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To: root@spiffy.cybernet.com
cc: hackers@freebsd.org
Subject: Re: Bragging rights.. 
In-reply-to: Your message of "Thu, 19 Oct 95 19:53:59 EDT."
             <XFMail.951019190335.root@spiffy.cybernet.com> 
From: David Greenman <davidg@Root.COM>
Reply-To: davidg@Root.COM
Date: Thu, 19 Oct 1995 16:37:01 -0700
Sender: owner-hackers@freebsd.org
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>
>On 10/19/95 20:25:56 David Greenman wrote:
>[clip]
>>
>>   Let me add a bit of sanity to this part of the discussion. 115200 baud async
>
>>will give you about 11.52Kbytes/second if you have no packet overhead. 115200
>>baud sync will give you 14.40Kbytes/second if you have no packet overhead.
>>   Why? Because we're talking bits - async is 8 data bits plus 1 start and 1
>>stop bit...10 bits. With synchronous serial, it's just 8 data bits. So sync
>>always has the potential to give you 25% more bytes throughput at the same bit
>>rate compared to async.
>>   Now with sync you'll also be running at a faster bit rate (128000bits/sec).
>>This is 16Kbytes/second. This is 38.9% faster.
>>
>>-DG
>
>
>As a slightly interested party, I'd like to ask:
>
>As mentioned recently on -hackers, isn't it possilbe to up the rate of the serial
>chip simply by doubling (or quadding) the rate of the xtal driving the chip?
>Many (most?) 16550 chips should be able to handle a Fmax higher than they are being
>driven, and with 16 byte FIFOS (set to trigger at 14 bytes), the interrupt overhead
>would not necessarily be increased.
>
>Is the same xtal trick applicable to sync serial, to get 32 KBytes/second @256000
>bits/sec (as opposed to 28.8 KBytes/sec async serial @230400 bits/sec)?

   Apparantly, *some* 16550 UARTs will do this, but as far as I know, this
would be overclocking most versions out there and might result in the part
overheating (or simply not working at 230K baud).

-DG