Interfacing ics for microprocessor

PERIPHERALS AND
INTERFACING

Prepared By,
R-THANDAIAH PRABU M.E.,
Lecturer - ECE
thandaiah@gmail.com

Introduction (cont’d)
65,536 possible I/O ports

Data transfer between ports and the processor is over data bus
8088 uses address bus A[15:0] to locate an I/O port
AL (or AX) is the processor register that takes input data (or provide
output data)
Data bus
AL
AX
I/O I/O I/O

8088

Address bus A[15:0]

Introduction
• I/O devices serve two main purposes
– To communicate with outside world
– To store data
• I/O controller acts as an interface between the
systems bus and I/O device
– Relieves the processor of low-level details
– Takes care of electrical interface
• I/O controllers have three types of registers
– Data
– Command
– Status

Introduction (cont’d)
• To communicate with an I/O device, we need
– Access to various registers (data, status,…)
• This access depends on I/O mapping
– Two basic ways
» Memory-mapped I/O
» Isolated I/O
– A protocol to communicate (to send data, …)
• Three types
– Programmed I/O
– Direct memory access (DMA)
– Interrupt-driven I/O

Accessing I/O Devices
• I/O address mapping
– Memory-mapped I/O
• Reading and writing are similar to memory read/write
• Uses same memory read and write signals
• Most processors use this I/O mapping
– Isolated I/O
• Separate I/O address space
• Separate I/O read and write signals are needed
• Pentium supports isolated I/O
– 64 KB address space
» Can be any combination of 8-, 16- and 32-bit I/O ports
– Also supports memory-mapped I/O
FFFFF FFFFF

Memory
addressing I/O
space FFFF I/O
addressing Memory addressing
space space
00000 00000
0000

Direct I/O Memory-mapped I/O

Accessing I/O Devices (cont’d)
• Accessing I/O ports in 80x86
– Register I/O instructions
in accumulator, port8 ; direct format
– Useful to access first 256 ports
in accumulator,DX ; indirect format
– DX gives the port address
– Block I/O instructions
• ins and outs
– Both take no operands---as in string instructions
• ins: port address in DX, memory address in ES:(E)DI
• outs: port address in DX, memory address in ES:(E)SI

8088 Port Addressing Space
Addressing Space  Accessing directly by instructions
FFFF
IN AL, 80H
IN AX, 6H
OUT 3CH, AL
OUT 0A0H, AX
Accessed
through
DX  Accessing through DX
00FF

00F8 Accessed IN AL, DX
directly by IN AX, DX
instructions OUT DX, AL
0000 OUT DX, AX

Input Port Implementation
Data Bus

Gating Input
8088 device
Address bus
Decoder

Other control
signals

— The outputs of the gating device are high impedance when the processor is not
accessing the input port

— When the processor is accessing the input port, the gating device transfers input
data to CPU data bus

— The decoding circuit controls when the gating device has high impedance output
and when it transfers input data to data bus

Input Port Implementation
 Circuit Implementation

A7 Tri-state
A6 Data bus buffer Input data
A5
A4
A3 CE
A2
A1
A0

RD IO/M

Output Port Implementation
 Circuit Implementation

A7
A6 Data bus Latch Output data
A5
A4
A3 CLK
A2
A1
A0

WR IO/M

An Example I/O Device

• Keyboard
– Keyboard controller scans and reports
– Key depressions and releases
• Supplies key identity as a scan code
– Scan code is like a sequence number of the key
» Key’s scan code depends on its position on the keyboard
» No relation to the ASCII value of the key

Interfacing the Keyboard to 8051 microcontroller

• The key board here we are interfacing is a matrix keyboard.
This key board is designed with a particular rows and columns.
These rows and columns are connected to the microcontroller
through its ports of the micro controller 8051. We normally use
8*8 matrix key board. So only two ports of 8051 can be easily
connected to the rows and columns of the key board.

• When ever a key is pressed, a row and a column gets shorted
through that pressed key and all the other keys are left open.
When a key is pressed only a bit in the port goes high. Which
indicates microcontroller that the key is pressed. By this high
on the bit key in the corresponding column is identified.

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• Once we are sure that one of key in the key board is pressed next our
aim is to identify that key. To do this we firstly check for particular row
and then we check the corresponding column the key board.

• To check the row of the pressed key in the keyboard, one of the row is
made high by making one of bit in the output port of 8051 high . This
is done until the row is found out. Once we get the row next out job is
to find out the column of the pressed key. The column is detected by
contents in the input ports with the help of a counter. The content of the
input port is rotated with carry until the carry bit is set.

• The contents of the counter is then compared and displayed in the
display. This display is designed using a seven segment display and a
BCD to seven segment decoder IC 7447.

