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Cypress EZ USB FX2 I/O Problems

Hi,

I'm trying to read data from I/O port C on the FX2 development board into my computer and then immediately write the received data back onto port E using the CyAPI library. I'd like to be able to run this cycle of data transfer (IOC->BulkInEndPt->PC console application->BulkOutEndPt->IOE) at a few kHz, but so far I've had no luck. I'm using two 512 byte 4x buffered bulk endpoints. My TD_Poll() function in my firmware looks like this:

void TD_Poll(void) // Called repeatedly while the device is idle

{ xdata unsigned char test = IOC & 0x01;

//Reads status of pins on i/o port C and readys the data to be transferred to the computer

if(!(EP2468STAT & bmEP6FULL)) { FIFORESET = 0x80; SYNCDELAY; FIFORESET = 0x06; SYNCDELAY; EP6FIFOBUF[0] = test; SYNCDELAY; EP6BCH = 0x02; //BCL and BCH regard endpoint buffer size SYNCDELAY; EP6BCL = 0x00; SYNCDELAY; EP6BCL = 0x00; SYNCDELAY; EP6BCL = 0x00; SYNCDELAY; EP6BCL = 0x00; SYNCDELAY; OUTPKTEND = 0x82; SYNCDELAY; FIFORESET = 0x00; SYNCDELAY; }

//Reads data sent from the computer and writes it into i/o port E. if(!(EP2468STAT & bmEP2EMPTY)) { xdata unsigned char *src = EP2FIFOBUF; xdata unsigned char value = *src;

IOE = value; FIFORESET = 0x80; SYNCDELAY; FIFORESET = 0x02; SYNCDELAY; FIFORESET = 0x00; SYNCDELAY; EP2BCL = 0x80; SYNCDELAY; EP2BCL = 0x80; SYNCDELAY; EP2BCL = 0x80; SYNCDELAY; EP2BCL = 0x80; SYNCDELAY; } }

And the following is the relevant code in my console application:

if(USBDevice->BulkOutEndPt && USBDevice->BulkInEndPt) { cout BulkInEndPt->XferData(buffer, length); USBDevice->BulkOutEndPt->XferData(buffer, length); } }

As you can see, whatever data is on port C should be immediately looped through the endpoints and computer to port E. However, I get about a 500 microsecond delay delay between the rising edge of a generated waveform on port C and the corresponding edge on port E. Can you make any suggestions for speeding this up? I'd like to get the delay as small as possible, so that port E can effectively follow port C at a higher frequency like 8 kHz. Let me know if you need any clarification.

Thanks so much, Zach

Reply to
stiggz
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It don't work that way... though like you I only figured that out after getting less than expected performance (from a USBserial) and searching for the explanation.

USB has high throughput, but it also has high latency.

It's great for pushing large amounts of data per comparatively small amounts of time, but it's not designed to move small amounts of data immediately. In a way, this isn't even a uniquely USB problem - as processor cores get faster, they tend to get more decoupled from relatively slow I/O, and unless careful attention is paid throughout the design process you end up with a blindingly fast computer that can be crippled by relatively high I/O servicing latency the minute someone tries to get it to interact in a constant bi-directional conversation with an external device.

Since you can't really change the design of USB, you'll need to look at finding a way to move some of the smarts of your application down to the fx2, so that it can act as a kind of executive engine on behalf of the PC and get more work done per each send/receive cycle on the wire. Sometimes that's as simple as making a bit-banging application a byte-banging one and letting the fx2 clock out the bits, other times if more "thought" is necessary to figure out how to respond to the external input it will be more tricky.

Reply to
cs_posting

That's disappointing to hear, I appreciate your response though. I'll need to look into redesigning my firmware. I may post a new response in the future if I tun into any more similar difficulty.

Thanks again for your help, Zach

Reply to
stiggz

Try using the DLL's asynchronous routines so that the read is set up just before the write.

Andrew

Reply to
Andrew Jackson

That's disappointing to hear, I appreciate your response though. I'll need to look into redesigning my firmware. I may post a new response in the future if I tun into any more similar difficulty.

Thanks again for your help, Zach

Reply to
stiggz

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