前一篇《UART Explained》介绍了UART的基本信息,重点分析了UART的信号。本文摘录的文章则重点介绍了波特率(Baud Rate)相关的内容,波特率越高,传输速度越快,但实际使用时波特率是越高越好吗,多少合适?文中给出了答案,具体如下。

  Although the PIC32 is an elegant and powerful microcontroller, it doesn't stand so tall when compared against a PC in terms of raw computing power! Sometimes it makes sense to use the feasability of the microcontroller to work as a sensor or an embedded controller of some type but still be able to communicate with a more powerful computer. Sometimes you might just want a quick visual display or a printf terminal for debugging. Whatever the reason, this tutorial will provide a basic guide to PIC32 / PC communication.

UART

  The module used for such communication is the PIC's Universal Asynchronous Receiver/Transmitter (UART) pronounced "you-art". For those familiar with computer music, the MIDI protocol uses a UART to do its serial communication. For those familiar with the Arduino, the TX/RX pins on an Arduino are the two main UART pins. In its most basic form the UART hardware consists of a transmit (TX) line and a receive (RX) line. The software configures how fast data is sent (the baud rate) and the specifics of the protocol.

Baud Rate

  Since the UART module is asynchronous, there is no external clock line like the synchronous protocols SPI or I2C. Instead, both communicating devices must have their own clock sources configured to the same frequency. As long as these clocks match one another, data should be transferred smoothly. On the PIC32, the UxBRG special function register configures this rate called the baud rate. Note that the x in UxBRG is a placeholder for the reference number of the UART that is being configured. Common baud rates include 4800, 9600, 19200, 57600, and 115200 bits/s. Different baud rates can be chosen by the PIC32 but you must always make sure both devices are very stable at the selected rate. Sometimes it's best to select a lower baud rate to reduce the chance of errors. Since there is no shared clock, the hardware is simpler but there is an increased chance for the clocks to be out of sync causing incorrect data to be interpreted on the receiving end. For this reason, lower baud rates like 9600 are very popular.

  The equation to configure the baud rate using the UxBRG register is below:

8N1

  Like with most peripherals, the configuration options can be overwhelming! We'll be configuring our UART to use the standard 8N1 format. 8N1 meaning that each data trasfer consists of 8 bits of data, no parity bits, and 1 stop bit.

  The parity bit provides an added layer of error detection. The parity bit is generated by the transmitting unit through a calculation done with the data being sent. If the receiving unit then calculates the same parity bit from the received data, it is assumed that everything is A-O-K. If a different parity bit is calculated, an error is triggered. For example, if odd parity checking is used, the transmitting unit would add up all "1" bits from the data being sent and set the parity bit to 1 if they are odd (like 01001100) or to 0 if even (like 01001101). The receiving unit would use the same calculation and compare the received parity bit to the calculated parity bit. Note that parity checking is done in hardware by the UART unit. However, once a parity error is flagged by the hardware, the programmer must deal with the error in software. Since parity checking will only take you so far (two corrupted bits is treated as no error!), you may desire something more powerful like a Cyclic Redundancy Check (CRC) which is outside the scope of this tutorial.

  The stop bit signifies the end of a data transfer. There will almost always be either one or two stop bits. If the stop bits read by the receiving unit don't match those expected, the receiver will assume its clock drifted out of phase and a the software must deal with this error. It is also implied that a start bit is being sent as well with each transmission. The start bit allows the clock on the receiving end to do a phase lock with the data being received. Otherwise, the receiving end might end up sampling on the data transition edges instead of right in the middle of each bit. The data received would then look like utter nonsense!

  So, in an 8N1 configuration, each byte of data sent is transmitted in the form of a 10-bit frame with the addition of one start bit, one stop bit, and no parity bits.

  The example below configures the PIC32's UART1 module to an 8N1 format with a baud rate of 19200 bits/s and enables both transmit and receive functions (in full duplex mode).

int FPb = 40000000; // peripheral bus clock frequency set at max SYSCLK

int desired_baud = 19200;

U1MODE = 0; // disable autobaud, TX and RX enabled only, 8N1, idle=HIGH

U1STA = 0x1400; // enable TX and RX

U1MODESET = 0x8000; // enable UART1

  Note that in the example above, we are using the standard baud mode set whenUxMODE<3> = 0. The UART clock is only able to take on specific frequencies and this bit helps determine how close the desired baud rate will be to the actual baud rate. When BRGH = 1 (i.e. UxMODE<3> = 1), high-speed mode is activated and the actual baud rate will probably be closer to the desired baud rate. However, each bit received on the UxRX pin will only be sampled once. When BRGH = 0 (i.e. UxMODE<3> = 0), multiple samples are taken on the UxRX pin to determine whether a HIGH or LOW is present. Even though the high-speed mode may have a more accurate baud rate clock, in standard-mode there is less chance to make receiving errors. See the UART reference manual section 21.6 for more information.

  The following table provides a set of desired baud rates, the value you would set BRGH to for this baud rate and the error.

