Bus Operations and Interfacing
Overview
• Focus on the microprocessor bus
o
o
o
o
Bus operations in general
Device addressing and decoding
Timing diagrams and timing requirements
External devices:
PRU, memory, other support chips
• Readings:
o Text: Chap 5, Chap 6 (Sec 2); Chap 7, Sec 1-5
o HC11: Sec 2.6
o E9: Appendix A
BUS Operations
• This section focuses on the ability to interface
external devices to the microprocessor
• Interfacing requires
o Hardware interface -- electrical and mechanical considerations
of the interface
o Software interface -- programming necessary to permit the
external device and the processor exchange information
• The "bus"
o The set of signal lines used to connect the processor and the
peripheral devices using read/write operations
Cont..
BUS Operations
o Can be a simple extension
of the processor pins or
can consist of
modifications of the pins
o Bus signals can be divided
into 3 categories
 Address
 Data
 Control
• Figure 5.1 Processor
bus organization
Cont..
BUS Operations
• Many microcontrollers use a multiplexed address
and data bus
o The data bus and (part of) the address bus use the same
physical lines
o Address and data signals cannot appear on the bus at the same
time
o Requires extra logic to demultiplex
 Usually need to latch the address bits
 (Temporarily put them in a register)
o Why do we use a multiplexed bus?
Cont..
BUS Operations
• General bus operation:
o Processor places desired peripheral's address onto address bus
o Processor (or peripheral) places data onto data bus for a write
(read) operation
o Peripheral (processor) gates the data into its internal registers to
complete the operation
o Operation is directed by the various control lines that are
included in the bus
 Clock signals
 Address strobe / latch
 Device enable signals
Cont..
BUS Operations
 Data direction signals -- read vs. write
 Type of reference -- standard or memory mapped
I/O -- IO/M*
 Data ready
• The bus is not a static connection mechanism
o Peripheral devices must be enabled (given access to write
to the bus) only when they are participating in a data
transfer
o Only 1 device can be driving the bus at any given time -otherwise bus contention results
o 3-state (= tristate) devices are commonly used
 Normal logic levels if enabled
 High-impedance state if disabled
Cont..
BUS Operations
o Figure 5.3 Three-State
Logic
Cont..
BUS Operations
• Enable signals
o Usually an active-low signal
o Referred to as E*
 Sometimes S* or CS*
o Device must be enabled before you can write to it or read from it
o E* is usually derived from address lines
o Other signals
 R/W*
• Read and Write may be separate signals
 G*
• Synchronizes data transfer
• Sometimes CLK or E
 Other address lines
Cont..
BUS Operations
• Address decoding
o To communicate with a particular peripheral device, the
processor places the device's address on the address bus
o All peripherals must examine the address and decide if they are
being referenced . . .
o Address decoding: Usually use digital logic to generate the
CS* signal
• Full (exhaustive) decoding
o Each peripheral is assigned to a unique address
o All address bits must be used to define the referenced location
o Typically used for memory devices
Cont..
BUS Operations
• Partial decoding
o
o
o
o
Not all address bits are used in the decoding process
Peripheral can respond to more than 1 address
Main advantage is decreased circuit complexity in the decoder
Disadvantages:
 Must guard against inadvertent accesses due to multiple
addresses
 Somewhat inefficient use of overall address space
• Example: Interface a peripheral that only uses 16
addresses
o Requires 4 address bits to select 1 of the 16 internal addresses
o Other system address bits used for the chip enable signal
Cont.
BUS Operations
o 16-bit system --12 bits for
enable!
o Try 6 bits for decoder
instead
• 74138 decoder chip is
commonly used for address
selection (a “1-of-8
decoder”)
o 3 address bit input
o 8 output select lines
(active low)
o 3 chip select lines (to
select the 74138)
• Figure 5.5 74138 Decode
Cheap
Cont.
BUS Operations
• Example: Interface two
2Kx8 memory chips to a
16-bit address system
o Starting addresses for the
chips should be $5000 and
$6800
o Each chip has a chip
enable (select) signal, E1*
(active low)
o Address ranges for the 2
chips are shown below
 Fixed bits are routed
to the decoder inputs
 Varying bits are routed
to the chip address
lines
Cont.
BUS Operations
Cont.
BUS Operations
• Now, use '138 output Y3 to select a 512x8 memory chip
o Y3 asserted for addresses in the range $5800 to $5FFF (2KB)
o 512 addresses requires 9 address bits, A0 to A8; bits A9 and
A10 are not needed
o As a result, partial address decoding is being used where each
physical address will respond to 4 addresses
 Example: address 1 in the chip is accessed whenever
addresses 01011 xx 000000001 are applied to the memory
system
 $5801
 $5A01
 $5C01
 $5E01
Cont..
