Using Timer with event

You're adding new event handler every time with line:
rate.Tick += new EventHandler(writeData);

This means that you've subscribed to Tick event as many times as timer ticked. Results of this situation become difficult to track, because writeData toggles start/stop indicator whenever entered - now it enters once the first time, twice the second time switching the timer off but first entrance started timer again, and so on. A bit untraceable kind of code.

Also, you should not call btnStartStop_Click(sender, e); from writeData because sender and e from writeData do not apply to btnStartStop_Click, although it compiles.

To correct the issue, do the following:
1. Subscribe to Tick event only at the same place where you instantiate the timer. Instantiating the object as you did is a bit troublesome (I personally never use it). Better off, create timer in constructor, and subscribe to Tick event there.
2. Create a new method which starts timer. Call it from btnStartStop_Click when applicable, and call it from writeData.