《Code and Life – AVR ATtiny USB Tutorial Part 4》.pdf

Home | Electronics | Raspberry Pi | V-USB tutorials | Reviews | General | Web coding February AVR ATtiny USB Tutorial Part 4 4 2012 Electronics , V-USB tutorials 59 Comments or trackbacks All right. Now that we got the basic USB code working in part 3, it’s time to wrap things up in this tutorial series. This fourth section will explain how to send data from your device to PC and also the other way around. I may later do a fifth part on how to make a USB HID device like a keyboard or mouse, so if you haven’t already, I’d recommend subscribing to the RSS feed to get updates. Sending data f rom device to PC If you look carefully at our command-line client code, you probably noticed that the control messages sent to toggle the led are of type USB_ENDPOINT_IN and we have a 256-byte buffer in place to receive any data the device sends. So far we have not received any data and the return value stored in nBytes has been zero. Let’s change that. If you read through the documentation in usbdrv/usbdrv.h, more specifically the comments for usbFunctionSetup() we used to toggle our LED on device side, you can see that there are two ways to return data from usbFunctionSetup(): 1. Set the global pointer usbMsgPtr to a (static RAM) memory block and return the length of data from the function 2. Define usbFunctionRead() function to do it yourself We will be using the first method. First we define an additional message USB_DATA_OUT and a static buffer to store the data we want to send from device to PC: #define USB_DATA_OUT 2 static uchar replyBuf[16] Hello, USB!; Then we add a new case to usbFunctionSetup() to return the contents of that buffer: case USB_DATA_OUT: // send data to PC usbMsgPtr replyBuf; return sizeof(replyBuf); Whoa, that was easy! V-USB does all the rest for us. Now