The simple method described here evaluates a one-wire device that uses pulse-width-encoded
communications from a PC serial port. Communications errors such as parity and
acknowledge are monitored. The hardware interface to the PC is a MAX232 driver/receiver,
and the slave in this example case is a Texas Instruments TMP141 temperature
sensor. The circuit can be expanded to four sensors on the same bus by using
the TMP141's four programmable addresses derived from A0 and A1 (Fig.
1).
The main scheme for this method is to use the built-in timing of the RS-232
baud-rate control. Three "bit codes" are required for most communications: (Start
bit), data (bit 0), and data (bit 1) (see the table
and Fig. 2). Using mscomm in Visual Basic,
the PC COM port is configured to output 8 data bits, parity None, 1 Stop bit,
and flow control none. The setting of COM port data bits along with the baud
rate determines the bit-code widths. Baud rate is changed between bits to create
the different pulse-width codes.
Reprogramming the PC COM port baud rate can create a delay of 20 to 30 ms between
bits, but this is okay because there's no time-out event in the slave. Yet the
delays can make it a challenge to read the output on a logic analyzer. The Visual
Basic program controls two-way communication to the TMP141 sensor. The program
allows internal registers to be read and programmed. Details of communication
execution are in the Visual Basic program, which can be downloaded at ED Online
12237 at www.electronicdesign.com.