Application Note AN897 "MC68008 minimum

AN897
MO~OROLA
@
Appliution
Semiconductor Products Inc.
Note
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MC68008 MINIMUM CONFIGURATION
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Prepared by
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Geoffrey Brown and Kyle Harper
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Advanced Microcomputer Applications Engineering
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Microprocessor Division
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Motorola Inc.
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Austin, Texas
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and are terminated by the assertion of
INTRODUCTION
data twer’acknowledge
(DTACK) by the peripheral or
This application note demonstrates the design of a simple
m~~o~’’:device being addressed. Figures 1-4 show the
high-performance MC68~8 system that uses the MC68681
.~~n~um hardware necessary for an Mc68w8 system conDual Universal
Asynchronous
Receiver Transmitter
~~vs;~ng ok
(DUART) to interface with external devices. The MC68W8 is
*),J
~T*’*~ddress
decode logic,
an exce~ent low-cost alternative to the MC68W
and
.~\,~\
,>
,..t!’.)
features an 8-bit data bus while maintaining software com“S$~~~YrDTACK generation logic,
patibility with the rest of the M68~ Family. TheMC68681
.$ “
Reset logic,
DUART is an M68~
Family data communications X,@jp Bus error generation logic,
i*,\N
i’<.
that features:
..,,., *1
System memory,
TWO independent asynchronous sefial ch~nels
,$@~~&.
,,.+?.
*$):~\*
Interrupt handting logic, and
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~. ,k.
A programmable l~bit counter/timer,
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~~**
...,.<.+
+I$++<,i:$
An
MC68681 interface.
.. ‘{:>,..$,
.
A 6-bit pardel input port, and
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‘+...-.
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:2$%,
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The following paragraphs detail the required hardware as apf$;i,..J.
:$.~
An 8-bit pardel output port.
pfied to the design concept described in this application note.
Emphasisin this design concept is pl~@~~~& performance,
expandabihty, and low chip COU~$<,~?~):,,,3
The M68~ system design p#@&~~ei’demonstrated in this
application note include: ,,,~t$:e>~kvs~
Interrupt hardware, ~ ‘?~~.*
.i~>;~’.
Peripheral interfac~@ ,,
Memory inter f~ce;~e&&ques,
Memory ref&@~,@itration in an M68~ system, and
Efficient,<~{#,&O software.
The s~te~~’~escnbed in this design concept, features the
foHo,#Wwme:
,ig&~~*
MC68W
microprocessor,
+$~%ytes of ROM,
&K bytes of dynamic RAM with no wait states, and
An MC68681 DUART.
The following paragraphs describe the hardware required
for a h~gh-perforrnance, expandable, low chip count
MC68M8 system fouowed by a description of the software
necessary to initialize and drive the MC68681 DUART.
HARDWA~
REQU=MENTS
The MC68~8 has an asynchronous bus structure in which
bus cycles are initiated by the assertion of address strobe
Address Decode Logic
The only tricky part of address decoding for an MC68W8
system is that the system ROM must be mapped to address
$m
at reset. It would be impractical to fti the ROM at the
bottom of the address map, as this would not tiow for
dynamic programming of interrupt vectors. To provide
dynamic mapping of these interrupt vectors, an SN74LS164
shift register (U28) is used to generate a signal, MAP, which
is low for the first eight memory cycles after reset (the
number of cycles necessry to fetch the reset vector and stack
pointer). U28 is reset rdong with the processor and is clocked
by the rising edge of ~. The ~
signrd generated by U28
is used by the address decoding circuitry to force selection of
ROM when MAP is low and to dow normal memory
decoding when ~
is high.
In the design given in this application note, address
decoding is accomplished by a PAL16L8 (U22). This PAL is
programmed to generate eight chip-select signals from ten input signals. The inputs to the PAL are the upper eight address tines (A12-A19), IACK (the NAND of the MC68W8
function code lines, FC&FC2), and the MAP signal. Four of
the PAL-generated chip-select fines are used in this design to
locate RAM at the address $m,
ROM at $A~,
and the
MC68681 at $F~.