• The BCD equivalent number of counter is sent through output part of
8051 displays the number of pressed key.

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Circuit diagram of INTERFACING KEY BOARD TO 8051.

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• Keyboard is organized in a matrix of rows and columns as shown in the
figure. The microcontroller accesses both rows and columns through the
port.

• The 8051 has 4 I/O ports P0 to P3 each with 8 I/O pins, P0.0 to P0.7,P1.0 to
P1.7, P2.0 to P2.7, P3.0 to P3.7. The one of the port P1 (it understood that
P1 means P1.0 to P1.7) as an I/P port for microcontroller 8051, port P0 as
an O/P port of microcontroller 8051 and port P2 is used for displaying the
number of pressed key.

• Make all rows of port P0 high so that it gives high signal when key is
pressed.

• See if any key is pressed by scanning the port P1 by checking all columns
for non zero condition.

• If any key is pressed, to identify which key is pressed make one row high at
a time.

• Initiate a counter to hold the count so that each key is counted.

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• Check port P1 for nonzero condition. If any nonzero number is
there in [accumulator], start column scanning by following step
9.

• Otherwise make next row high in port P1.

• Add a count of 08h to the counter to move to the next row by
repeating steps from step 6.

• If any key pressed is found, the [accumulator] content is rotated
right through the carry until carry bit sets, while doing this
increment the count in the counter till carry is found.

• Move the content in the counter to display in data field or to
memory location

• To repeat the procedures go to step 2.
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A “short list” of embedded systems
Anti-lock brakes
Modems
Auto-focus cameras MPEG decoders
Automatic teller machines Network cards
Automatic toll systems Network switches/routers
Automatic transmission On-board navigation
Avionic systems Pagers
Battery chargers Photocopiers
Camcorders Point-of-sale systems
Cell phones Portable video games
Printers
Cell-phone base stations Satellite phones
Cordless phones Scanners
Cruise control Smart ovens/dishwashers
Curbside check-in systems Speech recognizers
Digital cameras Stereo systems
Disk drives Teleconferencing systems
Electronic card readers Televisions
Temperature controllers
Electronic instruments Theft tracking systems
Electronic toys/games TV set-top boxes
Factory control VCR’s, DVD players
Fax machines Video game consoles And the list goes
Fingerprint identifiers Video phones
Home security systems
Life-support systems
Washers and dryers
on and on
Medical testing systems

Programmable Keyboard/Display Interface -
8279

• A programmable keyboard and display interfacing chip.
• Scans and encodes up to a 64-key keyboard. And Controls up to
a 16-digit numerical display.

• Keyboard section has a built-in FIFO 8 character buffer.

• The display is controlled from an internal 16x8 RAM that stores the coded
display information.
• 8279 has 8 control words to be considered before It is programmed

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PIN DESCRIPTION
RETURN LINE

RETURN LINE
3 Mhz
Interrupt request CONTROL STROB
SHIFT KEY
RETURN LINE
Scan line

OUTPUT
DISPLAY

DATA BUS

BLANK DISPLAY
CHIP SELECT

SJCET 0 DATA 1 CONTROL

Interfacing the 8279 to the Microprocessor

• The 8279 is decoded to function at 8-bit I/O address 10H & 11H

• 10H – data port

• 11H – control port

• PAL16l8 is used to decode the I/O address for 8279

• A0 selects either the data or control port

• IRQ, since it is a interrupt pin, this signal is not connected to
microprocessor.
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8279 interfaced to 8088 microprocessor :

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Keyboard Interface:
• The keyboard matrix can be any size from 2x2 to 8x8.
• The I/O port number decoded is the same, 10H & 11H
• The 74LS138 drives active low column strobe signals for the
keyboard on one line at a time .
• Selection Pins SL2-SL0 sequentially scan each column of the
keyboard
• The internal circuitry of 8279 scans RL pins, searches for key
closure.
• RL pins incorporate internal pull-ups, no need for external
resistor pull-ups.

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Programming the Keyboard Interface :

• before any keystroke is detected, the 8279 must be programmed

• the first 3 bits of the number sent to the control port (11H) select
one of the 8 different control words

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Programming the Keyboard Interface :

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Control Word Description:

• First three bits given below select one of 8 control
registers (opcode).
 000DDMMM
• Mode set: Opcode 000.
DD sets displays mode.
MMM sets keyboard mode.
• DD field selects either:
• 8- or 16-digit display
• Whether new data are entered to the rightmost or
leftmost display position.
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I/O Interface
• Control Word Description:
• MMM field:

