Subject: Re: Max serial port speed on 425?
To: Kevin Currie <kcurrie@on.bell.ca>
From: michael smith <miff@spam.frisbee.net.au>
List: port-hp300
Date: 03/15/1997 00:33:50
```Kevin Currie wrote:
>
>         What is it?  I thought I had read soemwhere that the 425`s hw was
> able to do 460K-- this can't be right is it? (Please say yes!)

It's a 16550 (I have the top off mine at the moment*), but the clock
source is unclear (probably derived from the ISA clock?).  Going by
the dcareg header :

/*
* 16 bit baud rate divisor (lower byte in dca_data, upper in dca_ier)
* NB: This constant is for a 7.3728 clock frequency. The 300 clock
*     frequency is 2.4576, giving a constant of 153600.
*/
#ifdef hp300
#define DCABRD(x)       (153600 / (x))
#endif
#ifdef hp700
#define DCABRD(x)       (460800 / (x))
#endif

it is still unclear, but I suspect the lower.  This gives you a
maximum (useful) baudrate of 38400 (divisor of 4).  The 700
has a more 'standard' clock frequency, and thus will do 57600,
115200, etc.

> |  Kevin Currie       |    kcurrie@on.bell.ca         |      Ottawa, Ontario |

(*)  To answer my earlier question about overclocking a 425;
the addition of a 66MHz crystal and a fan blowing on the
CPU appears to have done the trick.  Without the fan the CPU
is untouchable after only a few minutes.

Anything else different between a 425 and a 433?  What's that
jumper just next to the CPU?

--
Mike Smith  *BSD hack  Unix hardware collector
The question "why are the fundamental laws of nature mathematical"
invites the trivial response "because we define as fundamental those
laws which are mathematical".  Paul Davies, _The_Mind_of_God_
```