I MODE MAX)
MINIMUM AND MAXIMUM MODES
The requirements for supporting minimum and maxi-mum 80C88A systems are sufficiently different that they cannot be done efficiently with 40 uniquely de-fined pins. Conseque~, the 80C88A is equipped with a strap pin (MN/MX) which defines the system configuration. The definition of a certain subset of the pins changes, dependent on the condition of the strap pin. When the MN/MX pin is strapped to GND, the 80C88A defines pins 24 thrQ!!gh 31 and 34 in maximum mode. When the MN/MX pin is strapped to Vee, the 80C88A generates bus control signals itself on pins 24 through 31 and 34.
The minimum mode 80C88A can be used with either a multiplexed or demultiplexed bus. The multiplexed bus configuration is compatible with the MCS@-85
multiplexed bus peripherals (8155, 8156, 8355, 8755A, and 8185). This configuration (See Figure 5) provides the user with a minimum chip count sys-tem. This architecture provides the 80C88A pro-cessing power in a highly integrated form.
The demultiplexed mode requires one latch (for 64k addressability) or two latches (for a full megabyte of addressing). A third latch can be used for buffering if the address bus loading requires it. A transceiver can also be used if data bus bufferin~ required.
(See Figure 6.) The 80C88A provides DEN and DT /
R
to control the transceiver, and ALE to latch the>addresses. This configuration of the minimum mode provides the standard demultiplexed bus structure with heavy bus buffering and relaxed bus timing re-quirements.
The maximum mode employs the 82C88 bus con-troller. (See Figure 7.) The 82C88 decodes status lines
SO,
51, and 52, and provides the system with all bus control signals. Moving the bus control to the 82C88 provides better source and sink current capa-bility to the control lines, and frees the 80C88A pins for extended large system features. Hardware lock, queue status, and two request/grant interfaces are provided by the 80C88A in maximum mode. These features allow co-processors in local bus and re-mote bus configurations.2-126
inter ·80C88A
v v
..
~..
""'~i"r
.- ~
paR!~
WR PORT
~
AD 8155 B
J ' ALE PORT ~
.. DATAl C
,
, ADDR IN+-IOlii TIMER
RESET OUT ~
..
~A.~A" ADDR
J ~ AD
CLK AD.-AD, ADDR/DATA ALE
~
r - -
, ::!
PORTCE A
8OC88A
- - .. ~
A.-l0-
RDY 8355/8755AMN/MX f--Vee
Vee
'" r D 1
CLK",
RES ALE WRRti - - -
~ f - -- ,
RESET 101M PORT B~
VeeRDV
-
lolii-
iOii ~r----
RESv!vljJROG 82C84A
T
RES-OND WR
AD CE, 8185
.-
CS, ALE CE,.-
J.. A,.
A,AD .. ,
I'
r
i J
Vss Vee
... " ... ,..
240028-5
Figure 5. Multiplexed Bus Configuration
INT.
DT/A
om
-
CPUINT
1----1+-+1
Figure 6. Demultlplexed Bus Configuration
If, MiWC He
~ ===3~;;
DEN CTALR ~ ~me~:::::::;+=::::::::=+~::::::~::::::~;::-
iOWCI-DT'" ~ NC
Figure 7. Fully Buffered System Using Bus Controller
240028-6
240028-7
intJ 80C88A
Bus Operation
The 80C88A address/ data bus is broken into three parts-the lower eight address/data bits (ADO-AD7), the middle eight address bits (A8-A 15), and the up-per four address bits (A16-A19). The address/data bits and the highest four address bits are time multi-plexed. This technique provides the most efficient use of pins on the processor. The middle eight ad-dress bits are not multiplexed, i.e. they remain valid throughout each bus cycle. In addition, the bus can be demultiplexed at the processor with a single ad-dress latch if a standard, non-multiplexed bus is de-sired for the system.
Each processor bus cycle consists of at least four ClK cycles. These are referred to as T1, T2, T3, and T 4. (See Figure 8). The address is .emitted from the processor during T1 and data transfer occurs on the bus during T3 and T 4. T2 is used primarily for chang-ing the direction of the bus durchang-ing .read operations. In the event that a "NOT READY" indication is given by the addressed device, "wait" states (Tw) are in-serted between T3 and T 4. Each inin-serted "wait"
state is of the same duration as a CLK cycle. Periods can occur between 80C88A driven bus cycles.
These are referred to as "idle" states (Ti), or inac-tive ClK cycles. The processor uses these cycles for internal housekeeping.
! - - - C 4 + N w A I T ) - T C y - - - l - - - C 4 + N w A l d - ' C Y - - - !
T,
T.I " '.
T1 12 TI T.",K
\
ADDIU8T~TUS
AD.R
ADDJVDATA
---8~_·_·T_A_OU_T(_ •• _DoI_-...J)--~
jiji,iNii
READY
''--_ _ I
240028-8
Figure 8. Basic System Timing
2-129
During T1 of any bus cycle, the ALE (address latch enable) signal is emitted (by either the processor or the 82C88 bus controller, depending on the. MN/MX strap). At the trailing edge· of this pulse, a valid ad-dress and certain status information for the cycle may be latched.
Status bits
SO,
51, and 52 are used by the bus con-troller, in maximum mode, to identify the type of bus transaction according to the following table:S2 51
So CHARACTERISTICS Status bits 53 through 56 are multiplexed with high order address bits and are therefore valid during T2 through T 4. 53 and S4 indicate which segment reg-ister was used for this bus cycle in forming the ad-dress according to the following table:S4 S3 CHARACTERISTICS
o
(LOW) 0 Alternate Data (extra segment)0 1 Stack
1 (HIGH) 0 Code or None
1 1 Data
S5 is a reflection of the PSW interrupt enable bit. S6 is equal to
o.
110 ADDRESSING
In the 80C88A, 110 operations can address up to a maximum of 64k 110 registers. The 110 address ap-pears in the same format as the memory address on bus lines A15-AO. The address lines A19-A16 are zero in 110 operations. The variable 110 instructions, which use· register OX as a pointer, have full address
capability, while the direct 110 instructions dirf;lctly address one or two of the 256 110 byte locations in page 0 of the 110 address space. 110 ports are ad-dressed in the same manner as memory locations.
Designers familiar with the 8085 or upgrading an 8085 design should note that the 8085 addresses I/O with an 8-bit address on both·halves of the 16-bit address ·bus. The 80C88Auses a (ull 16-16-bit ad-dress on its lower 16 adad-dress lines.
EXTERNAL INTERFACE
PROCESSOR RESET AND INITIALIZATION