From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On
Behalf Of Chaipi Wijnbergen
 Subject: [EE]: USB transmission time jitter ?

> I am considering using PIC 16C765 with USB interface and have followed
some
> of the previous discussions on the actual expected throughput to a single
> device.

> In my project it is important for me to get a command from the PC in
> constant time from the time that my PC program decided to issue such
> command. By constant, I mean less then 1mS.


Sounds like you are trying to do real time on a PC.  First, a PIC16C765 is a
low speed device.  As such, the minimum time you could get that would be
repeatable (on a periodic interrupt) would be 10 msec on an interrupt
endpoint.  Low-speed interrupt endpoints are guaranteed that the system will
send or read data at least as often as 10 msec.  This is programmable up to
255 mSec.  Actually, that turns out to be 8 mSec in practice since it is a
power of 2.  However, it could be less or more as long as it was less than
or equal to 10 msec.  A high speed USB device could go down to 1 mSec.
However, this is the frequency of packets arriving.

But, you asked about latency.  I.e., how long between the time the host
application sent the data and the time it was received at the device.  This
is not determinate; it could be 10's of mSec.  In general it would certainly
be multiple mSec.  That's just the nature of Windows.  It is not meant to be
a real time OS.

There is a way to send data so it will arrive at a particular time, this can
be set to within a mSec.  However, the latency is still indeterminate, only
the delivery time is guaranteed (assuming it could arrive within the latency
constraints).  For this, you would need to use an Isochronous endpoint,
which requires a high-speed device.

I am not sure if you could do better on interrupt endpoint with a USB 2.0
device (I'd have to check the spec. ), but, in any case, the PIC16C765 (or
any low speed device) will not.

Marc Reinig
System Solutions

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