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Salvatore Damino


We have already learned the management of 7 segments display, the real time clock, the serial EEPROM and the I/O lines through the really comfortable I2C-BUS. In this issue we'll get the knowledge of other two interesting components of this big family: the PCF 8991 manufactured by PHILIPS and the DS 1621 manufactured by DALLAS.


A/D and D/A converter - PCF 8591

The PCF 8591 is a simple peripheral device that includes 4 A/D converter lines with 8 bits resolution and 1 D/A converter line always with 8 bits resolution. As You already know only 2 wires are sufficient for connection and management of the included I2C-BUS interface.
Through the 4 analog input lines and proper adjustment circuits can be easily measured analog sensors as those for temperatures, voltages, currents, pressures, humidity, etc. The 4 input lines of the A/D converter section are connected to low profile, 20 pins, connector CN1 respectively on pins 11, 12, 13 and 14.
Instead the D/A converter section generates an analog signal that, through a proper interface circuit, drives DC motors, sluice valve, proportional regulators, audio controllers, etc. The D/A converter line is connected to CN1 connector, on pin 16.


Dallas DS 1621

The DS 1621 is an interesting component from Dallas with a double functionality. The first one gets and measures temperatures from -55 °C to +125 °C with a 0.5 °C resolution. It is really interesting the possibility to use the component in a very small 8 pins, SOIC package. This feature let You integrate the component even in the shortest apparatus without functional limitations. The second feature, with the same importance, is the possibility to work as a thermostat, in the previous temperature range. This functionality drives an output (Tout) for the external field, placed on pin 3 of the component.
Summarizing in a comfortable 8 pins package, You can find everything necessary to measure and control a temperature in a 180 °C total range.
The temperature is acquired each second and it is returned with a 9 bits data stream.
The very interesting feature of DS 1621 is the format of the returned value in fact it is directly the binary code of the measured temperature. So it is not required the usual transformations as, for example, those required with an external temperature probe acquired through one of the PCF 8591 A/D converter inputs.
Everything result extremely linear and easy. The temperature is acquired by the CPU that drives the circuit using the I2C-BUS communication, in the same mode used for the output setting.


Thermostat section

It is a section of DS 1621 completely autonomous that shares with the first section some indispensable interface and communication modalities.
It has a non volatile memory, EEPROM type, where the set point value for thermostat intervent, can be setted.
The thermostat can work alone by simply programming its EEPROM, so it can be used in simple circuits that must mantain a fixed temperature without any interactive modifications of the working data. Instead when this components is connected to CPU (even the smallest one), You obtain complex and sophisticated system right for any temperature control and conditioning.
The Tout output of the DS 1621 is connected on pin 9 of

CN1 and its status is shown by the dedicated LED LD2.

Data sheets

- 4 A/D and 1 D/A converter lines with 8 bits resolution, PCF 8591.
- Temperatre measurement and thermostat management, DS 1621.
- Thermostat output, displayed by LED, available on connector.

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Page up-dated at August 29, 2004

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