• Encoded Mode: SL outputs are active-high, follow binary bit
pattern 0-7 or 0-15 depending on 8 or 16 digit display.
• Decoded Mode: SL outputs are active-low (only one of the four
outputs will be low at any time).Pattern output: 1110, 1101,
1011, 0111.
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I/O Interface
FIFO status register

•Code given in text for reading keyboard.
•Data returned from 8279 contains row data that need to be
translated to ASCII:

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I/O Interface

Display Interface

• Six Digit Display Interface of 8279

• The Interface uses a PAL16L8 to decode the 8279 at I/O Ports
• 20H for Data
• 21H for Control/Status

• The Segment data are supplied to the displays thru the OUTA & OUTB of
8279

• Bits are buffered by a segment driver (2003A) to drive the segment inputs to
the display

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I/O Interface

Six Digit Display Interface of 8279

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8251 UART
• Universal asynchronous receiver
transmitter (UART) : provides serial
communication.
• 8251 functions are integrated into
standard PC interface chip.
– 8255 used to a be a parallel interface
• Allows many communication
parameters to be programmed.

Serial communication

• Characters are transmitted separately:

No char
start bit 0 bit 1 ... bit n-1 stop

time

Serial communication
parameters
• Baud (bit) rate.
• Number of bits per character.
• Parity/no parity.
• Even/odd parity.
• Length of stop bit (1, 1.5, 2 bits).

8251 CPU interface

status
(8 bit)
CPU Tx /
8251
Rx
data serial
(8 bit) port

Programming I/O
• Two types of instructions can support I/O:
– special-purpose I/O instructions;
– memory-mapped load/store instructions.

• Intel x86 provides in, out instructions.
Most other CPUs use memory-mapped
I/O.
• I/O instructions do not preclude memory-
mapped I/O.

8251 USART Interface
8251 RS232
D[7:0] TxD

RD RD RxD
WR WR
A0 C/D
TxC
CLK CLK RxC

A7
A6
A5
A4
A3
A2
A1
IO/M

Programming 8251
8251 mode register

7 6 5 4 3 2 1 0 Mode register

Number of Baud Rate
Stop bits Parity enable
0: disable 00: Syn. Mode
00: invalid 1: enable 01: x1 clock
01: 1 bit 10: x16 clock
10: 1.5 bits Character length 11: x64 clock
11: 2 bits
00: 5 bits
01: 6 bits
Parity 10: 7 bits
0: odd 11: 8 bits
1: even

Programming 8251
8251 command register

EH IR RTS ER SBRK RxE DTR TxE command register

TxE: transmit enable
DTR: data terminal ready, DTR pin will be low
RxE: receiver enable
SBPRK: send break character, TxD pin will be low
ER: error reset
RTS: request to send, CTS pin will be low
IR: internal reset
EH: enter hunt mode

Programming 8251
 8251 status register

DSR SYNDET FE OE PE TxEMPTY RxRDY TxRDY status register

TxRDY: transmit ready
RxRDY: receiver ready
TxEMPTY: transmitter empty
PE: parity error
OE: overrun error
FE: framing error
SYNDET: sync. character detected
DSR: data set ready

PROGRAMMABLE DMA
CONTROLLER 8257

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DMA
• Direct memory access (DMA)
– Problems with programmed I/O
• Processor wastes time polling
– In our example
» Waiting for a key to be pressed,
» Waiting for it to be released
• May not satisfy timing constraints associated with some
devices
– Disk read or write
– DMA
• Frees the processor of the data transfer responsibility

DMA Example
• A hard disk data transfer rate of 5MB/s
– One byte every 200 ns !!

• A microprocessor hardly can execute even
one instruction in 200 ns.
– Multiple instructions would be required to
accomplish data transfer
• read the byte from the hard disk
• place it in memory
• increment a memory pointer
• test for another byte to read

DMA
• DMA is implemented using a DMA controller
– DMA controller
• Acts as slave to processor
• Receives instructions from processor
• Example: Reading from an I/O device
– Processor gives details to the DMA controller
» I/O device number
» Main memory buffer address
» Number of bytes to transfer
» Direction of transfer (memory → I/O device, or vice
versa)

DMA
• Steps in a DMA operation
– Processor initiates the DMA controller
• Gives device number, memory buffer pointer, …
– Called channel initialization
• Once initialized, it is ready for data transfer
– When ready, I/O device informs the DMA controller
• DMA controller starts the data transfer process
– Obtains bus by going through bus arbitration
– Places memory address and appropriate control signals
– Completes transfer and releases the bus
– Updates memory address and count value
– If more to read, loops back to repeat the process
– Notify the processor when done
• Typically uses an interrupt

