly through the same I2C-BUS interface. These 256 bytes of RAM can mantain the saved information by taking advantage from the clock battery. In this way the external CPU can manage different allarm conditions or the activation of external I/O devices as: the illumination; the garden irrigation control; the room heating; etc.
A further line managed by PCF 8583 is pin 7 that is an /INT signal capable to drive an external system, when a proper internal register is correctly setted. Moreover this line is connected to a visualization LED that shows its status.
The current time is displayed through the four digits driven by SAA 1064, while the four push buttons available on the card are used to manage the clock setting modalities.


SAA 1064 controller

The controller for 4 digits, 7 segments, LEDs display named SAA 1064 is another device driven with I2C-BUS protocol. As You can see in the block diagram, the SAA 1064 includes all the driving circuit, with constant current generator, for 4 digits 7 segments display and it needs only two external transistors.
By using dedicated I2C-BUS commands it is possible to select between different brightness levels.



Among the devices managed through I2C-BUS, independently by the used CPU, the classic serial EEPROM is certainly needed.
The circuit used in the K51-AVR card is capable to manage up to 1 Kbyte of serial EEPROM with I2C-BUS modality. This mean that the device up to 24C08 size can be mounted.
To simplify the understanding of the management tecniques two separate programs have been developed: one reads the data saved on EEPROM and the other writes data. The visualization of the data is performed on the 4 displays of the card while the commands are entered through the 4 push buttons available on the board. With these features the programs require only a power supply voltage.


PCF 8574

The PCF 8574 is a device that manages 8 bidirectional I/O lines via I2C-BUS protocol. The PCF 8574 allows to drive individually the 8 lines available on this integrated circuit by setting the direction (input or output) too. Also in this circumstance only the K51-AVR resources are used both for data visualization and line setting. To check the I/O lines status can be used the comfortable TI/O 16 card, a little board self made with 8 LEDs and 8 keys or a normal tester.
By using the provided electric diagram the user can add the suitable number of PCF 8574, add a buffer circuit for the output lines and an optocoupled interface for the input lines. The resulting I/O unit can be connected to external systems without problems, even if the systems are very complicated.


Data sheets

- Real Time Clock PCF 8583 with lithium battery, RAM and alarm.
- 4 digits, 7 segments, LEDs display controller, SAA 1064.
- Serial EEPROM, type 24C08.
- 8 bidirectional I/O lines controller, PCF 8574.
Technical documentations available on

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