Why aren't there 5v between SVcc and Gnd at msp430 ?

I have a msp430a320 microcontroler . I want to connect a DS1820 temperature senzor to the msp430 .I haven't connected them phisically yet , but I found in the documentation a scheme which is described below:
Vdd from senzor goes to SVcc of the msp430 , GND from senzor goes to Gnd of the msp430 and DQ (data ) from senzor goes to P0.Y (from digital signals pins group) of the msp430 . So my senzor could be powered from the msp430 , the only problem is that when I put the voltmeter between GND and SVcc of the msp430 ,I get nothing (the needle doesn't move), and I should get 5V , so I won't be able to power my DS1820 senzor when I'll connect it phisically to the msp430 . The msp430 was connected to the computer via serial and a demo program was running when I made the measurement . Is it possible that SVcc 5V should be enabled from program or something like that ? If not then where should I enable it from ?

This is a Dallas 1-wire device. Check the datasheet for using it in parasite mode if that's what you intended to do. In regular mode the sensor Vdd and GND pins go to your power supply rails. If you don't have 5V at your power supply then you've got it turned off.

Also, you need a pullup resistor on the DQ line.

Mike

Why do I need the pullup resistor at DQ ?

aral said:

Why do I need the pullup resistor at DQ ?

Click to expand...

So that the bus doesn't float.

Here's the datasheet:


Mike

In case you wanted more information...

All communication is done on the DQ line. And as such, any unknown condition can cause the device to get confused. By tying this line to a known state (5V) through a resistor you force it to stay at 5V until the controlling device (PIC) alters the line (pulls it low) for communication.

This is the function of pullup/pulldown resistors; to force to a known state.

(Some devices have internal pullups/pulldowns.)

-Mike

The Pull-up resistor is required for 1-wire bus. Because all devices are communicating by grounding the Bus-pin, Therefore you must add log1 using resistor (and 0 by communicating devices). The communication itself if based on measuring of LOG0 (less time for L bit, more time for H bit).