linux-kernel - Re: Serial custom speed deprecated?

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Message-ID: <20060826193513.27481.qmail@science.horizon.com>
Date:	26 Aug 2006 15:35:13 -0400
From:	linux@...izon.com
To:	linux-kernel@...r.kernel.org
Cc:	stuartm@...necttech.com
Subject: Re: Serial custom speed deprecated?

> Also, Oxford's 16PCI95x family has three different points of altering
> the clock; the clock prescaler, the actual sample rate (which is the
> classic /16 that most are used to), and the actual divisor. That can
> produce pretty much any baud rate, albeit with some error.

Their data sheet doesn't explain it too well, but it looks like they
use a similar implementation to the MSP430 UARTs, which are designed
to work with very low clock rates.  (32768 Hz watch crystals are
popular.)

Just as a reminder, UART receive operation is generally:
- After detecting a falling edge, wait half a bit time.
- Sample the input 3x and majority-vote.  If the input
  line is still low, call this a start bit and prepare to
  receive a character.
- Wait one bit time, sample 3x, and call the majority the start bit.
- Repeat for all data bits and the stop bit.  If the stop bit isn't
  1 as expected, note a framing error.

This is traditionally done by running a 16x clock sampling the
input, and after detecting a start bit on one clock (call that
clock 0), sample the start bit on clocks 7, 8 and 9, the first bit
on clocks 23, 24 and 25, etc.

MSP430 UARTs have a fully programmable clocks/bit number (no fixed /16,
although you want a few clocks per bit so the standard "sample 3x and
majority vote" algorithm works), and can add +1 clock to individual
bit times to approximate a desired baud rate more closely by dithering.
Oh, and they use both edges of the clock.

So it can go to a baud rate 1/3 of the input baud clock (sample the
start bit at 1, 1.5 and 2 clocks, then the first bit at 4, 4.5 and 5,
etc.), and proceed to 9/24, 10/24, etc.
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