The four remaining chip-select tines are
avaflable for future system expansion.
m
I
ROMDTACK
5
.
RAMDTACK
DUARTDTACK
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U21
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RESET
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32 i
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, -,.=
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Ul, U2, Ull,
U3-U1O
U12
U13
U14
U15, U16
U18
U19
U20
U21, UM
U22
U23
U24
U25
U26, U27
U28
U29
U30, U31
U32, U36
U33, U35
U37
u%
U39
U17
SN74S 153
MCM666E j-15
McW~l
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SN“
-. .74 LS146
SN74LS136
MCM68766
MC66661
CK17AI
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SN74LS04
PAL16L8A
SN74LS05
SN74LS08
SN74S1O
SN74532
SN74LS164
MCN56
.SN74F74
DDu4-51m
SN74LS175
K1114A, 8 MHz OSC
MC1466
MC1489
DTACK Generation Logic
There are three possible sources of DTACK:
DUART DTACK,
asserted during both types of RAM cycles (normal and
refresh). CAS is generated only during normal cycles and
must be held asserted until the processor removes ~. In
order to accomphsh this, the appropriate delay tap (80
RAM DTACK, and
nanoseconds) is used to clock the SELECT signrd through a
ROM DTACK.
flip-flop (U31B). This fllp-flop is cleared when ~
is
The DUART generates its own ~K,
=
for RAM
negated. The output of this flip-flop is used for both the
is generated by the RAM control circuitry, and DTACK for
CAS signal and for the RAM DTACK. The 8 MHz MC68008
ROM is generated by an SN74LS175 quad flip-flop (U33).
allows the DTACK to be asserted up to 90 nanoseconds
These three DTACK sources are NANDed together by U25
before data from memory is vafid on a read cyc~,,The
and U21 to generate one processor DTACK.
specifications
for MCM6665L15
dynamic ,,*”~ *ies
guarantee that data is valid 75 nanoseconds af&g@$
.
9
Reset Logic
The memory refresh controller operates o~~~~~d~ciple of
There are two sources of system reset:
cycle stealing. Refresh requests may o~,!.er
between
Power-up reset, and
MPU bus cycles. If an MPU RAM c~@~~e&.&stoccurs durPushbutton reset.
ing a refresh cycle, it will not be starte~~~~l the refresh cycle
is finished. At periodic interv~~~
~‘Free-running clock
Power-up reset is generated by the timer (U29B) which pro(U29A) clocks a fip-flop (U3$,A) ~#generate a refresh reduces an active high pulse of approximately one-half second
quest. This refresh reque$+~~+l~chronized with the MPU
duration. The pushbutton reset, which allows the user to
~w~~t
clock by two fip-flOpS
reset the system without powering down, is generated by a
The MC68008 ac ,ati’i~l
specifications guarantee one
debounced switch. These two reset signals drive RESET and
falling
clock
edge
d~*]~e~S
high time and that there will
HALT through SN74LS05 open-collector drivers (U23).
beat least a on~%~df~ck period of ~S high time following
Bus Error Generation Logic
that clock ed@~$j*bitration between MPU and refresh reThe bus error signal, BERR, is generated by an
quests ocw%:~~ing this one-half clock period. The refresh
SN74LS175 quad fip-flop (U35). U35 counts clock pulses
reques~ijskc~nizer
consists of two SN74F74 flip-flops
that occur after IS becomes asserted. If ~ is stilI asserted
(U30)St~#e*~rst flip-flop (U30A) has as its input the refresh
after four rising edges of the E clock (between 5 and 6.5
r~hest ~%gndfrom the refresh request flip-flop (U3 1A) and
microseconds), U35 will generate BERR.
