一，串口介绍
1.1 串口简介
串行接口简称串口，也称串行通信接口或串行通讯接口（通常指COM接口），是采用串行通信方式的扩展接口；
串行接口（SerialInterface）是指数据一位一位地顺序传送。其特点是通信线路简单，只要一对传输线就可以实现双向通信（可以直接利用电话线作为传输线），从而大大降低了成本，特别适用于远距离通信，但传送速度较慢；
1.2 串口使用场景
串口是一种用于android开发板对硬件设备通信的一种协议，通过发送某种指令控制硬件设备，通常用于物联网设备的信息传输，比如切割器，打印机，ATM吐卡机、IC/ID卡读卡等。
1.3 波特率
波特率表示串口传输速率，用来衡量数据传输的快慢，即单位时间内载波参数变化的次数，如每秒钟传送240个字符，而每个字符格式包含10位（1个起始位，1个停止位，8个数据位），这时的波特率为240Bd，比特率为10位*240个/秒=2400bps。波特率与距离成反比，波特率越大传输距离相应的就越短；
1.4 数据位
这是衡量通信中实际数据位的参数。当计算机发送一个信息包，实际的数据往往不会是8位的，标准的值是6、7和8位。如何设置取决于你想传送的信息；
1.5 停止位
用于表示单个包的最后一位。典型的值为1，1.5和2位。由于数据是在传输线上定时的，并且每一个设备有其自己的时钟，很可能在通信中两台设备间出现了小小的不同步。因此停止位不仅仅是表示传输的结束，并且提供计算机校正时钟同步的机会。适用于停止位的位数越多，不同时钟同步的容忍程度越大，但是数据传输率同时也越慢；
1.6 校验位
在串口通信中一种简单的检错方式。有四种检错方式：偶、奇、高和低。当然没有校验位也是可以的。对于偶和奇校验的情况，串口会设置校验位（数据位后面的一位），用一个值确保传输的数据有偶个或者奇个逻辑高位；
1.7 串口地址
不同操作系统的串口地址，Android是基于Linux的所以一般情况下使用Android系统的设备串口地址为/dev/ttyS0；
- /dev的串口包括：虚拟串口，真实串口，USB转串口
- 真实串口：/dev/tty0..tty1这个一般为机器自带COM口
- 虚拟串口：/dev/ttyS1...ttyS2...ttyS3...均为虚拟console，同样可以作为输入输出口
- USB转串口：/dev/tty/USB0
二 Android中串口的实践
2.1 由于串口底层需要调用C代码，所以需要用jni来进行C交互，下面是全部的C代码，以及JNI调用
2.1 SerialPort.h
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class android_serialport_SerialPort */
#ifndef _Included_android_serialport_SerialPort
#define _Included_android_serialport_SerialPort
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: android_serialport_SerialPort
* Method: open
* Signature: (Ljava/lang/String;IIIII)Ljava/io/FileDescriptor;
*/
JNIEXPORT jobject JNICALL Java_android_serialport_SerialPort_open
(JNIEnv *, jobject, jstring, jint, jint, jint, jint, jint);
/*
* Class: android_serialport_SerialPort
* Method: close
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_android_serialport_SerialPort_close
(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
/* Header for class android_serialport_SerialPort_Builder */
#ifndef _Included_android_serialport_SerialPort_Builder
#define _Included_android_serialport_SerialPort_Builder
#ifdef __cplusplus
extern "C" {
#endif
#ifdef __cplusplus
}
#endif
#endif
2.2 SerialPort.c
#include <termios.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <jni.h>
#include "SerialPort.h"
#include "android/log.h"
static const char *TAG = "serial_port";
#define LOGI(fmt, args...) __android_log_print(ANDROID_LOG_INFO, TAG, fmt, ##args)
#define LOGD(fmt, args...) __android_log_print(ANDROID_LOG_DEBUG, TAG, fmt, ##args)
#define LOGE(fmt, args...) __android_log_print(ANDROID_LOG_ERROR, TAG, fmt, ##args)
static speed_t getBaudrate(jint baudrate) {
switch (baudrate) {
case 0:
return B0;
case 50:
return B50;
case 75:
return B75;
case 110:
return B110;
case 134:
return B134;
case 150:
return B150;
case 200:
return B200;
case 300:
return B300;
case 600:
return B600;
case 1200:
return B1200;
case 1800:
return B1800;
case 2400:
return B2400;
case 4800:
return B4800;
case 9600:
return B9600;
case 19200:
return B19200;
case 38400:
return B38400;
case 57600:
return B57600;
case 115200:
return B115200;
case 230400:
return B230400;
case 460800:
return B460800;
case 500000:
return B500000;
case 576000:
return B576000;
case 921600:
return B921600;
case 1000000:
return B1000000;
case 1152000:
return B1152000;
case 1500000:
return B1500000;
case 2000000:
return B2000000;
case 2500000:
return B2500000;
case 3000000:
return B3000000;
case 3500000:
return B3500000;
case 4000000:
return B4000000;
default:
return -1;
}
}
/*
* Class: android_serialport_SerialPort
* Method: open
* Signature: (Ljava/lang/String;II)Ljava/io/FileDescriptor;
*/
JNIEXPORT jobject JNICALL Java_android_serialport_SerialPort_open
