Re: [Ftdi-usb-sio-devel] Resetting the FTDI

[Chris S. <chr...@ya...>]

BR> CS> My goal is to get a message to the FT245RL and back to the host
BR> CS> within each 1ms full speed frame.
BR>
BR> I see. That will be quite tricky I suspect.

About half the time I can do it... at least according to my interpretation of the following:

I have an FPGA device that uses FT245RL part.  It waits for a 2 byte "request" and ~immediately replies with a 62 byte message.  62 bytes was chosen because, once the 2 bytes of USB status message is appended, will fill up the outgoing FTDI buffer and not need to await a latency timer event.

I want to maximize the "ping pong rate" -- the rate at which I can send a request and get a response to one request and one reply per 1ms USB full speed frame (the part runs at full speed).  Here is pseudocode for a program I wrote to measure this rate (source attached):

fd = Open("ftdi_sio device");

set_serial_parms();

while (true)
{
  // busyloop calling write() until all data written
  write(fd, "two byte packet requesting 62 bytes");

  // busyloop calling read() until 62 read
  read(fd, "62 bytes");
}

Results:

The "ping pong rate" is 1000Hz for a few seconds, implying a reply and response within a 1ms frame.  The rate then switches to 300-700Hz for a few seconds, then back to 1000Hz.

source code: attached

program output:
1000.370137
1000.050003
1000.050003
1000.050003
1000.050003
1000.040002
980.430605
900.941484
613.549630
499.990000
393.720938
335.562587
645.177940
1000.030001
1000.060004
1000.030001
1000.040002
1000.050003
1000.040002

ftdi_sio debug output: attached.  It implies that when things are running at 1KHz I am sending the OUT and receiving the IN message in one frame.  When things are not running at 1KHz, it appears the IN message comes in on the subsequent frame.

I have a Beagle USB 480 which I am going to use to try to figure out exactly what is going on. http://www.totalphase.com/products/beagle_usb480/

My next idea is to start an asynchronous read before sending the write.

Your thoughts are appreciated!

Chris