~i,r~[%%fked by the MPU clock. The second flip-flop (U30B)
~~h% as its input the output of the first synchronizer fip-flo~
System Memory
‘*,~k{.#30A) and is clocked by the MPU clock qualified by ~
The MC68008 system presented here has a system memory
‘~~$~~~~$
high. This two-level synchronizer is used to ensure that there
that consists of 16K bytes of ROM and 64K bytes of dynamic
RAM (see Figure 2). Because system performance is critical ,,$ ‘: will be no risk of the first synchronizer flip-flop (U30A)
entering a metastable state due to a missed setup time, The
in this design, a fairly complicated, but fast, dynamic *
‘
output of the synchronizer fllp-flop (U30B) is the SELECT
control circuit has been designed (see Figure 3). This.wlr~t
signal. Al three fllp-flops of the refresh circuitry are cleared
uses tw~elay
lines to sequence RAS and to a~wy,the
after the C=/RAMDTACK
flip-flop (U31 B) has been
MUX, CAS, and DTACK sigmds. Delay lines @S,,Y~@ssary
clocked during the refresh cycle. Address multiplexing for
in order to optimize memory cycle times and %~$~~$~ossible
the RAM is done by four SN74S153 multiplexer (Ul, U2,
to design the memory controller such th~$x~ystem
can
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,
U11, and U17) with the appropriate addresses routed to the
operate without wait states.
\\,
RAMs by SELECT and MUX. Refresh addresses are
A description of RAM refresh req~~~$hronization
and
generated by an SN74LS393 dual 4-bit counter (U19) which
arbitration is given in the followin&)}~a@graphs. Note that,
is clocked by the refresh clock.
for now, a signal called SELE@~;~$3umed
which initiates
refresh cycles. The principl~,~#~i&rnents of this signal are
Interrupt Handting Logic
that it occurs periodically~d
‘&at it becomes asserted only
The interrupt handling logic must prioritize incoming
while ~ is negated. IR’add)~~n, the RAM decode signal is
interrupt requests and generate interrupt acknowledge
quafified with =S i~~&&#e$stocreate a RAM request signal.
signals back to the interrupt sources. Interrupt prioritization
Either the SELE.&~$$kl
or the RAM request signal may
is accomplished with an SN74LS148 8-to-3 priority encoder
initiate a RA~,&~G~&R
(U13). The MC68008 supports three of the M68000 interrupt
The front<~n@\~~%heRAM controller consists of three OR
levels (interrupt levels two, five, and seven); therefore, only
gates (U* @o&ed by a three input NAND gate (U25)
two of the outputsofU13 are connected to the MC68008. An
which in ~~~eeds into the first delay line (U32). Each of the
SN74LS138 3-to-8 demultiplexer (U14) is used to generate
thre~,~~ ~%+eshas as one of its inputs a signal from the secIACK signals for interrupting devices. The SN74LS138 is
o~~ %~y:]ine (u36) which changes state in the midde of the
enabled when ~S is asserted and FCO-FC2 are all high
rn~ti
cycle. The other inputs to these OR gates consist of
(indicating an interrupt acknowledge cycle). Because the
S~ECT,
RAM request, and an inverted feedback path
MC68681 uses ody one of the interrupt levels (interrupt level
from the output of the three input NAND. The initiation of a
five), the remaining two levels are available for future system
RAM cycle via either SELECT or RAM request causes the
expansion.
output of the NAND gate to go high. The output of the
The MC68681 Interface
NAND gate is then held high by the inverted feedback path
until the feedback from the second delay line forces it low.