  可以看到,实际波特率和目标波特率会有一些误差,如果通信双方的误差都有点大,那就很容易导致通信错误,所以很多串口通信的协议里都会有校验码,如GPS协议,通过数据帧的校验码可以过滤掉因传输导致的错误,避免异常。

  原文中还有参考代码和调试串口的神器,具体参见: http://umassamherstm5.org/tech-tutorials/pic32-tutorials/pic32mx220-tutorials/uart-to-serial-terminal

  另外,有关串口通信时序错误的详细解析,可参考:http://www.robotroom.com/Asynchronous-Serial-Communication-2.html(好似被墙,pdf档在此)

  波特率计算工具:http://mspgcc.sourceforge.net/baudrate.html

UART to Serial Terminal(转载)的更多相关文章

  1. RS-232 vs. TTL Serial Communication(转载)

    RS-232串口一度像现在的USB接口一样,是PC的标准接口,用来连接打印机.Modem和其他一些外设.后来逐渐被USB接口所取代,现在PC上已经看不到它的身影了.开发调试时如果用到串口,一般都是用U ...

  2. 鸿蒙内核源码分析(字符设备篇) | 字节为单位读写的设备 | 百篇博客分析OpenHarmony源码 | v67.01

    百篇博客系列篇.本篇为: v67.xx 鸿蒙内核源码分析(字符设备篇) | 字节为单位读写的设备 | 51.c.h.o 文件系统相关篇为: v62.xx 鸿蒙内核源码分析(文件概念篇) | 为什么说一 ...

  3. JTAG 引脚自动识别 JTAG Finder, JTAG Pinout Tool, JTAG Pin Finder, JTAG pinout detector, JTAGULATOR, Easy-JTAG, JTAG Enumeration

    JTAG Finder Figuring out the JTAG Pinouts on a Device is usually the most time-consuming and frustra ...

  4. Keil debugging techniques and alternative printf (SWO function)

    One of the basic needs of the embedded software development through the terminal to output debugging ...

  5. 随想录(skyeye中的soc仿真)

    [ 声明:版权所有,欢迎转载,请勿用于商业用途. 联系信箱:feixiaoxing @163.com] 想学好soc,再怎么看芯片手册和linux kernel都不为过.但是要学习好kernel,那再 ...

  6. The TTY demystified

    http://www.linusakesson.net/programming/tty/index.php The TTY demystified Real teletypes in the 1940 ...

  7. nRF52-PCA10040——Overview

    Overview Zephyr applications use the nrf52_pca10040 board configuration to run on the nRF52 Developm ...

  8. [Intel Edison开发板] 02、Edison开发板入门

    一.前言 Start from the link: 开始学习的链接 上面链接是官网的教程,按照教程可以开发板入门: 其中第一步是了解开发板,涉及到如何组装.如何连线.一些主要的接口简单介绍等信息: 第 ...

  9. 【图像处理】【SEED-VPM】4.串口调试信息

    —————————————————————————————————————————————————————————————————————— 串口返回正确的信息 Booting PSP Boot Lo ...

随机推荐

  1. Mondrian – 开源的矢量图形 Web 应用程序

    Mondrian 是一个免费矢量图形 Web 应用程序,类似 Adobe Illustrator 或 Inkscape.Mondrian 提供所有所需的工具来创建.修改和导出简单的 SVG 文件,过历 ...

  2. spring(4)——自动装配

    set注入和构造注入有时在做配置时比较麻烦.所以框架为了提高开发效率,提供自动装配功能,简化配置.spring框架式默认不支持自动装配的,要想使用自动装配需要修改spring配置文件中<bean ...

  3. windows server2008 安装问题、sqlserver安装设置默认账户问题

    1.Bios中的satadata设置开启 2账户和密码最好与本机相同

  4. 在内网中OWA第一次访问速度慢的问题

      当网络环境为内网时,有时访问OWA站点一直卡在Office Web App 那里. 这是因为SharePoint有一个证书需要联网检索 此环境为SharePoint 2013 通过下面的三个步骤 ...

  5. SharePoint Online 创建门户网站系列之定制栏目

    前 言 SharePoint Online自带的库就带有二级页面和详细页面,也就是Allitems页面和DispForm页面,但是实在不够美观,尤其对于门户网站这一企业门面来说,更是无法接受. 下面, ...

  6. ora-00119和ora-00132解决方案

    win7 64位    oracle 11g  先登录到sqlplus: sqlplus /nolog; 登录数据库: conn system/manager as sysdba; 然后启动数据库: ...

  7. [android]AndroidInject框架——我的第一个android小型框架

    作为一个移动应用开发者,随着需求的日益增多,Android项目的越来越臃肿,代码量越来越大, 现在冷静下来回头看看我们的代码,有多少代码跟业务逻辑没什么关系的 所以,本人自不量力,在github上建了 ...

  8. Swift tour

    输出函数: print(“hello world!") 无需引入函数库,无须使用“;”作为语句结尾,也无须写跟其它语言一样的main()函数,Swift中,全局区的代码就是程序入口.You ...

  9. Android 用Fragment创建一个选项卡

    本文结合之前的动态创建fragment来进行一个实践,来实现用Fragment创建一个选项卡 本文地址:http://www.cnblogs.com/wuyudong/p/5898075.html,转 ...

  10. Android微信登陆

    前言 分享到微信朋友圈的功能早已经有了,但微信登录推出并不久,文档写的也并不是很清楚,这里记录分享一下. 声明 欢迎转载,但请保留文章原始出处:)  博客园:http://www.cnblogs.co ...