Memory Interface Circuits 80x86 Processors: Ref: Online Course On EE-390, KFUPM
Memory Interface Circuits 80x86 Processors: Ref: Online Course On EE-390, KFUPM
Memory Interface Circuits 80x86 Processors: Ref: Online Course On EE-390, KFUPM
on EE-390, KFUPM
Handout 16
Objective:
C
- The output control (‘OC’)
1D 1Q signal is used to assign
high impedance state to the
2D 2Q
output lines (Q’s),
3D 3Q
4D 4Q
Inputs Output
5D 74F373 5Q
‘OC’ ‘C’ D’s Q’s
6D 6Q
L H H H
7D 7Q
8Q
L H L L
8D
L L x Q0
OC
H x x high-Z
Note: the circuitry used to construct this IC and their operating details
are discussed in the following slides. Note that if the clock (C) input is
logic low, the output logic level (Q’s) remained unchanged or unaffected
by inputted data (D’s).
-
Slide 2: Circuit diagram of 8-bit Address latch IC “74F373”:
D flip-flops
Buffers
D F/F
D
flip-flop
- The main components of this circuit are ‘D’ type flip-flops (in
blue) and buffers (in brown)
Note: before explaining the operation of this circuit, lets discuss the
construction and operation of these components.
Slide 3: ‘Flip-flops and Buffers used in Address bus latch circuit:
Logic ‘0’
ALE signal
from CPU
Animate
D
F/F
Latched
Valid P.A. P.A. send
from CPU to memory
NOTE: If valid physical address is applied to the eight input pins (D’s) of the IC
and then ALE pulse is applied into the clock input of the IC, the D flip-flops will be
activated and latch the inputted information.
Due to ‘Logic 0’ input of the output control pin, the buffers are activated and
behave in transparent mode. Thus, the latched 8-bit physical address is
outputted through the Q-pins.
Slide 5: Operation of 20-bit Address bus latch circuit:
The complete memory interface with this circuit component as "Address Bus
Latch" is show in next figure.
Note: Note that each of the 74F373 IC’s are used to latch 8-bit
information as per ALE pulses, as buffers works in transparent
mode. This diagram consists the Address bus latch block, of
the memory interface discussed before. Click the link to see
this complete circuit.
CLK A17L-A19L Address
CLK
Decoder
Address CE0 - CE7
AD0-AD15 A0 - A19
A16 -A19 Bus
latch A1L- A16L
BHE A0L
CLK
BHEL
8086 Bank
write WRU
MPU CLK CLK ALE
control WRL
Bus Controller logic
8288 Memory
MWTC Subsystem
S0 S0 Bank
S1 S1 MRDC Read
RDU
S2 S2 control
RDL
DT/R DEN logic
DIR EN
Data
D0-D15 Bus
MN/MX Transceive
buffer D0-D15
Ready Ready
Animate
Note:. The figures above shows the block and circuit diagram of the 8-bit Data
bus transceiver buffer IC. Also note that G bar input is used to enable the buffer
operation, whereas DIR input selects the direction of intended data transfer.
Assume that the device is enabled by applying G bar = 0. Now if DIR is set to
logic 0, the output of AND gate 1 will be 0 and all the odd numbered buffers (G3,
G5, G7 and so on) will be off. So the data path from An to Bn will be disabled.
But the output of AND gate 2 will be logic 1 and all the even numbered buffers
(G4, G6, G8 and so on) will be ON. Consequently, the data path from Bn to An
will be ENABLED.
Similarly, for G bar = 0 and DIR = logic 1, data path from An to Bn will be
ENABLED.,
Slide 7: Operation of Data bus transceiver buffer (cont’d):
The complete memory interface with this circuit component as "Data Bus
Transceiver Buffer" is show in next figure.
Note: Here two octal buffer IC’s are parallely connected to achieve
16-bit bidirectional data bus transceiver buffer operation. The
complete memory interface circuit with this transceiver buffer
component can be seen by clicking the given link.
Note that the enable signal (G) that comes from the DEN
signal of the CPU, requires logic -1 to enable the buffer, unlike
that of 8088.
CLK A17L-A19L Address
CLK
Decoder
Address CE0 - CE7
AD0-AD15 A0 - A19
A16 -A19 Bus
latch A1L- A16L
BHE A0L
CLK
BHEL
8086 Bank
write WRU
MPU CLK CLK ALE
control WRL
Bus Controller logic
8288 Memory
MWTC Subsystem
S0 S0 Bank
S1 S1 MRDC Read
RDU
S2 S2 control
RDL
DT/R DEN logic
DIR EN
Data
D0-D15 Bus
MN/MX Transceive
buffer D0-D15
Ready Ready
- Note that G input is the control input that enables the decoder.
Once enabled, any of the four outputs (Y0 to Y3) are selected
depending on logic levels of inputs 'A’ & ‘B’, as shown in Table
Note: Note that due to the output inverters of the IC, when selected, the
outputs generated a logic low signal.
_____________________________________________________
Slide 10: Decoder Circuits used in Memory Interface (Cont’d):
Note: Each decoder has an active LOW Enable input, which can be used to
activate the decoder.
Slide11: Controller circuits of 8086 Memory Interface:
- Bank read and write control logic circuits enables even and odd
address byte transfer, as per logic levels of BHE and A0 signals.
Note: Detail discussion in this topic can be found in next lecture.
- Read and Write bank control logic circuits are as given below:
Note: Note proper read or write signal should also be generated by the CPU
to activate these controllers.
The complete memory interface circuit where this above component is used is:
BHE A0L
CLK
BHEL
8086 Bank
write WRU
MPU CLK CLK ALE
control WRL
Bus Controller logic
8288 Memory
MWTC Subsystem
S0 S0 Bank
S1 S1 MRDC Read
RDU
S2 S2 control
RDL
DT/R DEN logic
DIR EN
Data
D0-D15 Bus
MN/MX Transceive
buffer D0-D15
Ready Ready
Slide 12: Example 1: Design a Decoder circuit
Using eight buffer circuits, design a 4-bit data bus transceiver buffer
circuit that once enabled with G = 1, transfers data according to
following DIR input.
SOLUTION: