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DR 6502 AER 201S Engineering Design 6502 Execution Simulator
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Supplementary Notes By: M.J.Malone
Trouble Shooting the 6502 Project Board
=======================================
Quick Checks
============
There is only one important thing to keep in mind when faced
with a non-functioning 6502 board. The board is just a series of
traces. The errors in the trace pattern are well known and once
fixed using the jumpers described in the project board notes, their
is no reason for a board not to work except for reasons of software,
power, bad chips and wiring mistakes. The quick diagnosis methods
and quick are given below.
A Software Error
----------------
Many times when a design group is trying something the test
fails, the software person is blamed. After a recheck of the
program source code, the programmer says confidently, "Just look at
this code, it is so simple how could there be an error? It must be
'the board' !". After several hours the software person discovers
that they have typed:
lda DDRA_Port_Mask ; The mask for the I/O pattern of Port A
sta DDRA
instead of:
lda #DDRA_Port_Mask ; The mask for the I/O pattern of Port A
sta DDRA
If the bit pattern 'DDRA_Port_Mask' were interpreted as an address
as in the first example, of course nothing works.
There are many such one character errors that are very
difficult to see late at night on the fourth check of the source
code especially when the checker is convinced there is no error.
The solution is to have a test EPROM around at all times that checks
all components of the computer. Whenever a dispute of 'Is the error
in hardware or software?' arises simply plug in the test EPROM and
see if the board performs as expected or not. An example test EPROM
program is given in BOARDTST.ASM. If your board has failed such a
test as the one given in BOARDTST.ASM then continue.
page 2
Bad/No Power
------------
Check the power supply, batteries and connections. Check WITH
A VOLTMETER the actual voltage being produced by the 7805 power
regulator. The +5 volt trace is the one connected to the regulator
pin next to the edge of the board. This trace runs the length of
the board and has connections to several 10K resistors and at least
one capacitor. The ground trace is connected to the middle pin on
the power regulator. If the voltage is greater than 5 volts (like
maybe 7-12 volts) see: 'Over Voltage' below. If the voltage is less
than 5 volts (like maybe 0-4 volts) see 'Under Voltage' below. If
the voltage is 5 +/- .1 volts then proceed.
Bad Chip
--------
It is possible that you have a bad chip. First check that your
check EPROM is good by checking it on someone else's computer
board. If the EPROM is OK then put it back in the non-functioning
board and proceed to swap chips with the working board ONE AT A
TIME. If at some time the working board stops working or the
non-functioning board checks out you can determine which of and how
many bad chips you have.
Wiring Mistakes
---------------
Probably the fastest way to check if there is a wiring mistake
is to have someone else look at your computer board, preferably
comparing it to a functioning board. Check the EPROM jumper is
correct and in place and measures 5 volts. Check that there are two
jumpers near the 74xx04. Check that one has a polarity of +5volts
and the other GND. Check that there is a jumper near the 74xx74.
Check that this jumper has a voltage of GND. Check that there is a
jumper on the bottom of the 6502 and that it measures 5 volts.
Check the polarity of the tantalum capacitors.
Trouble Shooting Procedure
==========================
Though all non-functioning computers will fail to function from
one of the above causes, a more detailed trouble-shooting procedure
will be developed. After this several symptom-solution scenarios
will be presented.
1) Connect the computer board to +12 and GND connections on the
power supply. Place you finger on the back of the 7805 power
regulator and turn on the power supply. If the regulator becomes
uncomfortable to touch then go to 'High Current Drain'. If the
regulator does not become hot then continue.
page 3
2) First we will check the output of the 2 MHz clock circuit. Check
pin #6 on the 74xx04 chip with an oscilloscope set to 1 microsecond
per division and 1 volt per division with the GND level at the
bottom of the screen. If the pin shows an AC signal that is a
rounded square wave of 2 MHz frequency (two complete cycles per
division) then advance to step 5. If the signal is at either 0
volts or 5 volts (almost exactly) then go to step 3. If the signal
is between .7 volts and 2.7 volts then go to 4).