BUS Operations
•
Modes of operation
o Internal parts of MCU:
 CPU, memory, registers
o External parts:
 Pins for I/O and bus signals
o To reduce pin count, some pins may have more than one
function
o For HC11, operating mode determines how pins are used
 Select the operating mode using MODA and MODB pins at
reset
Cont..
BUS Operations
•
Single-chip mode
o No external memory or I/O chips
 Don’t need external bus
 Ports B and C are used as I/O ports
o Reduced system cost
o Limited to on-chip RAM, ROM and EEPROM
• Expanded multiplexed mode
o Ports B and C used as address and data bus
 Allows connections to external memory and I/O chips
 Port B = A15 - A8
 Port C = AD7-AD0 (A7-A0 multiplexed with D7-D0)
o Control bus signals
Cont..
BUS Operations
 Strobe A/address strobe pin (STRA/AS) used as AS
 Strobe B/read/write pin (STRB/R/W*) used as R/W*
• Special bootstrap mode
o Test mode
o Generally used to load in a test program, EEPROM
programming, or running a monitor program
o On reset, HC11 executes code located in the Boot ROM
(BF00-BFFF for 68HC11E9)
 Loads more program code using serial interface
 Used by PCBUG11
 Listing of code in Reference Manual (Appendix B)
Cont..
BUS Operations
• Special test mode
o Intended for use by manufacturer only
o Not much documentation available
o Used to test the chip
• Chip specifications
o Appendix A of Technical Data manual
 Maximum ratings
 Recommended operating conditions
 DC electrical characteristics
 AC electrical characteristics
 Power dissipation
Cont..
BUS Operations
o Important when interfacing with other devices; Be aware of
 Current limits
 Voltage limits
 Fan in/fan out
• DC electrical characteristics
VDD
5V 3V
VOL Maximum low-level output voltage 0.1V 0.1V
VOH Minimum high-level output voltage 4.9V 2.9V
VIL Maximum low-level input voltage 1.0V 0.6V
VIH Minimum high-level input voltage 3.5V 2.1V
o VOL = 0.1 V
Cont..
BUS operations
o
o
o
o
VOH = VDD - 0.8 V (ILOAD = -0.8 mA)
VIL = 0.2 x VDD
VIH = 0.7 x VDD
ID = 25 mA
• AC electrical characteristics
o Timing information
Timing diagrams and timing
requirements
• Timing diagrams show
the changes that occur
in a signal or group of
signals over time
• Figure 7.1 Basic timing
diagram information
Cont..
Timing diagrams and timing
requirements
• System clock
o Bus transitions occur in relation to system clock
o Called the E clock in 68HC11
 1/4 crystal frequency
 Low - internal process
 High - reading or writing data
• Some definitions:
o Propagation delay: amount of time used by a device to change
its output in response to an input change
o Setup time: length of time that an input to a device must be
stable before a clock transition
o Hold time: length of time that an input to a device must remain
stable after a clock transition
Cont..
Timing diagrams and timing
requirements
• Figure 7.2 Example
timing diagram for a
write cycle
Cont..
Timing diagrams and timing
requirements
tcyc 500 ns min PWEH 222 ns
min
tf 20 ns max tPDSU 100 ns min
tr 20 ns max tPDH 50 ns min
PWEL 227 ns min
Cont..
Timing diagrams and timing
requirements
• 68HC11 timing
specifications (E-series
manual pages A18, 20)
Cont..
Timing diagrams and timing
requirements
Cont..
Timing diagrams and timing
requirements
• Timing analysis and interfacing external devices to
the 68HC11
o General observation: the timing characteristics of the external
device (e.g., memory unit) must meet or exceed the timing
requirements of the HC11
 Must compare the related timing values in the HC11
read/write timing diagram to the values in the device's
diagram
 Must take into account any delays due to external circuitry
such as the decoder
o Note that in the HC11 . . .
 Address information is provided to the external device
Cont..
Timing diagrams and timing
requirements
o (using the multiplexed address/data bus) in the low
half cycle of the E-clock
 Data to be read/written is placed on the data bus only in the
high half cycle of the Eclock
 All read and write operations MUST take place in 1 Ecycle
• External devices and circuitry must be designed to meet
this requirement
• Cannot use “wait states” as you can in other
microprocessor systems
o Intel 8085: Slower devices can use READY input to request
wait states
o Processor maintains address, data, and control signals
Cont..
