Since each symbol represents one bit, the bit rate equals the baud rate. Basically, the baud rate indicates how many times the lines can change state (high or low) per second. Baud rate: Baud rate is the number of symbols or modulations per second.There are five parameters which must be configured correctly to establish a basic serial connection: UART peripherals typically have several configurable parameters required to support different standards. Universal asynchronous receiver/transmitter (UART) We will look in more detail at the MSP430 implementation later on. Therefore, we won’t cover how to build it in this tutorial, but if you would like to know more feel free to shoot me an email. Here is an example of one I have built:įortunately, the MSP430 Launchpad has a serial to USB converter built right onto the the board so this additional equipment is not required. It is extremely simple to use and can be easily breadboarded. The MAX232 and variants are some of of the most common RS-232 transceivers on the market. If you try to connect RS-232 directly to the MSP430 or most other microcontrollers it will not work and likely cause some damage. When hooking up RS-232 to an MCU it is important to remember that the voltage levels supported by the IO are different (0V – 3.3V), so an external transceiver is required to convert the signals to the appropriate levels. At least one stop bit is required, but two stop bits are often supported as well. To end a transmission, a stop bit (mark) is sent to the receiver, and the held in the idle state. The data can be in several possible formats depending what is supported by both devices. To begin a transmission of data, a start bit (space) is sent to the receiver. The positive voltage is called a ‘space’, and indicates a logic zero (0). The idle state of the line is at the negative voltage level and is referred to as a ‘mark’. The standard defines signals which typically vary from -5V to +5V, but can as much as -15V to +15V. RS-232 signals are different than than what we are used to in the digital world because the voltage switches between negative and positive values. We are not going to go into details on all the possible configurations, however you can read about it here if you are interested. Typically RS-232 is used without hardware flow control and at full duplex. Two optional lines RTS and CTS are provided for this function. Hardware flow control can also be enabled in order to mitigate the flow of data. However, depending on the higher level protocols, there may be a need to block the transmitter while receiving. This is called full-duplex and it is the standard means for communicating over serial. The separate RX and TX lines mean that data can flow in both directions at the same time. The minimum connection required for bidirectional communication is three signals: transmit (TX), receive (RX), and ground. RS-232 is a point-to-point signalling standard, meaning only two devices can be connected to each other. The UART module may support several features which allow it to interface with various signaling standard such as RS-232 or RS-485 – another serial interface commonly used in industrial applications. RS-232 has no dependency on any higher level protocol, however it does have a simple layer 1 (physical layer) set of standards which must be followed. The UART is the peripheral on the microcontroller which can send and receive serial data asynchronously, while RS-232 is a signalling standard. It is important to distinguish the difference between the terms UART and RS-232. In this lesson, we will learn a bit about the theory behind UART and RS-232, learn how to write a simple UART driver for the MSP430, and create a menu which gives the user the ability to change the frequency of the blinking LED during runtime. These days most PCs not longer have a serial port, so it is more commonly seen implemented using USB, however the concept is the same. The debug console is a serial interface which historically is implemented as RS-232 to connect with a PC serial port. When a board arrives in the hands of the software/firmware team, the first step is typically to get the debug console functional. One of the most common interfaces used in embedded systems is the universal asynchronous receiver/transmitter (UART). It could be to transferring data to another device, sending and receiving commands, or simply for debugging purposes. An embedded system often requires a means for communicating with the external world for a number of possible reasons.
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