Hardware connection with sensor¶
To be able to successfully use sensors and other devices with embedded systems, these needs to be physically wired with embedded system (or PC).
Target devices (usually sensors or memory devices) are connected to master host device using single wire (from here protocol name One Wire) for communication only. There are also voltage and ground lines, marked as VCC and GND, respectively.
At this point, we assume you are familiar with UART protocol and you understand it has
2 independent lines, one for transmitting data (TX) and second to receive data (RX).
For successful communication with sensors, bi-directional support is necessary to be implemented, but there is only
1 wire available to do so. It might sound complicated at this point.
OneWire data line is by default in open-drain mode. This means that:
Any device connected to data line can at any time pull line to GND without fear of short circuit
None of the devices are allowed to force high state on the line. Application must use external pull-up resistor to do so.
How to send data over TX pin if application cannot force high level on the line? There are
Configure UART TX pin to open-drain mode
Use push-pull to open-drain converter using 2 mosfets and 1 resistor
Since many latest embedded systems allow you to configure TX pin to open-drain mode natively, you may consider second option instead.
Application must assure that TX pin is always configured to open-drain mode, either with push-pull to open-drain converter or directly configured in the system.
TX and RX pins¶
Every communication starts by master initiating it. To transfer data over UART, application uses TX pin and RX pin is used to read data. With 1-Wire protocol, application needs to transfer data and read them back in real-time. This is also called loop-back mode.
Let’s take reset sequence as an example. By specifications, UART has to be configured in
9600 bauds and master needs to send single UART byte with value of
0xF0. If there is any slave connected, slave must pull line to GND during transmision 0f
0xF part of byte. Master needs to identify this by using RX pin of the UART.
Please check official document on Maxim website to understand why