Hardware interfacing
Supplying Clock & Power
 Buses and bridges
 DC/AC analysis
 Timing analysis
 Design considerations
 Design for worst case

Supplying power

Power circuitry
–
–
–
–
–
What voltages do you need?
How much power?
DC2DC
Battery power
Filter to bypass Power Supply Noise (0.01-0.001
microfarad)
– Avoid ground loops.

Power saving techniques
– Power consumption proportional to the clock frequency
– Choice of components
– Power saving modes
Supplying clock

Frequency
– Minimum (some devices may require minimal
clock in order to maintain internal state)
– Maximum

Duty cycle
– Usually symmetrical but may be asymmetrical
as well
Buses and bridges

Focus on the microprocessor bus and its operation
–
–
–
–

General bus operations
Device addressing and decoding
Timing diagrams and timing requirements
External devices: PRU, memory, other support chips
Bridge is device transparently connecting two or
more buses.
– Buses can be different or the same
• Example PCI/PCI or PCI/EISA
Buses and bridges
General bus operation
– Processor places desired peripheral's address onto
address bus
– Processor (or peripheral) places data onto data bus for a
write (read) operation
– Peripheral (processor) gates the data into its internal
registers to complete the operation
– Operation is directed by the various control lines that
are included in the bus
•
•
•
•
•
•
Clock signals
Address strobe / latch
Device enable signals
Data direction signals -- read vs. write operations
Type of reference -- standard or memory mapped I/O -- IO/M*
Data ready
Timing analysis
Just as in comedy, timing is essential to the
success of microcomputer design
 When timing or loading problems show up
in design they usually appear as intermittent
failures or sensitivity to power supply
fluctuation, temperature and so on.

Timing diagram Notation
Convention
Valid
high
Floating
Not
Driven
Tri-state
High-Z
Transition
Low
Active
Valid
Stable
Data
Valid
low
Changin or
Undefined
Data
Transition
high
Valid high
Rise and Fall Times
Raise Time
logical 1
80% of logical 1
20% logical 1
logical 0
Fall Time
Timing analysis

Propagation delays
– Asymmetrical for high to low and low to high
transitions
– Setup and hold time
• Setup time is amount of time a sampled input signal must be
valid and stable prior to a clock signal transition
• Hold time is amount of time that sampled signal must be held
valid after the clock transition occures
– If setup or hold time requirement not met it causes
metastability – state unpredictable and may be unstable
Timing diagrams

System clock
– Bus transitions occur in relation to system clock
– Called the E clock in 68HC11
• 1/4 crystal frequency
• Low - internal process
• High - reading or writing data

Some definitions:
– Setup time : time for a device to change its output in
response to an input change
– Hold time: length of time a device will maintain its last
output in response to a request to change it
Example
Fan-Out and Loading analysis
DC and AC

The main question can this output drive all
the inputs I want connect to it?
DC Specifications
VDD
5V 3V
VOL
VOH
VIL
VIH
Maximum low-level
output voltage
Minimum high-level
output voltage
Maximum low-level
input voltage
Minimum high-level
input voltage
0.1V 0.1V
4.9V 2.9V
1.0V 0.6V
3.5V 2.1V
DC

The maximum current that can be produced by
output
– Minimum output low (sink) current for valid 0
output voltage - IOLmin
– Minimum output high (source) current for a valid
one output voltage - IOHmin

Maximum current required to drive an input
– Maximum input low current for valid zero input
voltage – IILmax
– Maximum input high current for a valid one input
voltage - IIHmax
AC




CL – The load capacitance that an output is
specified to drive
Cin – Maximum input capacitance of a driven
input load
Cstray – Wiring and stray capacitance can be
approximated to be in the range of 1 to 2
picofarads per inch of wiring on a typical PC
board.
Driving device spec CL > actual Cload=Cin1 +
Cin2 + …+Cwiring
68HC11 Memory cycle
Note that in the HC11



Address information is provided to the external device (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 E-clock
All read and write operations MUST take place in 1 E-cycle
– External devices and circuitry must be designed to meet this
requirement
– Cannot use “wait states” as you can in other microprocessor
systems
• 8085: Slower devices can use READY input to request wait
states
• Processor maintains address, data, and control signals
Expanded multiplexed mode

