What does the capacitor do?

Hi,
I have a Bluefruit EZ-Link - Bluetooth Serial Link & Arduino Programmer - v1.3. It works great . However, I am wondering why it needs a 1uF capacitor between the DTR and the reset pin on the Arduino, what does it do? and why it is 1uF?

HI.

The capacitor in series blocks DC and will let AC or pulses through.
The DTR signal is “abused” here.
Changing the level of that pin connected to the capacitor will generate a pulse at the other side of that capacitor, which is connected to the controller’s reset pin.
This pulse is used as a reset pulse, which is needed for the programming to be initiated.

The value is chosen so that the pulse lasts long enough, but not too long to generate a time out with your programming software.

MAS3:
HI.

The capacitor in series blocks DC and will let AC or pulses through.
The DTR signal is "abused" here.
Changing the level of that pin connected to the capacitor will generate a pulse at the other side of that capacitor, which is connected to the controller's reset pin.
This pulse is used as a reset pulse, which is needed for the programming to be initiated.

The value is chosen so that the pulse lasts long enough, but not too long to generate a time out with your programming software.

Thank you for your reply. More questions, Where does this AC come from? Is the DTR signal pulse AC? And what do you mean by the DTR signal is abused? Thanks again

Is the DTR signal pulse AC?

No it is a DC signal that changes. Any DC signal that changes can be regarded as AC. A capacitor lets through changes but does not let through steady levels.

And what do you mean by the DTR signal is abused?

He means it is used for purposes that it was not designed to be used for. Only in the Arduino world is the DTR signal used like this. Its real use is for a modem handshaking line, it stands for "Data Terminal Ready" and is used to indicate that the ......... guess.

It's an RS232 STANDARD signal, but judging from your questions , you probably don't want to go there just yet. For the most part, RS-232 has been replaced by USB but I think there are still some rs232 based equipment in operation. One of the advantages is the range is greater than USB. It is easy to get a 100' RS232 link working if it is running off at least +/- 12V. Long range links have for most part been replaced by RS485 , partly for the range but mostly for the speed.

It offers data transmission speeds of 35 Mbit/s up to 10 m and 100 kbit/s at 1200 m. Since it uses a differential balanced line over twisted pair (like RS-422), it can span relatively large distances (up to 4,000 feet (1,200 m)). A rule of thumb is that the speed in bit/s multiplied by the length in meters should not exceed 108. Thus a 50 meter cable should not signal faster than 2 Mbit/s.[2]