0016_Isochronous_Transfers.md

Isochronous Transfers

参考文档

• Isochronous Transfers:
  An Isochronous pipe sends a new data packet in every frame, regardless of the success or failure of the last packet.
• USB Protocol

A special packet is the Start-of-Frame packet (SOF) that splits the USB bus into time segments. Each pipe is allocated a slot in each frame. The Start-of-Frame packet is sent every 1ms on full speed links. At high speed, the 1ms frame is divided into 8 microframes of 125μs each. A Start-of-Frame packet is sent at the beginning of each microframe using the same frame number. The frame number increments every 1ms.

Isochronous IN

• 处理同步数据方法

  [...省略]
  case Alt6_IsocINOUT:
  {
      // Send data on EP2
      while(!(EP2468STAT & bmEP2FULL))
      {
          EP2FIFOBUF[0] = LSB(mycount);
          EP2FIFOBUF[1] = MSB(mycount);
          EP2FIFOBUF[2] = USBFRAMEL;
          EP2FIFOBUF[3] = USBFRAMEH;
          EP2FIFOBUF[4] = MICROFRAME;
  
          EP2BCH = 0x04;
          EP2BCL = 0x00;
  
          mycount++;
      }
  
      // check EP6 EMPTY(busy) bit in EP2468STAT (SFR), core set's this bit when FIFO is empty
      if(!(EP2468STAT & bmEP6EMPTY))
      { 
          EP6BCL = 0x80;          // re(arm) EP6OUT
      }
  }
  break;
  [...省略]

• 运行效果
  
• 目前不知道为什么端点缓冲区是1k Byte，但是发送的数据却至少要8k Byte的倍数，难道是因为至少用1ms来算？暂不追查原理。
• StreamExample采用的是16k Byte的参数；

Isochronous OUT

Buffer Data

初始化端点2的4个buffer中的数据

void TD_Init(void)             // Called once at startup
{
   [...省略]

   // Prepare data
   for (i=1;i<5;i++)
   {
      EP2FIFOBUF[0] = LSB(mycount);
      EP2FIFOBUF[1] = MSB(mycount);
      EP2FIFOBUF[2] = USBFRAMEL;
      EP2FIFOBUF[3] = USBFRAMEH;
      EP2FIFOBUF[4] = MICROFRAME;
      for (j=5;j<1024;j++)
      {
         EP2FIFOBUF[j] = i;
      }
      EP2BCH = 0x02;
      EP2BCL = 0x00;
   }

   [...省略]
}