>>>>> "KrishnaMohan" == KrishnaMohan Bandi <krishnamohan.bandi@xxxxxxxxx>
>>>>> writes:
KrishnaMohan> --20cf3040e9c665eee404ad850ce7 Content-Type: text/plain;
KrishnaMohan> charset=ISO-8859-1
KrishnaMohan> Hi all,
KrishnaMohan> I have a program that uses libftdi-0.19 library to read
KrishnaMohan> and write from/to a FTDI device, which on the other end is
KrishnaMohan> connected to an FPGA. I do not have more than 20 bytes of
KrishnaMohan> data at any time to send/receive to/from the FTDI device,
KrishnaMohan> however I do very frequent reads and writes (approximately
KrishnaMohan> every 15ms) from/to the FTDI device to read the GPS clock
KrishnaMohan> time and some other registers from the FPGA.
KrishnaMohan> The read and write are together taking about 5-8ms and
KrishnaMohan> sometimes it goes up to as high as 15ms. This means the
KrishnaMohan> transfer rate is not even 9600bits/sec. However, the baud
KrishnaMohan> rate has been set to 921600. On the FPGA side, the time to
KrishnaMohan> read, process, and write back the data to the FTDI does
KrishnaMohan> not take more than 100uS. So, for the program the data is
KrishnaMohan> available as soon as the data is written to the FTDI
KrishnaMohan> device.
KrishnaMohan> The strange behaviour I noticed is that when
KrishnaMohan> ftdi_read_data() is called, it returns zero as the number
KrishnaMohan> of bytes read.
Are you sure the data has already been delivered to the FTDI chip? Have a
deep look at
http://www.ftdichip.com/Support/Documents/AppNotes/AN232B-04_DataLatencyFlow.pdf
to understand the issues involved.
Chapter 3.1, point 2 might help: Toggle some status line when the device has
send the data to the FTDI chip.
KrishnaMohan> Only, the third (always) time the
KrishnaMohan> ftdi_read_data() actually reads the data and about 3ms is
KrishnaMohan> elapsed between the time the data is available and the
KrishnaMohan> ftdi_read_data() is actually able to read the data from
KrishnaMohan> the FIFO.
KrishnaMohan> What I want to know from you guys is why is
KrishnaMohan> ftdi_read_data() takes so much time to read small amount
KrishnaMohan> of data? What should be done to reduce the read/write time
KrishnaMohan> to the FTDI?
Is this with a x232H chip or an old type USB full speed chip. In the
latter case, add a USB2.0 High-Speed Hub in between.
...
KrishnaMohan> Hi all,<br><br>=A0=A0 I have a program that uses
Please, no HTML in Mail.
--
Uwe Bonnes bon@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------
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