EPIA-M serial port information

[Niki Waibel]

> Based on some emails from Andrew I started poking around into the
> epia-m serial port problem. Here are my observations:
> The 9 pin internal serial port is part of the VT1211 chip, not
> the VT8235, or other chips with serial ports.
> 
> Under linux all baud rates are 1/2 what they should be. So if I pick
> 115200, I get 57600. If I pick 19200, I get 9600, etc.
> 
> I can fix linux with
> setserial /dev/ttyS0 baud_base 57600
> Then the actual baud rate matches what I expect. 38400 and 115200 cannot be
> achieved though.
> 
> Andrew says if he boots with Award and the warm boots into linuxbios, the
> serial port is 115200.

confirmed (epia-m @ 600mhz).

if i power off / on the epia-m then i get the lower speed.

> On the epia-m motherboard near the VT1211 chip is a crystal oscillator labeled
> KTS24.5X1K, which suggests it is 24.5 mhz. The VT1211 spec calls for a 48 mhz
> clock. Don't know if this is the cause of the problem, VT1211 being driven by
> 1/2 speed clock.
> 
> My guess is there is some register in the VT1211 that has to be tweaked to
> specify the serial port clock divisor. linuxbios doesn't touch this at all.
> award bios sets it to match the 24.5 mhz crystal. On cold startup the vt1211
> defaults to assuming 48 mhz input clock. This explains all observations, but
> doesn't solve the problem...
> 
> I just put a scope on pin 17 of the vt1211, it appears to be 48 mhz though so
> that blows that theory.

niki