Timing diagrams and timing
requirements
• Expanded multiplexed mode
o 68HC11 supplies external bus signals
 Port B = A15-A8
 Port C = A7-A0 multiplexed with D7-D0
o Address usually must be valid during entire operation
 Need to latch A7-A0 (using 74HC373, for example)
o Use external logic to derive control signals
 Chip enable/select
 Read/Write
 Output enable
Cont..
Timing diagrams and timing
requirements
• Read operation
o Memory puts data on
bus when clock rises
o MCU latches data when
clock falls
Cont..
Timing diagrams and timing
requirements
• Write operation
o MCU puts data on bus
when clock rises
o Memory latches data
when clock falls
Cont..
Timing diagrams and timing
requirements
• Example analysis:
consider the following
very general circuit
layout that interfaces
the HC11 to a 6264
Fast Static Ram (8k x
8) (figure shows a 6164
but we’ll use 6264 in
lab)
• Figure 7.6 Expanded
mode operation
Cont..
Timing diagrams and timing
requirements
o Observations on the circuit
 74373 is used as the address latch to “save” the lower 8
bits of the address that are on Port C only during the first
half of the E-clock cycle
 Discrete logic is used to derive the write and read enable
signals for the memory chip (W* and G*)
• Both can only be asserted in the second half of the E-clock
cycle
 74138 is used for address decoding to generate a memory
chip enable (chip select) signal (E1*)
• Since the E-clock enables the 138, the decoder is only
active in 2nd half cycle
• Memory chip can not be enabled in the 1st half cycle
o This impacts some of the timing relationships
Cont..
Timing diagrams and timing
requirements
o Timing relationships are derived by comparing the timing
diagrams of the memory chip and the HC11 and considering
the external circuitry where necessary.
o Read operation:
 HC11:
• Expects external device to place data on data bus in
time for it to be read
• External device must hold data until E-clock falls, but
must remove it (and go to high impedance state)
before HC11 places next address on bus
 6264:
• Outputs data after receiving the address and the E*
and G* signals
Cont..
Timing diagrams and timing
requirements
• Timing constraints
o How long does 6264 take to output data after
receiving address and enable signals?
o How long does it keep data on the bus?
o Write operation:
 6264:
• Needs address, data, E*, and W* signals
o Write occurs only when both E* and W* are low
o Data must be held on bus until either E* or W* rises
 HC11:
• Places address on address bus and latch
• After E-clock rises, places data on data bus and holds it
Cont..
Timing diagrams and timing
requirements
o Have to take into
account the propagation
delays due to the
external circuitry
 Decoder (74138): PDDEC =
25 ns
 Inverter (7404): PDINV = 15
ns
 Latch (74573): PDLATCH =
23 ns
 Nand (7400): PDNAND = 15
ns
o Read operations (read cycle 2
of 6264)
Cont..
Timing diagrams and timing
requirements
o Timing relationships for
read operation:
Cont..
Timing diagrams and timing
requirements
o Write operations (write
cycle 2 of 6264)
Cont..
Timing diagrams and timing
requirements
o Timing relationship for
write operation:
Cont..
Timing diagrams and timing
requirements
 Timing operation for
write operation(cont’d):
Cont..
Timing diagrams and timing
requirements
• In the lab, you will be required to design and
construct an expanded mode memory interface
o Strongly encourage you to use Figure 2-23 in the HC11
Reference Manual as a guide
 In this figure, the memory chip is always enabled
 The 138 decoder is used to derive read and write enables
and thus replaces the discrete logic used in Spasov’s
example
o Both methods will (and have) worked in the lab but using the
138 for write/read signals is recommended
 Fewer chips -- all fit on a single strip of protoboard
 Shorter runs of inter-chip wires
o Timing analysis equations must be updated to reflect the
circuit that you design!
Cont..
Timing diagrams and timing
requirements
• Cycle-by-cycle
operation
o Appendix A of Reference
Manual shows cycle-bycycle execution for each
instruction
o Shows contents of
address and data buses
during each cycle
o Example: STAA (ext)
Cont..
Timing diagrams and timing
requirements
Cycle
1
2
3
4
Addr Bus
OP
OP+1
OP+2
hhll
Data Bus
B7
hh
ll
(A)
R/W*
1
1
1
0
• Example: STAA (IND, X)
Cycle Addr Bus
Data Bus
1
OP
A7
2
OP+1
ff
3
FFFF
-4
X+ff
(A)
R/W*
1
1
1
0
Cont..
Timing diagrams and timing
requirements
• General ideas of bus expansion and interfacing
o
o
o
o
Bus composition and components
Addressing bus devices
Decoding bus addresses
Timing diagrams
• HC11 interfacing and timing requirements
• Modes of operation