On 8 Mar, Greg KH wrote:
> On Wed, Mar 07, 2001 at 02:34:26PM -0500, [EMAIL PROTECTED] wrote:
>> I found that a native UART has a latency of 1-2ms for a single byte.
>> That latency increases with packet size. When run over the 8U232AM
>> the 1 byte latency is about 16-17ms and remains about 15ms longer than
>> the native UART for larger packet sizes. Is this latency somehow
>> tunable?
>
> USB to serial devices will always be much slower (have much greater
> latency) than UARTs. This is due to the fact that you add at least 1
> USB transaction to the whole time. And since you are doing a loopback
> you have the following sequence to go through:
> - data across USB to UART in device
> - data out of UART
> - data back into UART
> - data back across USB to host
I anticipated this and came up with about 5ms worst case:
Frame action
1 Submit packet to USB drivers
2 data to UART
3 data through loopback
4 data from UART
5 data from USB drivers
I'm quite open to 2 frames for the data go get through the loopback,
but it shouldn't take more than that and we are still at 6ms which is
less than half of what I am observing.
I'm pretty ignorant about how the USB drivers work so I could be way
off here, but 15ms seems kinda long for a system that has 1ms frames.
I'd be really interested in any insite people might have on this as
round trip and/or one way latency is very important for some possible
upcoming projects as well as understanding this UART. The latency of
the UART isn't a killer, but I would like to understand it and it may
be useful to reduce it if doing so is reasonable.
> Some USB to serial devices handle latencies better than others. But of
> course they cost more :)
Naturally, ;-) ...I'm using stock Intel UHCI.
> I'll let Bill Ryder answer the odd data characters, as he's the main
> author and maintainer of the driver, and I don't have one of these
> devices.
>
> thanks,
>
> 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
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