Nuts & Volts - February 2004

A Simple Capacitance Meter

FIGURE 2. The location of the first channel of A/D and the first channel of D/A on the evaluation kit.

(Counting from the pin that connects to the first channel of the A/D, it is the ninth pin over.) See Figure 2.

The Serial Interface

When using the development kit, the serial interface is used for two purposes — downloading the program and communicating with the ADuC812 while the program is running. Downloading programs to the ADuC812 development kit is easy. First, connect the serial cable that comes with the kit to the kit itself (being careful to match up the pins on the board with the markings on the cable). Then connect the cable to COM1. (For some reason, the downloader that comes with the development kit does not seem to work from COM2.) Make sure that the jumper at location LK3 (PSEN) on the evaluation board is in. Run WSD, the Windows serial downloader. Go to the configuration menu of the downloader and make sure that it is set to COM1. Next click on DOWNLOAD. Find the program that you want to download and download it. Finally, click on the RUN button. At this point, the program is running on the ADuC812.

The program that is running on the ADuC812 expects to communicate with the outside world through the COM port, too. The program sets the ADuC812's internal UART to transmit and receive at 19200 baud with one stop bit and no parity bit. Additionally, no flow control is used. Thus, the host computer must be set up similarly. I generally use Hyperterminal (a Windows utility) to communicate with the ADuC812. When using Hyperterminal, you must tell Hyperterminal which COM port you are using (COM1), and then you must set the properties of the COM port to 19200 baud, one stop bit, no parity, and no flow control.

Once the set-up is taken care of, to initiate a measurement all that you need to do is to type a lower case 'm' in the Hyperterminal window. The ADuC812 will measure the capacitance and will send the capacitance value to the COM port. Hyperterminal will pick up the value and display it on your screen.

For those who do not want to type, the ADuC812 has been set to start a measurement when the INT0 button on the development kit is pressed. In order to see the value of the capacitance, you must still use Hyperterminal.

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