Re: Building a 6502 peripheral - timing

From: silverdr_at_wfmh.org.pl
Date: Thu, 15 Mar 2018 00:10:38 +0100
Message-Id: <18907DC6-3402-4709-A436-0297F0CAC1E6@wfmh.org.pl>
> On 2018-03-14, at 23:59, Mia Magnusson <mia@plea.se> wrote:
> 
>> On the wave of the recent 6509 / 6502 discussion... when building or
>> connecting a peripheral that would allow both reading and writing,
>> what is basically needed are the following signals:
>> 
>> * _CS (from address decoder)
>> * R_W (from 6502)
>> * D[0..7]
>> * possibly some low address bus lines A[0..3] for example
>> * some chips take PHI2
>> 
>> http://archive.6502.org/datasheets/mos_6500_mpu_nov_1985.pdf
>> 
>> shows timing diagrams. I wonder what would be the required timing for
>> a peripheral without PHI2 clock input. I suspect something similar to
>> what SRAM chips specify. Any hints as where to put attention? Like
>> relations between _CS, R_W, ... ?
> 
> Somewhere on the net I've seen schematics of using 16550 UART's on a
> C64. Although the 6510 differs slightly from a 6502, it could probably
> be used as some kind of guide.
> 
> 16550 has Intel timing, it's just a fancier 8250 UART which is part of
> the 82** family of peripheral ICs that are ment both for 8080/8085 and
> 8088.
> 
> For the Intel style peripherals I think you should be careful so they
> don't get a short read before and/or after a write. One way to generate
> the required read/write signals is to feed R/_W, PHI2 and maybe other
> signals to a 74*128/74*138 3-to-8/dual 2-to-4 decoder.
> 
> It would be easier to answer if you specify some example of what you
> want to hook up.

Actually I want to "build" a CPLD based peripheral chip. Let's say a port like 6526 but with single port and preferably without feeding it with PHI2.

> (SRAMs is afaik never affected by spurious short reads, so with those
> you might get away with timing that don't work well with various I/O
> stuff).

Sorry for being ignorant here, but what exactly are "spurious short reads"? Something like reading chip's own output from the bus?

-- 
SD! - http://e4aws.silverdr.com/
Received on 2018-03-15 01:01:21

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