With these logic circuits in place, interfacing the MC68681
The purpose of the feedback path from the second delay line
to the MC68008 is trivial (see Figure 4). The RESET, R/~,
is to guarantee that the delay lines will be cleared and ready
and data bus tines (DO-D7) are connected directly between
to begin another RAM cycle at the end of a cycle. The outthe MC68681 and the MC68008. The 1/0 chip-select line
puts of the first delay line generate the RAS, MUX, and CAS
generated by the address decode logic (1/0) is connected to
signals. Both the RAS and address multiplex signals are
the MC68681 chip-select (C=) pin. These address lines are
6
used instead of AO-A3 in order to maintain hardware design
consistency with the other M68000 Family microprocessors
(which do not have address fine AO). The MC68681 interrupt
(IRQ) and interrupt acknowledge (IACK) pins are tied to the
~ and IACK5 lines of the interrupt handhng logic, respectively, thus assigning the MC68681 interrupt a level 5 priority. Finally, a 3.68@ MHz crystal is connected between the
MC68681 X1/CLK and X2 pins. The crystal is required for
the on-chip baud-rate generator. 15 pF and 5 pF shunt
capacitors must dso be connected between the crystal and
ground as shown to ensure proper operation of the oscillator.
The MC68681 serial channels are connected to external
devices via RS-232 drivers and DB-25 connectors. The
MC68681
—
— OPO, _IPO, OP1, and IP1 pins are used as the
RTSA, CTSA, RTSB, and CTSB handshake fines, respectively; therefore, they too are routed via the RS-232 drivers
to their. respective connectors.
DINIT is the DUART initialization routine and is executed
upon system power-up. After DINIT initirdizes the DUART
channels, it enables channel A and channel B in normal
operation mode. INCH is the input character routine. Upon
entry, INCH requires the channel base address in address
register AO. Upon return, the lower byte of data register DO
will contain the received character. OUTCH is the output
character routine. Upon entry, OUTCH requires the channel
THE DUART SO~WA~
——
This design will use both of the channels and the RTS/CTS
handshake capabilities of the DUART. The interface software required for this design is flowcharted in Figure 5 and is
listed at the end of this document, There are three routines:
DINIT, INCH, and OUTCH.
OUTCHR
&&
FIGURE 5 – MC-
Minimum System Sotiare
Flowchans
MOTOROLA
M68000
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12
13
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16
17
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20
21
22
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27
28
29
30
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INTERRUPT
INTERRuPT
ST4TUS
REGISTER
MASK
REGISTER
REGISTER
REGISTER
CTUR
CLS9
CURRENT
COUNTER/TIMER
COUNTER/TIMER
UPPER
CURRENT
COUNTER/TIMER
MOST
REGISTER
LEAST
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COUNTER/TIMER
LOWER
REGISTER
41
42
0UART+16
0UART+16
CHANNEL
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MOOE
REGISTER
AODRESS
19
43
44
4s
DUAQT+16
OUART+lB
MODE
29
DUARTt13
DUART+20
46
47
0UART+22
0UART+22
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REGISTER
STATUS
REGISTER
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CLOCK-SELECT
REGISTER
coMhANo
RECEIVER
REGISTER
BuFFER
TRANSMITTER
SIGNIFICANT
SIGNIFICANT
8
a
a
aUFFER
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EQu
DUART+24
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0UART+26
INTERRUPT
INPUT
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EQU
EQU
0UART+26
DUART+2a
OUTPUT
PORT
START-COUNTER
CONFIGURATION
COMMAND
EQU
EQU
0UART+2a
DUARTt30
OUTPUT
PORT
REGISTER
aIT
SET
EQU
0UART+30
STOP-COUNTER
OUTPUT
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COMMAND
REGISTER
BIT
RESET
VECTOR
REGISTER
(UNLATCHED)
REGISTER
COMMANO
COMMANO
BYTE
aYTE
w
98
91
93
.
.
This information haa bean carefully chackad and Is believed to ba entirely relisble. However, no responsibility is assumed for inaccuracies. Motorola reserves
the right to make changes to any products herein to improve rallability, function ordeslgn. Motorola does not assume any liability arising out of the application
or use of any product or circuit described herein. No license is conveyad under patent rights in any form. When this document contains information on a new
product, specifications herein are subject to change without notice.
MOTOROLA
@
Semiconductor
Products Inc.
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Printed in Switzerland