I built a "logic analyzer" many years ago. I used the printer port on a
Cromemco CP/M computer as the logic inputs. I watched for a change in the
byte I read from the port, then saved the new value to memory and bumped a
pointer. Once I had finished a capture, I printed out all the data in
binary, one byte per line. I used this to reverse engineer the serial bus
on a Commodore 1541 floppy disk drive.

For your application, I think it'd be easier to add external RAM to use
during capture, then send the data after capture. I've got a project using
a 1MB static RAM that's easy to interface to. I used it on a PIC24 which
did not have the parallel master port, so I bit banged it. On the PIC32,
you could use the PMP for a fair amount of the work (though you'd have to
manually control higher address lines).

Here's some of my UsbTask code from a project:

    if(!USBHandleBusy(USBGenericOutHandle)){	// If it's safe to look at
the receive buffer
      if(count=3DUSBHandleGetLength(USBGenericOutHandle)){    // if there's
something in the receive buffer
        if(count<=3DpUsbRxFifo->BytesFree){   // we have room in the fifo
          index=3D0;
          while(count--){
            PutFifo(pUsbRxFifo,OUTPacket[index++]);     // get data from
usb buffer to fifo
          }
          USBGenericOutHandle =3D
USBGenRead(USBGEN_EP_NUM,(BYTE*)&OUTPacket,USBGEN_EP_SIZE); //
ok to read more
        }// endif we had room
      }// endif there was something there
    }// endif not busy
    if(!USBHandleBusy(USBGenericInHandle)){ // 0 if ok to transmit
      if(count=3DpUsbTxFifo->BytesInBuf){  // we have something to send
        if(count>=3Dsizeof(INPacket)){
          count=3Dsizeof(INPacket);         // limit how many bytes we send
to size of output buffer
        }
        index=3D0;
        NumBytes=3Dcount;       // remember how many bytes we're sending
        while(count--){
          INPacket[index++]=3DGetFifo(pUsbTxFifo);  // fill the transmit
buffer from the fifo
        }
        USBGenericInHandle =3D
USBGenWrite(USBGEN_EP_NUM,(BYTE*)&INPacket,NumBytes); // send the
bytes
      }
    }



Harold

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