Newsgroups: comp.robotics
Path: brunix!uunet!bloom-beacon.mit.edu!news.media.mit.edu!fredm
From: fredm@media.mit.edu (Fred G Martin)
Subject: Re: SPI ports on the Mini Board
Message-ID: <1993Jul27.134347.9248@news.media.mit.edu>
Sender: news@news.media.mit.edu (USENET News System)
Organization: MIT Media Laboratory
References: <m53ci3INN2l8@appserv.Eng.Sun.COM> <1993Jul26.133455.16295@news.media.mit.edu> <93207.173950LEEK@QUCDN.QueensU.CA>
Date: Tue, 27 Jul 1993 13:43:47 GMT
Lines: 27

In article <93207.173950LEEK@QUCDN.QueensU.CA> <LEEK@QUCDN.QueensU.CA>
writes:

>Fred said:
>>If I understand the SPI correctly, tying the SS pins together wouldn't
>>work, because the SPI master also uses the SS line as an input (to
>>perform multiple master collision detection).  So you'd need another
>>681 output to drive the slave SPI's.  The only logical choice for
>
>If you tied all the /SS line together, with all of them in wire-or mode and
>a pull-up resistor, the master can set the bit 5 of the port D direction
>register to enable /SS as a output port and set /SS to 0. This would
>disable mode fault as well as controlling /SS on the slaves. Only 1 I/O
>is used.

Mr. Lee, can you explain the utility of having the master use the /SS
line as an output, since that disables its own mode-fault circuitry?
Surely Motorola didn't design the SPI system with the expectation that
people would built multiple master networks in this fashion.

Of course, the Mini Board ties /SS to ground, so no one's going to get
the chance to use their mode-fault circuit, but in all of your
message, nowhere did I see the SPI configuration that would let you
use the mode-fault.

	-Fred