68HC11 supplies external bus signals
– Port B = A15-A8
– Port C = A7-A0 multiplexed with D7-D0

Address usually must be valid during entire
operation
– Need to latch A7-A0 (using 74HC373, for example)

Use external logic to derive control signals
– Chip enable/select
– Read/Write
– Output enable
Error detection and correction

Errors
– Soft error
– Hard error

Confidence Checks
–
–
–
–
Parity
Hamming code
Checksum
CRC
Test study

consider the following very general circuit
layout that interfaces the 68HC11 to a 6264
Fast Static Ram (8k x 8)
Example



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
Timing relationship


Timing relationships are derived by comparing the
timing diagrams of the memory chip and the HC11 and
considering the external circuitry where necessary.
Read operation HC11
• E-clock goes low
• Address information placed on address bus, AS
pulsed to trigger address latch
• E-clock goes high
• HC11 expects data to be placed on data bus
• E-clock goes low again and process repeats
Timing relationship

Read operation HC11
– HC11 expects data to be placed on the data
bus before E-clock goes low
– HC11 expects data to remain on data bus
until E-clock goes low
– External device must release data bus before
HC11 places next address on address bus
– Exact timing requirements are given in table
Timing relationship

6264 timing:
– Outputs data after receiving the address and the E* and G*
signals
– Timing constraints
• How long does 6264 take to output data after receiving
address and enable signals?
• How long does it keep data on the bus?
• Timings given in data sheet
– We also 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
• Values taken from data sheets
– Suggestion:
• For each timing parameter given in the RAM data
sheet, draw a new timing diagram that shows the
relation between the RAM’s signals and the HC11’s
signals
– Example: tELQV
•
•
•
•
•
This is the time from when the Enable signal (E1*) goes low
until the data is placed on the data bus (assumes the address is
already available and that the output enable G* is already low)
For this circuit, the E1* is connected to the output of the 138
decoder, so it changes state after the inputs to the decoder
change
Inputs to the decoder are A15-A13 and the E-clock
So, draw a timing diagram that shows E-clock, A15-A13, E1*,
and the data bus
Does the RAM put the data on the bus before the HC11
Read operation
Timing relationships for read
operation
6264
HC11
tELQV (CE to data valid) < tACCE -PDdec
tGLQV (OE to data valid) < tACCE - PDinv
tAVQV (A valid to data valid) < tACCA -PDlatch
tGHQZ (OD to data hi Z) > tDHR - PDinv,
< tMAD - PDinv
tEHQZ (CD to data hi Z) > tDHR -PDdec,
< tMAD -PDdec
Write operation - timing

HC11:
– Sequence of events
•
•
•
•
•

E-clock goes low
Address placed on address bus, AS pulsed to latch it
E-clock goes high
HC11 places data on data bus
E-clock goes low and cycle repeats
6264:
– Needs address, data, E*, and W* signals
• Write occurs only when both E* and W* are low
• Data must be held on bus until either E* or W* rises
– We need to make sure that
• HC11 places data on data bus in time for RAM to get it
• HC11 holds data long enough for write to complete
Timing relationship for write
operation
6264
HC11
tAVAV < tAVM + tr + PWEH + tf + tAH PDlatch
tAVEH < tAVM + tr + PWEH -PDlatch + PDdec
tAVWH < tAVM + tr + PWEH -PDlatch +
PDnand
tAVEL < tAVM + tr - PDlatch + PDdec
tELEH < PWEH
tELWH < PWEH - PDdec + PDnand
Timing operation for write
operation (cont’d)
tDVEH < PWEH - tDDW + PDdec
tDVWH < PWEH - tDDW + PDnand
tEHAX < tAH - PDdec
tWHAX < tAH - PDnand
tEHDX < tDHW - PDdec
tWHDX < tDHW - PDnand
Example modification
13
A12-A0
8K x 8 RAM
Address
Data
R/W*
A13
A14
E
A15
74HC138
A0
Y2
A1
A2
Y3
CS1
CS2*
CS3*
W*
G*
E*
8
Interrupts
Edge or Level?
 Interrupt aggregation and hierarchy

– Open collector
– Using external logic
• Latch and Status
• Make sure you latch it only once at source
• Make sure you can mask/unmask on each level
Assignment

Calculate what is the base address of 6164
in the circuit from Slide 22 and Example
modification.