Re: final cartridge for vic 20

From: Nate Lawson <nate_at_root.org>
Date: Fri, 11 Sep 2015 14:30:49 -0700
Message-Id: <C1BE4968-126B-4088-AD94-A29B7D71B56F@root.org>
> On Sep 10, 2015, at 7:28 AM, Greg King <greg.king5@verizon.net> wrote:
> 
> On 2015-09-10 1:24 AM, Jim Brain wrote:
>> On 9/10/2015 12:11 AM, Nate Lawson wrote:
>>> I think the problem is here:
>>> 
>>> ###
>>> assign reset_in =                (reset_en ? 0 : !reset); //if soft
>>> reset, 0, otherwise !reset
>>> register #(.WIDTH(6))                cart_config1_reg(clock, reset_in,
>>> cart_config1_reg_ce, data[5:0], cart_config1);  // active high reset.
>>> ###
>>> 
>>> If the external reset line goes low (which it does when you set
>>> reset_en to do a soft reset), reset_in goes high and the config
>>> register is reset. Just stop passing reset_in to the config register
>>> and get it from a different source.
>> If external reset lines goes low, but I did so with the soft reset
>> functionality, then reset_in will be 0, according to the above.  It will
>> be that way until the next falling clock, given the always block.
> 
> When is the reset line released?  Does it happen _before_ that falling clock?  Maybe, you have a race condition on the release:  reset_in sees the release before reset does; therefore, it gets a pulse from !reset.

Yes, I agree.

Continuous assignment, wires, etc. arenít registered, so you have to be sure about relative timing of the signals. Personally, Iím not comfortable with using continuous logic from external sources until itís been registered on a clock edge. Thereís too much potential for glitches from ringing etc.

-Nate
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Received on 2015-09-11 22:01:05

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