# Sleep Mode – Timer Setup

Upgraded to wordpress 3.2. Quite painless.

# Setting up the Timer

I received my components from digikey to finish the mesh based data collection prototyping. One of the components is the LTC6991 timer chip. I never soldered surface mount devices before so I did my share of reading up on it. Mine turned out functional but not the best looking. Hats off to those who make it look easy. The IC is TSOT-23 and I purchased a breadboard friendly board to solder it to.

Note US penny is same diameter as Canadian penny.

This chip is awesome and worked flawlessly. The datasheet has all the information and includes a sample-circuit for a self-resetting timer. The vendor also has an excel based calculator to facilitate resistor value calculations. It does not have provisions for the  resistor/capacitor portion self-resetting portion of the circuit. One can refer to the datasheet to get the formulas to compute those. For my experiment, the circuit used from the datasheet is reproduced below.

Solve the above equation for $R_{pw}$ to get

$R_{pw} = \frac{-t_{pulse}}{C_{pw}\cdot ln(1.429/3.3)}$  (1)

where
$C_{pw} = 4.3 nF$
$t_{pulse} = 55 to 56 ms$  I may have to lengthen the pulse duration later.

$R_{pw}$ came to $15.4 M\Omega$  and I had a couple of $7.68 M\Omega$ 1% resistors available. Close enough.

The rest of the components values where calculated from the the vendor tool.

$R_{set}= 118k\Omega \approx 120k\Omega$
$R1 = 392k\Omega$
$R2 = 1M\Omega$

I wanted a period of 10 seconds and active low. That is a very high duty cycle that you would not get with a 555 timer. The final solution will have around 5 minutes between pulses but for testing purposes, 10 seconds is easier.

The output from the scope below shows (click on it for zoom). The period is 10.1s.

The pulse came in at 57ms as shown below.

Now time to move on to setup the arduino and Xbee integration.