[Gmecm] Trip computer

[frederic]

> I noticed a flag in the '7749 definition file that reads "Calculate MPG for
> HUD" or thereabouts. Is this an actual working function of a 749 and if so,
> can you actually get it to output to some microcontroller with display?

If you attach an o-scope to the aldl line, and operate the 749 normally, 
you see a waveform that represents serial data as to be expected.  If you 
let the 749 run on the bench for a while, with pots for controls and a 555 
timer pulsing RPMs consistantly, the data appears to be consistant as 
viewed wtih teh scope.

Enable that flag, and the aldl data changes.  The pattern that recycles 
gets longer.  I have never attempted to reverse engineer the ALDL stream 
other than what freescan does, and freescan didn't display anything 
differently before or after the flag is set. But, from what I can tell, 
additional data is present in the aldl stream.

Since the HUD units on various cars use the ALDL stream to receive 
displayable data, as do a lot of the digital clusters (like anything 
Northstar, for example), it's likely that this additional data in the 
stream is speed, tach and other data being "offered" to a HUD device, or, 
it might just be a set of bytes telling the HUD "what is coming next pay 
attenton to".

Obviously, some additional efforts are required.  I counted 11 additional 
bytes in the stream, but using a scope I could be wrong.