(JNIEnv *env, jobject thiz, jstring path, jint baudrate, jint dataBits, jint parity,
jint stopBits,
jint flags) {
int fd;
speed_t speed;
jobject mFileDescriptor;
/* Check arguments */
{
speed = getBaudrate(baudrate);
if (speed == -1) {
/* TODO: throw an exception */
LOGE("Invalid baudrate");
return NULL;
}
}
/* Opening device */
{
jboolean iscopy;
const char *path_utf = (*env)->GetStringUTFChars(env, path, &iscopy);
LOGD("Opening serial port %s with flags 0x%x", path_utf, O_RDWR | flags);
fd = open(path_utf, O_RDWR | flags);
LOGD("open() fd = %d", fd);
(*env)->ReleaseStringUTFChars(env, path, path_utf);
if (fd == -1) {
/* Throw an exception */
LOGE("Cannot open port");
/* TODO: throw an exception */
return NULL;
}
}
/* Configure device */
{
struct termios cfg;
LOGD("Configuring serial port");
if (tcgetattr(fd, &cfg)) {
LOGE("tcgetattr() failed");
close(fd);
/* TODO: throw an exception */
return NULL;
}
cfmakeraw(&cfg);
cfsetispeed(&cfg, speed);
cfsetospeed(&cfg, speed);
cfg.c_cflag &= ~CSIZE;
switch (dataBits) {
case 5:
cfg.c_cflag |= CS5; //使用5位数据位
break;
case 6:
cfg.c_cflag |= CS6; //使用6位数据位
break;
case 7:
cfg.c_cflag |= CS7; //使用7位数据位
break;
case 8:
cfg.c_cflag |= CS8; //使用8位数据位
break;
default:
cfg.c_cflag |= CS8;
break;
}
switch (parity) {
case 0:
cfg.c_cflag &= ~PARENB; //无奇偶校验
break;
case 1:
cfg.c_cflag |= (PARODD | PARENB); //奇校验
break;
case 2:
cfg.c_iflag &= ~(IGNPAR | PARMRK); // 偶校验
cfg.c_iflag |= INPCK;
cfg.c_cflag |= PARENB;
cfg.c_cflag &= ~PARODD;
break;
default:
cfg.c_cflag &= ~PARENB;
break;
}
switch (stopBits) {
case 1:
cfg.c_cflag &= ~CSTOPB; //1位停止位
break;
case 2:
cfg.c_cflag |= CSTOPB; //2位停止位
break;
default:
cfg.c_cflag &= ~CSTOPB; //1位停止位
break;
}
if (tcsetattr(fd, TCSANOW, &cfg)) {
LOGE("tcsetattr() failed");
close(fd);
/* TODO: throw an exception */
return NULL;
}
}
/* Create a corresponding file descriptor */
{
jclass cFileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");
jmethodID iFileDescriptor = (*env)->GetMethodID(env, cFileDescriptor, "<init>", "()V");
jfieldID descriptorID = (*env)->GetFieldID(env, cFileDescriptor, "descriptor", "I");
mFileDescriptor = (*env)->NewObject(env, cFileDescriptor, iFileDescriptor);
(*env)->SetIntField(env, mFileDescriptor, descriptorID, (jint) fd);
}
return mFileDescriptor;
}
/*
* Class: cedric_serial_SerialPort
* Method: close
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_android_serialport_SerialPort_close
(JNIEnv *env, jobject thiz) {
jclass SerialPortClass = (*env)->GetObjectClass(env, thiz);
jclass FileDescriptorClass = (*env)->FindClass(env, "java/io/FileDescriptor");
jfieldID mFdID = (*env)->GetFieldID(env, SerialPortClass, "mFd", "Ljava/io/FileDescriptor;");
jfieldID descriptorID = (*env)->GetFieldID(env, FileDescriptorClass, "descriptor", "I");
jobject mFd = (*env)->GetObjectField(env, thiz, mFdID);
jint descriptor = (*env)->GetIntField(env, mFd, descriptorID);
LOGD("close(fd = %d)", descriptor);
close(descriptor);
}
2.3 gen_SerialPort_h.sh 生成java的文件目录
#!/bin/sh
javah -o SerialPort.h -jni -classpath ../java android.serialport.SerialPort
2.4 SerialPort.java jni对应的java文件
/*
* Copyright 2009 Cedric Priscal
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.serialport;
import android.util.Log;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileDescriptor;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
public final class SerialPort {
private static final String TAG = "SerialPort";
public static final String DEFAULT_SU_PATH = "/system/bin/su";
private static String sSuPath = DEFAULT_SU_PATH;
