Subject: Re: High serial port (output) speeds
To: Hauke Fath <hauke@Espresso.Rhein-Neckar.DE>
From: Bill Studenmund <>
List: port-sparc
Date: 10/23/1999 16:55:01
On Sat, 23 Oct 1999, Hauke Fath wrote:

> Actually, I was pretty surprised to hear the sparc port cuts off at 38k4.
> Is this an interrupt priority issue?

The actual cut-off is at 76800.

No, it (like this entire thread) is a clock issue.

The 8530 has four sources of its data clock:

1) PCLK fed through the Baud Rate Generator (BRG)

2) Receive clock (RTxC) fed through BRG

3) Receive clock

4) Transmit clock (TRxC)

The minimum division from the BRG is 4.

The sun & sparc use a PCLK of 4.9152 MHz (9600*512). 

Using 16x mode, the highest sparc baud rate is 9600*512/16/4 = 76800.

Both power and 68k macs use a 3.68 MHz receive clock (14400 * 256). So
from the BRG, the highest baud rate is 14400*256/16/4 = 57600.

What the mac can also do is just divide the recevie clock directly,
getting 14400*256/16 and 14400*256/32, = 230400 & 115200. That's what your
NAT box is doing. :-)

Take care,