On 8 Mar, Greg KH wrote:
> On Thu, Mar 08, 2001 at 10:47:30AM -0500, [EMAIL PROTECTED] wrote:
>> > Some USB to serial devices handle latencies better than others. But of
>> > course they cost more :)
>>
>> Naturally, ;-) ...I'm using stock Intel UHCI.
>
> No, I mean that the USB to serial device _itself_ is probably where the
Oops! I mis-read that.
> greatest latency happens. The cheaper devices use a very small and slow
> microcontroller. They more expensive ones don't.
This one claims to be real hardware and not a micro-controller. It
requires no prom or programming.
> If you have a test script or program, I'd be glad to run it on some of
> the devices I have here, on both OHCI and UHCI and see what the times
> are.
I have attached serial-test.tgz. It contains source and a stripped
binary. It also contains the patch I use to increase the scheduler
rate to 1024Hz.
The program is very simplisitic about how it takes command line
arguements.
argv[1] = device port name i.e. /dev/ttyS1 or /dev/usb/tts/01
argv[2] = baud rate
argv[3] = max packet size
argv[4] = Number of packets for each size
The program will start at a packet size of 1 and time argv[4] packets.
It will then do 2, 4, 8, 16 byte packets up to size<argv[3]+1.
Note that this program runs as a posix reatime task:
schparms.sched_priority = 10;
sched_setscheduler(0, SCHED_FIFO, &schparms);
As such it must be run as root. ...actually, I think it will run as
non-root but will fail to set the scheduler parameters. It also
measures time using both gettimeofday and the pentium cycle counter
because I wanted to check the accuracy of gettimeofday.
I attached tx to rx to create the loopback. No flow control is used.
In addition to the latency, I found that the USB-UART I am using started
dropping bytes above 115200 bps if the packets are sufficiently large.
I don't recall what sufficiently large is.
> greg k-h
Thanks!
Ty
--
Tyson D Sawyer iRobot Corporation
Senior Systems Engineer Real World Interface Div.
[EMAIL PROTECTED] Robots for the Real World
603-532-6900 ext 206 http://www.irobot.com
serial-test.tgz
