Re: 8ms Timer for serial port access

From: Jens Schumacher (jens.schumache_at_gmx.net)
Date: 10/29/03 

Date: Tue, 28 Oct 2003 19:41:48 -0500

Hello,

> You could loop on a blocking read. You could also use 'poll' to wait up
> to a certain amount of time. The key is that each time you call 'read', you
> *must* read all the data that's available before sleeping.

When I specify the blocking in the open function, read returns as late as
there are 24 bytes available, is this right?
 
> Do not try to teach the kernel that 24 bytes means something special to
> you. It doesn't care. If you get 8 bytes, wonderful. You'll get 16 more
> later. If you get 36 bytes, fine, process 24 of them now and save 12 for the
> next time you get some data.

I meet some Problems when I tried this. I tried to read the data and checked
how many bytes were read. When I had less than 24 I read again and asked for
the missing bytes. But somehow some bits were corrupt and gave me wrong
numbers.
This is why I checked the bytes in the buffer first and when there are 24
bytes I read them.

> No, you don't. You read the data when it's ready, not at some magic
> time. You can use 'poll' to tell when there's data ready or you can use a
> blocking read. Here's what you're doing:
>
> 1) Wait X time
> 2) Read Y bytes
> 3) Repeat.
>
> Here's what you should do:
>
> 1) Wait X time or block in 'poll' until data is available.
> 2) Read however many bytes there are.
> 3) Repeat.

1) Sounds reasonable to me...but how to block it. Sorry maybe a stupid
question.

2&3) I meet a problem described above.

>> I just need the data from the serial when a whole frame arrives every 8ms
>> and need to process the data directly. I never thought this would be a
>> problem on a modern system.
>
> It's not a problem, you'd just trying to teach the kernel that 8
> milliseconds and 24 bytes are special. It doesn't care and so won't do what
> you want it to do. Just read however much data it has whenever it has it and
> you'll be fine.
>
> You can even do this:
>
> 1) Wait 8 milliseconds (though it will probably really be ten)
> 2) Read as many bytes as are ready without blocking.
> 3) Process as many complete frames as you now have saving any leftover
> bytes for the next pass.

Then I will have another delay before I can process the data. I try to avoid
any delay like this.

>
> DS
>
>

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