前言
在Linux中对USB类型的CDC设备，默认识别为一个USBTTY设备，也就是串口设备，可以使用串口通讯的方式进行通讯，但在USB固件开发时，CDC类型的设备更容易实现，参考的代码也更多，实际并不是一个串口设备，或者想通过USB的EndPoint调用libusb的API直接进行通讯。
参考代码
以下是在linux中直接调用libusb进行USB通讯的参考代码：
/*
* This is a simple example to communicate with a CDC-ACM USB device
* using libusb.
*/
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <libusb-1.0/libusb.h>
/* You may want to change the VENDOR_ID and PRODUCT_ID
* depending on your device.
*/
#define VENDOR_ID 0x2341 // Arduino LLC
#define PRODUCT_ID 0x0034 // Arduino Leonardo
#define ACM_CTRL_DTR 0x01
#define ACM_CTRL_RTS 0x02
/* We use a global variable to keep the device handle
*/
static struct libusb_device_handle *devh = NULL;
/* The Endpoint address are hard coded. You should use lsusb -v to find
* the values corresponding to your device.
*/
static int ep_in_addr = 0x83;
static int ep_out_addr = 0x02;
void write_char(unsigned char c)
{
/* To send a char to the device simply initiate a bulk_transfer to the
* Endpoint with address ep_out_addr.
*/
int actual_length;
if (libusb_bulk_transfer(devh, ep_out_addr, &c, 1,
&actual_length, 0) < 0) {
fprintf(stderr, "Error while sending char\n");
}
}
int read_chars(unsigned char * data, int size)
{
/* To receive characters from the device initiate a bulk_transfer to the
* Endpoint with address ep_in_addr.
*/
int actual_length;
int rc = libusb_bulk_transfer(devh, ep_in_addr, data, size, &actual_length,
1000);
if (rc == LIBUSB_ERROR_TIMEOUT) {
printf("timeout (%d)\n", actual_length);
return -1;
} else if (rc < 0) {
fprintf(stderr, "Error while waiting for char\n");
return -1;
}
return actual_length;
}
int main(int argc, char **argv)
{
int rc;
/* Initialize libusb
*/
rc = libusb_init(NULL);
if (rc < 0) {
fprintf(stderr, "Error initializing libusb: %s\n", libusb_error_name(rc));
exit(1);
}
/* Set debugging output to max level.
*/
libusb_set_debug(NULL, 3);
/* Look for a specific device and open it.
*/
devh = libusb_open_device_with_vid_pid(NULL, VENDOR_ID, PRODUCT_ID);
if (!devh) {
fprintf(stderr, "Error finding USB device\n");
goto out;
}
/* As we are dealing with a CDC-ACM device, it's highly probable that
* Linux already attached the cdc-acm driver to this device.
* We need to detach the drivers from all the USB interfaces. The CDC-ACM
* Class defines two interfaces: the Control interface and the
* Data interface.
*/
for (int if_num = 0; if_num < 2; if_num++) {
if (libusb_kernel_driver_active(devh, if_num)) {
libusb_detach_kernel_driver(devh, if_num);
}
rc = libusb_claim_interface(devh, if_num);
if (rc < 0) {
fprintf(stderr, "Error claiming interface: %s\n",
libusb_error_name(rc));
goto out;
}
}
/* Start configuring the device:
* - set line state
*/
rc = libusb_control_transfer(devh, 0x21, 0x22, ACM_CTRL_DTR | ACM_CTRL_RTS,
0, NULL, 0, 0);
if (rc < 0) {
fprintf(stderr, "Error during control transfer: %s\n",
libusb_error_name(rc));
}
/* - set line encoding: here 9600 8N1
* 9600 = 0x2580 ~> 0x80, 0x25 in little endian
*/
unsigned char encoding[] = { 0x80, 0x25, 0x00, 0x00, 0x00, 0x00, 0x08 };
rc = libusb_control_transfer(devh, 0x21, 0x20, 0, 0, encoding,
sizeof(encoding), 0);
if (rc < 0) {
fprintf(stderr, "Error during control transfer: %s\n",
libusb_error_name(rc));
}
/* We can now start sending or receiving data to the device
*/
unsigned char buf[65];
int len;
while(1) {
write_char('t');
len = read_chars(buf, 64);
buf[len] = 0;
fprintf(stdout, "Received: \"%s\"\n", buf);
sleep(1);
}
libusb_release_interface(devh, 0);
out:
if (devh)
libusb_close(devh);
libusb_exit(NULL);
return rc;
}
代码解析
以上代码，初始化init和open都与正常调用libusb设备一样，不同的是如下代码：
for (int if_num = 0; if_num < 2; if_num++) {
if (libusb_kernel_driver_active(devh, if_num)) {
libusb_detach_kernel_driver(devh, if_num);
}
rc = libusb_claim_interface(devh, if_num);
if (rc < 0) {
fprintf(stderr, "Error claiming interface: %s\n",
libusb_error_name(rc));
goto out;
}
}
由于我们要使用CDC-ACM设备，很可能Linux已经将cdc-acm驱动程序连接到此设备，我们需要从所有USB接口上分离驱动程序。这里CDC-ACM类定义了两个接口：Control接口和数据接口。所以，这里循环遍历两个接口，将接口已经配置的内核驱动进行分离，然后claim接口就可以调用libusb与设备进行通讯了。
然后，再调用libusb_control_transfer对串口参数进行配置（如果需要）后，就可以试用libusb_bulk_transfer进行数据传输了。
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