| Note: This tutorial assumes that you have completed the previous tutorials: Tivaware and toolchain setup. |
 Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags. |
RGB subscriber
Description: EK-TM4C123GXL board over CDC USB device subscribes to ColorRGB msg type and change LED color.Tutorial Level: INTERMEDIATE
Next Tutorial: RGB service
Introduction
Having created several publishers on the previous tutorials, its about time we implement a subscriber.
Our application will subscribe to the topic /led and change the color accordingly to the data on the ColorRGBA message.
Source code explained
1 #include <stdbool.h>
2 #include <stdint.h>
3 // TivaC specific includes
4 extern "C"
5 {
6 #include <driverlib/sysctl.h>
7 #include <driverlib/gpio.h>
8 #include <rgb.h>
9 }
10 // ROS includes
11 #include <ros.h>
12 #include <std_msgs/ColorRGBA.h>
13
14 uint32_t current_color[3] = {0xFFFE, 0xFFFE, 0xFFFE};
15 float current_intensity = 1.f;
16 void color_cb(const std_msgs::ColorRGBA& msg)
17 {
18 current_color[0] = msg.r * 0xFFFE;
19 current_color[1] = msg.g * 0xFFFE;
20 current_color[2] = msg.b * 0xFFFE;
21 current_intensity = msg.a;
22 RGBSet(current_color, current_intensity);
23 }
24
25 // ROS nodehandle and subscriber
26 ros::NodeHandle nh;
27 ros::Subscriber<std_msgs::ColorRGBA> rgb_sub("led", &color_cb);
28
29 int main(void)
30 {
31 // TivaC application specific code
32 MAP_FPUEnable();
33 MAP_FPULazyStackingEnable();
34 // TivaC system clock configuration. Set to 80MHz.
35 MAP_SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_XTAL_16MHZ | SYSCTL_OSC_MAIN);
36
37 // Setup RGB driver
38 RGBInit(0);
39 RGBSet(current_color, current_intensity);
40
41 // ROS nodehandle initialization and topic registration
42 nh.initNode();
43 nh.subscribe(rgb_sub);
44 bool nh_prev_state = false;
45
46 while (1)
47 {
48 // If subscribed, enable RGB driver
49 if (nh.connected() && !nh_prev_state)
50 {
51 RGBEnable();
52 nh_prev_state = true;
53 }
54 if (!nh.connected() && nh_prev_state)
55 {
56 RGBDisable();
57 nh_prev_state = false;
58 }
59
60 // Handle all communications and callbacks.
61 nh.spinOnce();
62
63 // Delay for a bit.
64 nh.getHardware()->delay(1000);
65 }
66 }
Again, use the project rgb_led from `rosserial_tivac_tutorials package as reference to follow the tutorial.
As always, the node is initiated and all the publishers and subscribers must be registered.
Here we instantiate a new subscriber on the topic /led of message type ColorRGBA, handled by color_cb function.
14 uint32_t current_color[3] = {0xFFFE, 0xFFFE, 0xFFFE};
15 float current_intensity = 1.f;
16 void color_cb(const std_msgs::ColorRGBA& msg)
17 {
18 current_color[0] = msg.r * 0xFFFE;
19 current_color[1] = msg.g * 0xFFFE;
20 current_color[2] = msg.b * 0xFFFE;
21 current_intensity = msg.a;
22 RGBSet(current_color, current_intensity);
23 }
The callback function will be executed for each new received message. It takes color and intensity values in the range [0 1] and applies the color to the board LED using the board specific library.
46 while (1)
47 {
48 // If subscribed, enable RGB driver
49 if (nh.connected() && !nh_prev_state)
50 {
51 RGBEnable();
52 nh_prev_state = true;
53 }
54 if (!nh.connected() && nh_prev_state)
55 {
56 RGBDisable();
57 nh_prev_state = false;
58 }
59
60 // Handle all communications and callbacks.
61 nh.spinOnce();
62
63 // Delay for a bit.
64 nh.getHardware()->delay(1000);
65 }
Then execution is trapped inside a loop, continuously waiting for new messages on the topic.
However the LED will only be enabled when a valid node connection is present.
Build and flash
By now you're an expert at this. Build and flash the binary file into your board.
catkin_make catkin_make rosserial_tivac_tutorials_rgb_led_flash
Test it!
Don't forget to connect the device USB port to your computer!
Run roscore and run serial_node.py from rosserial_python on the correct port /dev/ttyACM#.
roscore
When you successfully connect using `serial_node the LED will light up.
rosrun rosserial_python serial_node.py _port:=/dev/ttyACM0
On a new terminal window, publish a new color.
rostopic pub /led std_msgs/ColorRGBA "r: 0.0 g: 1.0 b: 0.0 a: 1.0"
It should change to the brightest green.
