UWE Bristol Engineering showcase 2015

Nigel Godwin
BEng (hons) Electronic Engineering
Project Supervisor
Nigel Gunton
There are many requirements that a 6502
microprocessor clone should fulfil. These include
appearance, operation, behaviour and timings.
Hardwired Control Units
The control unit of each CPU uses combinatorial
and sequential logic to generate the CPU’s internal
and external control signals. These control signals
cause the CPU to perform the desired microoperations
in the correct order to fetch, decode
and execute the instructions in the processor’s
instruction set
Microsequencer Control
A Microsequencer stores its control signals in a
lookup ROM, the microcode memory. By accessing
the locations of the ROM in the correct order, the
lookup rom asserts the control signals in the
proper sequence to realize the instructions in the
processor’s instruction set.
The Decision
Microprocessor Type 6502 Clone
When considering the two design concepts
introduced in Chapter 6, a decision was made
relating to a desired approach toward the cloning
of the 6502. The decision was governed by the
result of the investigators research, coupled with
his personal career based needs and
requirements. With this in mind, it was decided
that a hardwired control system would be
implemented.
In my normal working capacity as a lecturer, I am
being considered for the delivery of an
introductory level of VHDL as a new subject area
of learning for my students. With this in mind, I
saw the possibility of improving my own
knowledge of the basic concepts of hardware
design language and its usage. I therefore decided
that a TTL logic design, originally constructed by
Ruud Baltissen would be implemented.
RTL Diagram of 74LS393
Euroboard 1
There was four connectors used in the design of
this board. They are identified by using the
prefixes: con1_, con2, con3, con4. Each connection
pin was then identified individually, e.g.,
con1_reset, con1_rdy, and so on.
The integrated circuit designs that were included
in this board were:
74ALS32
74ALS573
74LS688
74LD125
74ALS04
These were then converted to schematic symbols
and inserted into the schematic and connected
accordingly.
Enlarged section of Euroboard 1 showing
converted symbols for designed integrated circuits.
Project summary
An investigation has been performed to examine the
practicalities involved in cloning a 6502
microprocessor by using Hardware Description
Language. The cloned system is designed to use ‘Old
School’ techniques, rather than the on-board tools
that are incorporated into the Altera software
package that was used. Although the design is not
fully completed, all testing that has been carried out
has confirmed the viability of the approach. It is
considered that when completed, the design could be
used as an educational aid to introduce new students
to the possibilities that VHDL can offer when used in
its most basic form.
Project Objectives
The main aim of this investigation into cloning a 6502
microprocessor is do design a processor which, when
linked to the necessary peripherals will be able to
allow archive games for Commodore 64 and Atari
home computers to be played.
Project Conclusion
Although this investigation was never going to derive
a working 6502 due to the necessity of additional
peripheral components, it has fully accomplished its
desired outcomes.
The investigator, due to work requirements needed
to be able to embrace the topic of VHDL from the
point of basic principles and practices. By designing
the TTL logic integrated circuits from scratch, the
investigator has learned various methods of
combining bot combinational and sequential logic
within the VHDL environment.
The individual integrated circuit designs, when tested,
all worked correctly, therefore suggesting that when
connected into the schematic design files, a
successful result will ensue.
The exploration into the use of blocks and conduits to
combine individual schematic design files by way of
connectors was very successful, with the investigator
successfully connecting the necessary circuit board
designs, via the use of the backplane.