Re: 6809 assembly knowledge needed

From: Jim Brain <>
Date: Fri, 05 Feb 2010 01:00:57 -0600
Message-ID: <>
I've been teaching myself Verilog tonight, and I have made the design I 
wanted in the language.  Still, it'd be nice to know who on-list might 
be able to answer questions, as I have plenty.

What caught me tonight was this:

output wire p1_io1,p1_io2
reg [7:0] config
input wire io1,io2

// allow p1_io1 to go low when selected (config[0] = 0) and io1 is low 
and swap (config[2]) is low, or if config[2] is high and config[1] and 
io2 are both low

// this works
assign p1_io1 = (CONFIG[0] | io1 | config[2]) & (config[1] | io2 | 

// this does not:
always @(io1 or io2 or config1[2:0]) begin
         1'b0:  begin
             p1_io1 = io1 | config[0];
             p1_io2 = io2 | config[1];
         1'b1:  begin
             p1_io2 = io1 | config[0];
             p1_io1 = io2 | config[1];

I thought the latter was easier to read, but to use it, p1_io1 and 
p1_io2 have to be declared as reg.  I have a philosophical issue with 
that, as I view "reg" as a register, and I don't think the CPLD/FPLGA 
needs to dedicate a precious register to the value of p1_io1, etc.  I 
see in the documentation/web pages that the tool should determine that 
no state is kept, so it will just turn the case statement into 
combinatorial logic, but it worries me.

The final solution I used was:

assign p1_io2 = config[2] ? io2 | config[1] : io1 | config[0];

Thanks goodness for Icarus.  I'd have given up trying to learn in the 
vendor tools.

To keep things on topic, here's my idea:

I was thinking of adding some smarts to the dumb cart port expander.  
Use a CPLD to select/deselect the various control lines, and set the 
config via a set of registers that are exposed in IO2 in some temporary 
way.  The switches would still be there, but they would represent the 
"initial state" of the settings, which can be overriden via SW.



Jim Brain, Brain Innovations                                      (X)
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!

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Received on 2010-02-05 08:00:04

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