Many programmers have often taken to the mentality that it's a good thing to use buffers when dealing with USB. I strongly disagree. If you can do things with direct access to the hardware, it often behooves you to do so. Ordinarily, an ATMega32u2 would only be able to buffer ~300 WS2812 LEDs, or less depending on the USB stack.
By taking out the buffers and piplining the code, you can send over 1,000 LEDs in one go! AND! With less latency! All with just a single control message, so controlling it is trivial!
Turns out those "limitations" of the AVR like 1kB of RAM aren't really that big of a deal after all.
Check out the video here: https://www.youtube.com/watch?v=Y5ldIov4JmM
All you need for this project: An ATMega32u2 with a USB port attached to it, and a string of WS2812's hooked up to PORTD.3.
It's particularly easy to use. In libUSB, connect, then send it 0xA5 messages.
libusb_init(NULL)
devh = libusb_open_device_with_vid_pid( NULL, 0xabcd, 0xf003 );
libusb_control_transfer( devh,
0x40, //reqtype
0xA5, //request
0x0100, //wValue
0x0000, //wIndex
rbuffer, //LED Data
(LEDS*3)+1, //# of bytes for LEDs
1000 ); //TimeoutThe idea behind how this works is as follows (see usb.c)
- Put code in the AVR's ISR(USB_COM_vect).
- Compute how many blocks will be needed.
- Compute how much remainder there will be (if bytes transferred does not fill frame)
- Start reading data; interleving the WS2812 outputs.
...
buf[0] = UEDATX; //Read data from USB
SEND_WS(buf[0]); //Send WS2812 Data.
buf[1] = UEDATX; //Read a bunch more data.
buf[2] = UEDATX;
buf[3] = UEDATX;
buf[4] = UEDATX; //Can't wait too long before sending another WS2812 byte.
SEND_WS(buf[1]);
buf[5] = UEDATX;
buf[6] = UEDATX;
buf[7] = UEDATX;
buf[8] = UEDATX;
SEND_WS(buf[2]);
buf[9] = UEDATX;
buf[10] = UEDATX;
...
buf[59] = UEDATX; //We were able to sneak a lot of data out, while sending to the WS2812.
SEND_WS(buf[15]);
buf[60] = UEDATX;
buf[61] = UEDATX;
buf[62] = UEDATX;
buf[63] = UEDATX;
//In fact, a whole buffer's worth, we can ack it so the data keeps flowing.
if( l-i > 1 )
usb_ack_out();
//While the USB hardware is handling getting another buffer in, we can flush out the WS2812.
for( j = 16; j < 63; j++ )
{
SEND_WS( (buf[j]) );
_delay_us(4);
}
//That took a long time, so by now, the data should be ready to read!
if( l - i > 1 )
usb_wait_receive_out();
(Repeat) * USB Keyboard Example for Teensy USB Development Board
* http://www.pjrc.com/teensy/usb_keyboard.html
* Copyright (c) 2009 PJRC.COM, LLC
* Copyright (c) 2011-2014 <>< Charles Lohr
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