[Libusb-devel] USB Latency with a FTDI chip

[Sylvain [Bibi] <bi...@cr...>]

Hi,

I'm trying to develop a FPGA-based system which is analog to an audio 
acquisition card for some real time applications. The system is 
connected to a Macbook (or a PC if needed) through a FTDI chip. So I'm 
using the libftdi (and libusb) to send and receive data from the USB.

The acquisition card must send every 67uS (15kHz sampling frequency) 
about 22 bytes to the host. So, the data rate is about 330 kB/s, which 
seems correct for USB. But the problem is that sometimes there are some 
latencies, lasting sometimes about 200ms (!), which stop all the 
FTDI/host transfers. I have added a 65kB buffer in the FPGA to store the 
real-time data when the communication is stopped, but the buffer 
randomly overflows and I lost some values ...

I'm clearly new to the USB coding, and I suppose that such latencies are 
  "normal". But is there a way to reduce them ? Do you have some 
suggestions or solutions which could help me to avoid such data loss ?

Thanks for any advice.

-- 
Sylvain