private File device;
private int baudrate;
private int dataBits;
private int parity;
private int stopBits;
private int flags;
/**
* Set the su binary path, the default su binary path is {@link #DEFAULT_SU_PATH}
*
* @param suPath su binary path
*/
public static void setSuPath(@Nullable String suPath) {
if (suPath == null) {
return;
}
sSuPath = suPath;
}
/**
* Get the su binary path
*
* @return
*/
@NonNull
public static String getSuPath() {
return sSuPath;
}
/*
* Do not remove or rename the field mFd: it is used by native method close();
*/
private FileDescriptor mFd;
private FileInputStream mFileInputStream;
private FileOutputStream mFileOutputStream;
/**
* 串口
*
* @param device 串口设备文件
* @param baudrate 波特率
* @param dataBits 数据位；默认8,可选值为5~8
* @param parity 奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
* @param stopBits 停止位；默认1；1:1位停止位；2:2位停止位
* @param flags 默认0
* @throws SecurityException
* @throws IOException
*/
public SerialPort(@NonNull File device, int baudrate, int dataBits, int parity, int stopBits,
int flags) throws SecurityException, IOException {
this.device = device;
this.baudrate = baudrate;
this.dataBits = dataBits;
this.parity = parity;
this.stopBits = stopBits;
this.flags = flags;
/* Check access permission */
Log.e(TAG, "SerialPort: canRead" + device.canRead());
if (!device.canRead() || !device.canWrite()) {
try {
/* Missing read/write permission, trying to chmod the file */
Process su;
su = Runtime.getRuntime().exec(sSuPath);
String cmd = "chmod 666 " + device.getAbsolutePath() + "\n" + "exit\n";
su.getOutputStream().write(cmd.getBytes());
if ((su.waitFor() != 0) || !device.canRead() || !device.canWrite()) {
throw new SecurityException();
}
} catch (Exception e) {
e.printStackTrace();
throw new SecurityException();
}
}
mFd = open(device.getAbsolutePath(), baudrate, dataBits, parity, stopBits, flags);
if (mFd == null) {
Log.e(TAG, "native open returns null");
throw new IOException();
}
mFileInputStream = new FileInputStream(mFd);
mFileOutputStream = new FileOutputStream(mFd);
}
/**
* 串口，默认的8n1
*
* @param device 串口设备文件
* @param baudrate 波特率
* @throws SecurityException
* @throws IOException
*/
public SerialPort(@NonNull File device, int baudrate) throws SecurityException, IOException {
this(device, baudrate, 8, 0, 1, 0);
}
/**
* 串口
*
* @param device 串口设备文件
* @param baudrate 波特率
* @param dataBits 数据位；默认8,可选值为5~8
* @param parity 奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
* @param stopBits 停止位；默认1；1:1位停止位；2:2位停止位
* @throws SecurityException
* @throws IOException
*/
public SerialPort(@NonNull File device, int baudrate, int dataBits, int parity, int stopBits)
throws SecurityException, IOException {
this(device, baudrate, dataBits, parity, stopBits, 0);
}
// Getters and setters
@NonNull
public InputStream getInputStream() {
return mFileInputStream;
}
@NonNull
public OutputStream getOutputStream() {
return mFileOutputStream;
}
/**
* 串口设备文件
*/
@NonNull
public File getDevice() {
return device;
}
/**
* 波特率
*/
public int getBaudrate() {
return baudrate;
}
/**
* 数据位；默认8,可选值为5~8
*/
public int getDataBits() {
return dataBits;
}
/**
* 奇偶校验；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
*/
public int getParity() {
return parity;
}
/**
* 停止位；默认1；1:1位停止位；2:2位停止位
*/
public int getStopBits() {
return stopBits;
}
public int getFlags() {
return flags;
}
// JNI
private native FileDescriptor open(String absolutePath, int baudrate, int dataBits, int parity,
int stopBits, int flags);
public native void close();
/**
* 关闭流和串口，已经try-catch