3) You are at this step because the clock oscillator which is
supposed to be astable has latched into the high or low state. This
is simply not possible if everything is connected and the 74xx04 is
even close to working. Check the connection of the crystal, two
resistors and the .1 microF capacitor. Try also replacing the
74xx04 chip.
4) You are at this step because your clock signal is not starting by
itself but instead is sitting in an equilibrium state. We will try
disturbing its equilibrium a bit. Touch the leads of the crystal
with your finger while continuing to probe the 74xx04 chip. If an
AC signal develops then the solution to this problem is to turn off
the power, connect a 100-1000pF capacitor from the square GND area
to one of the leads of the crystal and then power up. If the AC
signal develops immediately then continue to step 5 otherwise
increase the capacitance and try again.
5) You now have a functioning 2 MHz clock unit. Probe pin #37 of
the 6502. The signal should be a square wave of frequency 1 MHz,
one cycle per division. If this is the case advance to step 8). If
the signal is a square wave but at a frequency of 2 MHz then go to
step 7) otherwise continue to step 6).
6) You are here because your 74xx74 chip is probably not
functioning. Check it by swapping the chip and also check the
jumper near the 74xx74 chip that it is 0 volts. Try removing the
6502 chip and see if the problem persists. If the clock signal
becomes normal then perhaps the 6502 is blown or the 74xx74 is
blown.
7) You are here because you have the 2 MHz clock problem as
described in the assembly notes. Try the solution as described in
the printed notes you received with your computer board.
8) You have a proven CPU clock. Next test to see if the 6502
properly resets. Check the voltage on pin #40 of the 6502 using an
oscilloscope. The voltage should be 5 volts. It is normal to see
small spikes in the 5 volt level of approximately +/-.3 volts.
Press the reset button and check that the voltage is 0 volts. If
the voltage is the opposite to that expected then it is probably
that you have a momentary open switch not a momentary closed switch
as specified. If this is the case then you can still test your
computer temporarily by holding the reset switch but you should
replace the switch at the earliest opportunity.
page 4
9) You now have a proven Reset circuit. Next test to see if the
6502 is executing instructions by probing the SYNC signal of the
CPU. The SYNC signal goes high every time the CPU fetches an opcode
of program. The SYNC signal can be monitored by probing pin #7 of
the 6502.
Note: to get the pulse pattern to be stationary on the screen
and not covered with ghosts and flickers, set the time base on 10
microseconds per division and then adjust the time base 'cal' knob
slightly until the pattern locks in with the fewest ghosts. Set the
times 10 sweep magnification on the time base. The time base should
be effectively just a bit more than 1 microsecond per division. By
adjusting the horizontal position knob, you will be able to scan
through the series of pulses.
The opcode fetch will last 1 microsecond. This pulse will be
followed by at least 1 microsecond low. NOP, LDA #, TAY, BNE (not
taken), SEC are all instructions that will take two machine cycles
(microseconds) and will be seen on the oscilloscope as one cycle
high followed by one cycle low. The longest instructions (cycles)
are RTS (6), BRK (8) and Reset (6). On average, most instructions
will show 3 or 4 cycles since all non-indexed memory fetches, jumps
and arithmetic or logical statements are 3 or 4 cycles. It may seem
for very long programs that it would be impossible to use this sort
of count the cycles type analysis but it is usually not. Since most
programs have one small segment of the code that is executed most of
the time (like the innermost nest of a delay loop) it is possible to
assume that most of what you see on the screen that small part of
the program with the ghosts being the other parts of the program.
By this method it is possible to determine whether the 6502 is
executing the program intended or not.
Summary
-------
If the above procedure does not lead to a cause of the problem
then the recommended procedure is to trim down the program to its
minimum, and attempt chip swapping again paying particular attention
to the VIA's, CPU, EPROM and RAM. It may also be possible that a
poor soldering job may have left some connection not connected. If
this is suspected then it is recommended that the board be inspected
by someone, preferably someone else, and suspicious areas be touched
up.
Your computer board will work eventually. If there are no
missing jumpers and no bad chips the test EPROM must check out.
