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Subject: RE: ANNOUNCE: Rockbox emulates M-Bus car CD changer (Alpine and OEM)
From: VanBaren, Gerald (AGRE) (Gerald.VanBaren_at_smiths-aerospace.com)
Date: 2004-02-13


Some thoughts on the Sony protocol using a UART to provide the bulk of the timing. This technique is not original, it is used as one alternative to interfacing to the Dallas/Maxim (www.maxim-ic.com) 1-wire interfaces (App Note 214)
  http://www.maxim-ic.com/appnotes.cfm/appnote_number/1189/ln/en

UARTs can typically do 8 bits and 7 data bits easily, and usually support 6 and 5 bits as well. You have to add a <start> bit, which is one bit time of zero, then 5..8 bits of data, followed by one bit time of ones known as the <stop> bit (which is identical to the gap between successive data frames.

Looking at how long in bit times 1200uSec is:
  9 bit times: 7500 baud
  8 bit times: 6667 baud
  7 bit times: 5833 baud
  6 bit times: 5000 baud

If you set your uart to 7500 baud, 1200uSec is 9 bit times (one start bit and 8 data bits). Thus, the 1200uSec "zero" will be seen by the uart as 0x00. The 600uSec "one" will be seen as 0xF8 (literally give or take a bit :-) since there will be one bit of start and then 3 bits (+/- a bit) of zeros followed by 4 bits of ones. An initialization pulse (2400uSec) will look like a <break>.

Transmitting a "zero" would consist of transmitting 0x00 and then waiting 600uSec for the inter-Sony-bit gap. Transmitting a "one" would consist of transmitting 0xF8 and then waiting 600uSec.

This assumes you can set your baud rate reasonably close to the required baud rate. Since there usually is a fair amount of tolerance built into this kind of protocol, it usually works if you are anywhere in the ballpark (7200 baud should work, 9600 might be a problem).

gvb

> -----Original Message-----
> From: rockbox-bounces_at_cool.haxx.se
> [mailto:rockbox-bounces_at_cool.haxx.se]On Behalf Of jobarjo
> Sent: Friday, February 13, 2004 12:34 PM
> To: Rockbox development
> Subject: Re: ANNOUNCE: Rockbox emulates M-Bus car CD changer
> (Alpine and
> OEM)
>
>
> > No, unfortunately the pin is not interrupt-capable. And
> this is where the
> > nice general concept collapses, sorry.
>
> Too BAD! ;-(
>
> I've read your source. Lucky that MBus encoding have fixed
> length bits.
> For the sony s-link it uses variable length pulses:
>
> http://www.undeadscientist.com/slink/encoding.html
>
> might still be possible to hack with the uart. But the BIG
> problem is that
> there is an uncertainty on the last bit value for each byte
> received. So it
> does not seems easily feasible.
>
> I dont like polling also.
>
> I will think about a solution.... or find an alpine cd pilot...
>
> Is it possible to program a 600us timer interrupt to sample
> the pin after
> the first falling edge?
>
> _______________________________________________
> http://cool.haxx.se/mailman/listinfo/rockbox
>

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