*/
public void tryClose() {
try {
mFileInputStream.close();
} catch (IOException e) {
//e.printStackTrace();
}
try {
mFileOutputStream.close();
} catch (IOException e) {
//e.printStackTrace();
}
try {
close();
} catch (Exception e) {
//e.printStackTrace();
}
}
static {
System.loadLibrary("serial_port");
}
public static Builder newBuilder(File device, int baudrate) {
return new Builder(device, baudrate);
}
public static Builder newBuilder(String devicePath, int baudrate) {
return new Builder(devicePath, baudrate);
}
public final static class Builder {
private File device;
private int baudrate;
private int dataBits = 8;
private int parity = 0;
private int stopBits = 1;
private int flags = 0;
private Builder(File device, int baudrate) {
this.device = device;
this.baudrate = baudrate;
}
private Builder(String devicePath, int baudrate) {
this(new File(devicePath), baudrate);
}
/**
* 数据位
*
* @param dataBits 默认8,可选值为5~8
* @return
*/
public Builder dataBits(int dataBits) {
this.dataBits = dataBits;
return this;
}
/**
* 校验位
*
* @param parity 0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
* @return
*/
public Builder parity(int parity) {
this.parity = parity;
return this;
}
/**
* 停止位
*
* @param stopBits 默认1；1:1位停止位；2:2位停止位
* @return
*/
public Builder stopBits(int stopBits) {
this.stopBits = stopBits;
return this;
}
/**
* 标志
*
* @param flags 默认0
* @return
*/
public Builder flags(int flags) {
this.flags = flags;
return this;
}
/**
* 打开并返回串口
*
* @return
* @throws SecurityException
* @throws IOException
*/
public SerialPort build() throws SecurityException, IOException {
return new SerialPort(device, baudrate, dataBits, parity, stopBits, flags);
}
}
public static void checkFilePermission(File file) {
Log.e(TAG, "canRead: " + file.canRead());
Log.e(TAG, "canWrite: " + file.canWrite());
if (!file.canRead() || !file.canWrite()) {
try {
/* Missing read/write permission, trying to chmod the file */
Process su;
su = Runtime.getRuntime().exec(sSuPath);
String cmd = "chmod 7777 " + file.getAbsolutePath() + "\n" + "exit\n";
su.getOutputStream().write(cmd.getBytes());
Log.e(TAG, "checkFilePermission: " + file.getAbsolutePath());
if ((su.waitFor() != 0) || !file.canRead() || !file.canWrite()) {
throw new SecurityException();
}
} catch (Exception e) {
e.printStackTrace();
Log.e(TAG, "checkFilePermission: Exception:" + e.getMessage());
throw new SecurityException();
}
}
}
//隐藏系统导航栏
public void hideBottomNavation() {
chmod("mount -o remount -w /system");
chmod("chmod 777 /system");
chmod("echo qemu.hw.mainkeys=1 >> /system/build.prop");
}
public void chmod(String instruct) {
try {
Process process = null;
DataOutputStream os = null;
process = Runtime.getRuntime().exec("su");
os = new DataOutputStream(process.getOutputStream());
os.writeBytes(instruct);
os.flush();
os.close();
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
2.5 CMakeLists.txt，编译c或c++程序的规则文件
Cmake在Jni那篇讲过，这个地方在讲下
CMake是一个可以跨平台的编译工具，可以用简单的语句来描述所有平台的编译过程。他能够输出各种各样的 makefile 或者工程文件。和make与makefile类似，我们在使用CMake时同样也需要一个文件来提供规则，这个文件就是CMakeLists
使用CMake编写跨平台工程的流程如下：
(1)编写源文件
(2)编写CMakeLists.txt
(3)由CMake根据CMakeLists.txt来生成相应的makefile文件
(4)使用make并根据makefile调用gcc来生成相应的可执行文件。
# Sets the minimum version of CMake required to build your native library.
# This ensures that a certain set of CMake features is available to
# your build.
cmake_minimum_required(VERSION 3.4.1)
# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add.library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.
add_library( # Specifies the name of the library.