Symptoms/Cures
==============
Symptom: Over Voltage
---------------------
First there are three possible causes for this. The power
regulator is designed to produce at least 5 volts. If there is
another source of voltage on the board of greater than 5 volts, the
page 5
system voltage may rise above 5 volts. This is commonly caused by
some user circuit or input to a CMOS VIA. The obvious remedy is to
turn off all external voltage sources and recheck the system
voltage.
The second problem may be simply that the +12 volt power supply
has been mistakenly connected to the +5 volt connection on the
bottom of the circuit board.
The third possible problem may be that the 7805 is blown and
not operating properly.
The over voltage condition is very serious and can lead to
destruction of ALL of the chips in the system (the $35 mistake).
Symptom: Under Voltage
----------------------
If the voltage in the system is low and the 7805 power
regulator is not getting uncomfortably warm then this can only mean
that the 7805 is defective or is not properly connected or the
measurement of voltage is not being done properly. If the 7805 is
getting uncomfortably hot to touch then go on to 'High Current
Drain'.
Symptom: High Current Drain
---------------------------
This assumes that the VIA's are not connected to any circuitry
which may be causing large currents to be drawn from the board. If
the VIA's are connected to external circuitry then disconnect them
and begin again.
If NMOS 6502 or 6522 chips have been used then it is quite
possible that there is nothing wrong at all. An computer with an
NMOS 6502 and two NMOS 6522s can easily draw 1 ampere as compared to
0.024 milliamperes (yes a factor of 40!) for an all CMOS computer.
This extra current draw is enough to make any 7805 a bit hot. For
testing, it is possible to bypass the regulator and feed the
computer board from a high current supply for testing purposes but
it is recommended that the NMOS components be replaced with CMOS at
the earliest possible opportunity. It is entirely possible that the
board may not work at all with too many NMOS components because of
voltage drops over the lengths of the fine traces used to make the
circuit.
This can be caused by a bad chip, a short circuit in the wiring
or a bad tantalum capacitor. To check to see if a bad chip is to
blame, try removing the chips one by one and trying again each
time. If the current drain disappears then the last chip removed
may be the cause. Another troubleshooting method recommended is to
bypass the 7805 and connect a high current 5 volt supply directly to
the 5 volt rail of the board. Turn on the power and quickly check
all chips with the back of a finger (because it is most sensitive to
heat) to look for one that may be heating. NMOS and TTL chips may
become warm but will never be hot under normal operation.
Transistors and regulators may become hot but will never be
uncomfortable to touch. If none of the chips seem to be heating
then leave the power on and check the voltage and current being
page 6
output by the supply. If the supply voltage is very close to zero
then there is likely a short in the wiring (see next heading). Wait
a short time and look for smoke (big white clouds of smoke) from one
of the tantalum capacitors. If one of the capacitors is bad it will
progress from smoke to flames quickly so stand by the off switch.
As soon as heat is detected from a chip or smoke from a capacitor
shut off the supply and replace the component. See if there is
still a problem with a high current drain.
Symptom: Unlocatable Short Circuit
----------------------------------
Sometimes there is an unlocatable short circuit somewhere on
the board, usually the result of a poor soldering job or a resistor
or capacitor lead bent over and touching another area. If the board
has been thoroughly inspected by several people and the error is not
obvious then there is another suggested course of action.
Remove all the chips from the board and bypass the regulator
with a high current variable supply. Adjust the current limiting or
the voltage until the current flowing through the board is something
between .05 and .1 amperes. Using a voltmeter set to read
millivolts, measure the voltage across the power supply. If the
voltage is too great to be read in millivolts do not become alarmed,
this is actually better.
The idea here is than any trace that has a current flowing
through it also has a small voltage drop along its length. If you
start where you have connected the -ve lead of the power supply and
test all paths leading from it there will be one with a significant
voltage difference in millivolts along some test length. This path
is the one that leads to the short circuit. Continue to leap frog
along until you find your way back to the +ve lead of the power
supply. Somewhere on the path you will discover a wrong connection
and there is your short.