（比较WXS师兄的）USB 同步传输和异步传输固件的区别

比较WXS师兄同步传输固件和异步传输固件的区别，发现dscr.a51、fw.c文件完全一致，tcxmaster.c有一行存在区别，位于初始化函数TD_Init(void)中：

1. 在同步固件中：IFCONFG=0x03；//0000 0011
2. 在异步固件中：IFCONFG=0xCB；//1100 1011

同步0x03，FIFO CLK来自外部，Slave FIFO工作在同步模式之下，Slave FIFO Interface；

异步0xCB，FIFO CLK来自内部，IFCLK频率选为48MHz，Slave FIFO工作在异步模式之下，Slave FIFO Interface；

 

下面是IFCONFG寄存器中bit 3的注释：

Bit3 ASYNC: Slave FIFO Asynchronous Mode.

0 The Slave FIFOs operate synchronously. A clock is supplied either internally or externally on the IFCLK pin; the FIFO control signals function as read and write enable signals for the clock signal.

1 The Slave FIFOs operate asynchronously. No clock signal input to IFCLK is required; the FIFO control signals function directly as read and write strobes.

下面是IFCONFG寄存器中bit 76的解释：

Bit7 IFCLKSRC: FIFO/GPIF Clock Source.

This bit selects the clock source for both the FIFOS and GPIF.

0 The external clock on the IFCLK pin is selected.

1 An internal 30 or 48 MHz (default) clock is used (default).

Bit6 3048MHZ: Internal FIFO/GPIF Clock Frequency

This bit selects the internal FIFO and GPIF clock frequency.

0 30 MHz (default)

1. 48 MHz

 

总结：由于Bit3=1时，FIFO工作在异步模式下，根据注释，IFCLK并不需要时钟信号输入，所以异步模式中IFCONFG(7,6)=11没有什么实际意义，为什么WSX师兄要写成0xCB而不是直接写成0x0B呢？