serial_port
# Sets the library as a shared library.
SHARED
# Provides a relative path to your source file(s).
src/main/cpp/SerialPort.c )
find_library( # Sets the name of the path variable.
log-lib
# Specifies the name of the NDK library that
# you want CMake to locate.
log )
# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.
target_link_libraries( # Specifies the target library.
serial_port
# Links the target library to the log library
# included in the NDK.
${log-lib} )
2.5 调用串口-连接，并获取输入输出流
Runnable serialConnectRunnable = new Runnable() {
@Override
public void run() {
try {
if (mSerialPort == null) {
mSerialPort = new SerialPort(new File(path), baudrate);
mOutputStream = mSerialPort.getOutputStream();
mInputStream = mSerialPort.getInputStream();
}
} catch (SecurityException e) {
ToastUtil.showToast(App.getInstance(), "You do not have read/write permission to the serial port.");
} catch (IOException e) {
ToastUtil.showToast(App.getInstance(), "The serial port can not be opened for an unknown reason.");
} catch (InvalidParameterException e) {
ToastUtil.showToast(App.getInstance(), "Please configure your serial port first.");
}
//Serial结束
}
};
2.6 读取串口消息
private class ReadThread extends Thread {
@Override
public void run() {
super.run();
while (!isInterrupted()) {
int size;
try {
byte[] buffer = new byte[512];
if (mInputStream == null) return;
size = mInputStream.read(buffer);
if (size > 0) {
String mReception=new String(buffer, 0, size);
String msg = mReception.toString().trim();
Log.e(TAG, "接收短消息：" + msg);
}
} catch (IOException e) {
e.printStackTrace();
return;
}
}
}
}
2.7 发送串口指令
private class WriteRunnable implements Runnable {
@Override
public void run() {
try {
String cmd="KZMT;";
Log.e(TAG, "发送短消息：" + cmd);
mOutputStream.write(cmd.getBytes());
mOutputStream.flush();
} catch (IOException e) {
}
}
}
2.8 断开关闭串口
/**
* 关闭串口连接
*/
public void closeSerialPortStream() {
try {
if (mOutputStream != null) {
mOutputStream.close();
mOutputStream = null;
}
if (mInputStream != null) {
mInputStream.close();
mInputStream = null;
}
if (mSerialPort != null) {
mSerialPort.close();
mSerialPort = null;
}
} catch (Exception e) {
e.printStackTrace();
}
}
三 google官方串口工具类
3.1 除了上面自己编程C底层文件，也可以直接用google官方的串口工具SDK(android-serialport-api)，Github串口Demo地址:https://github.com/licheedev/Android-SerialPort-API
3.2 依赖：
allprojects {
repositories {
...
jcenter()
mavenCentral() // since 2.1.3
}
}
dependencies {
implementation 'com.licheedev:android-serialport:2.1.3'
}
3.3 使用
// 默认8N1(8数据位、无校验位、1停止位)
// Default 8N1 (8 data bits, no parity bit, 1 stop bit)
SerialPort serialPort = new SerialPort(path, baudrate);
// 可选配置数据位、校验位、停止位 - 7E2(7数据位、偶校验、2停止位)
// or with builder (with optional configurations) - 7E2 (7 data bits, even parity, 2 stop bits)
SerialPort serialPort = SerialPort
.newBuilder(path, baudrate)
// 校验位；0:无校验位(NONE，默认)；1:奇校验位(ODD);2:偶校验位(EVEN)
// Check bit; 0: no check bit (NONE, default); 1: odd check bit (ODD); 2: even check bit (EVEN)
// .parity(2)
// 数据位,默认8；可选值为5~8
// Data bit, default 8; optional value is 5~8
// .dataBits(7)
// 停止位，默认1；1:1位停止位；2:2位停止位
// Stop bit, default 1; 1:1 stop bit; 2: 2 stop bit
// .stopBits(2)
.build();
// read/write to serial port - needs to be in different thread!
InputStream in = serialPort.getInputStream();
OutputStream out = serialPort.getOutputStream();
// close
serialPort.tryClose();
四 总结
串口通讯使用到进程、Linux指令、JNI等，但本质最终还是获得一个输入输出流去进行读写操作；
串口通讯对于Android开发者来说，仅需关注如何连接、操作(发送指令)、读取数据。
大部分的物联网通信本质上都是获取io流，通过io流进行数据的传输和读取，比如蓝牙，wifi等，只不过蓝牙，wifi是通过Socket协议维持一个长连接进行通信