The DS1621 is a very accurate and easy to interface chip. It has a built in temperature sensor and using I²C protocol, can send very accurate temperature readings. The interface of I²C is just 2 wires, one Data and one Clock. What makes this chip even more handful is that it comes with a 3-bit address. This means that with those 2 wires, up to 8 chips can be interfaced and accessed by it's address!

The DS1621 chip is very accurate

The DS1621 is a very simple-to-use chip for measuring temperatures. One great advantage is the high accuracy that it has and the wide measuring range. It can measure form -25oC to +110oC with maximum error 0.5oC near the edges. For room temperatures, the error is almost zero.

Furthermore, each chip is hardware addressable. Using just 2 communication lines from the microcontroller, up to 8 chips can be cascaded and separately controlled by it's pre-selected address. The communication is done using standard 2-wire ^I2C protocol.

The temperature is returned from the chip with 2 8-bit bytes. The first one is the temperature in degrees (no conversion needed). The second one is the half-degrees. Using only those registers, you can have a 0.5oC step. But there are 2 more registers in the chip that can be used, and using a formula, this step can be significantly decreases, increasing with this way the accuracy. This circuit will not use those registers and thus, a step of 0.5oC will be used, which for room temperature monitoring is already enough.

Finally, this chip can be a stand-alone pre-programmed thermostat. It has an output that becomes high when the temperature exceeds a pre-programmed value, and turns back low when the temperatures goes bellow another pre-programmed value.

What i have design here is a dual temperature meter. I use two DS1621 chips. The first one has the address 000 (0) and the other one the address 001 (1). The address can be hardware selected from the package pins A0 A1 and A2. The same circuit with very slight modifications in the PIC program can be used to interface up to 8 chips. For addressing, i use one 4-dip switch on each chip. I use of course only the 3 first dip switched. On the real PCB, this switch can be replaced with permanent wiring to reduce cost and size. Here is the schematic diagram:

As already has been told, the DS1621 uses only two wires for interface. The Data bus comes from the RC6 I/O of the PIC, and the Clock bus comes from the RC7 I/O. Furthermore, the Data bus needs a pull-up resistor. An 1K resistor is enough. Higher values may cause problems as the rise and fall times will be higher. Finally, the LED is blinking every time a new measurement is taken.

Here are all the files needed as well as the full assembly listing, to re-compile the program:

DS1621 PIC temperature meter

For those that do not want to recompile and wants it as-is, here is the compiled hex version to directly upload to the PIC:

DS1621 PIC temperature meter - hex

Resistors
R1	10 KOhm variable resistor
R2	100 Ohm variable resistor
R3	1 KOhm 1/4Watt 5% resistor
ICs
IC1	PIC 16F631 Microcontroller
IC2	DS1621 Digital Thermometer and Thermostat
IC3	DS1621 Digital Thermometer and Thermostat
Misc
LCD1	LCD 20x4 Character LCD w/ HD44780U1 controller
LED1	LED 3mm red

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