Re: lab4: what is the idle signal?

Ed Casas (edc@ece.ubc.ca) Sun, 27 Feb 2000 07:43:11 -0800 

Date: Sun, 27 Feb 2000 07:43:11 -0800
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: lab4: what is the idle signal?



On Sat, Feb 26, 2000 at 04:51:01PM -0800, plin@ece.ubc.ca wrote:
> I am having some problems with lab4, the serial output port.
> 
> 1. What exactly is the idle signal? Also, since we already create an
> idle state, can't we figure out the current status of the input from the
> idle state instead of the idle signal or we have to assign the following
> statement:
> 
>             idle_signal <= '1' when state = idle else '0';

The idle signal is exactly what you have described: a signal that
is asserted when the controller is in the idle state.

Your program has to check if the serial port is in the idle state
before writing another character to the data register.

We could have designed the serial port to report the state
directly (as a 4-bit value, for example) but since all we're
interested is idle/not-idle status, only one bit is used in this
design.

> 2. Also, for the data bus, is it an inout signal instead of an input
> signal?

Yes.

> From the description of the lab, it seems that we are reading
> and writing values into the data bus.

Yes, it is a bidirectional bus and you'll need to instantiate
tri-state buffers as described in the lecture notes.

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592