I/O Data Transfer (cont’d)
DMA controller details

8255 Programmable Peripheral
Interface

8255 Programmable Peripheral Interface

• 8255 PPI has three 8-bit registers
• Port A (PA)
• Port B (PB)
• Port C (PC)
– These ports are mapped as follows
8255 register Port address
PA (input port) 60H
PB (output port) 61H
PC (input port) 62H
Command register 63H

8255 Programmable Peripheral Interface
Data bus
D[7:0]
PA[7:0]
A0
8088 A1 PB[7:0]
RD Control port
WR PC[7:0]
RESET
A7 CS
A6
A5
A4
A3
A2 A1 A0 Port
IO/M
0 0 PA
0 1 PB
1 0 PC
1 1 Control

• Mapping I/O ports is similar to mapping memory
– Partial mapping
– Full mapping

• Keyboard scan code and status can be read from
port 60H
– 7-bit scan code is available from
• PA0 – PA6
– Key status is available from PA7
• PA7 = 0 – key depressed
• PA0 = 1 – key released

I/O Data Transfer
• Data transfer involves two phases
– A data transfer phase
• It can be done either by
– Programmed I/O
– DMA
– An end-notification phase
• Programmed I/O
• Interrupt

• Three basic techniques
– Programmed I/O
– DMA
– Interrupt-driven I/O

I/O Data Transfer (cont’d)
• Programmed I/O
– Done by busy-waiting
• This process is called polling
• Example
– Reading a key from the keyboard involves
• Waiting for PA7 bit to go low
– Indicates that a key is pressed
• Reading the key scan code
• Translating it to the ASCII value
• Waiting until the key is released

Programming 8255
 8255 has three operation modes: mode 0, mode 1, and mode 2

Programming 8255
 Mode 0:
— Ports A, B, and C can be individually programmed as input or output ports
— Port C is divided into two 4-bit ports which are independent from each other

 Mode 1:
— Ports A and B are programmed as input or output ports
— Port C is used for handshaking

PA[7:0] PA[7:0]
PC4 STBA PC7 OBFA
PC5 IBFA PC6 ACKA
PC3 INTRA PC3 INTRA
8255 PB[7:0] 8255 PB[7:0]
PC2 STBB PC2 OBFB
PC1 IBFB PC1 ACKB
PC0 INTRB PC0 INTRB
PC6, 7 PC4, 5

Interfacing ics for microprocessor

Programming 8255
Mode 2:

— PortA is programmed to be bi-directional
— Port C is for handshaking
— Port B can be either input or output in mode 0 or mode 1

PA[7:0]
PC7 OBFA
PC6 ACKA
PC4 STBA
8255 PC5 IBFA
PC3 INTRA
PC2 In Out STBB OBFB
PC1 In Out IBFB ACKB
PC0 In Out INTRB INTRB
PB[7:0]
Mode 0 Mode 1

INTERRUPT INTRODUCTION
• An interrupt is an event which informs the CPU that its
service (action) is needed.

• Sources of interrupts:
– internal fault (e.g.. divide by zero, overflow)
– software
– external hardware :
• maskable
• nonmaskable
– reset

Basic Procedure for Processing
Interrupts
• When an interrupt is executed, the mp:

– finishes executing its current instruction (if any).
– saves (PUSH) the flag register, IP and CS register in the stack.
– goes to a fixed memory location.
– reads the address of the associated ISR.
– Jumps to that address and executes the ISR.
– gets (PULL) the flag register, CS:IP register from the stack.
– continues executing the previous job (if any).

8088/86 Hardware Interrupts
pins
INTR: Interrupt Request.
– Input signal into the CPU
– If it is activated, the CPU will finish the current instruction and
respond with the interrupt acknowledge operation

• NMI: NonMaskable interrupt.
– Input signal
– Examples of use: power failed. Memory error

• INTA: Interrupt Acknowledge.
– Output signal

Programmable Interrupt
Controllers (PIC)

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8259 Chip
• The Intel 8259 is a family of
Programmable Interrupt Controllers
(PIC) designed and developed for use with
the Intel 8085 and Intel 8086
microprocessors. The 8259 acts as a
multiplexer, combining multiple interrupt
input sources into a single interrupt output
to interrupt a single device.

Pin description
• 8-bit bi-directional data bus, one address line is needed,
PIC has two control registers to be programmed

• The direction of data flow is controlled by RD and WR.

• CS is as usual connected to the output of the address decoder.

• Interrupt requests are output on INT which is connected to the INTR
of the processor. Int. acknowledgment is received by INTA.

• IR0-IR7 allow 8 separate interrupt requests to be inputted to the PIC.

• sp/en=1 for master , sp/en=0 for slave.

• CAS0-3 inputs/outputs are used when more than one PIC to cascaded.

• It is a tool for managing the interrupt requests.

• 8259 is a very flexible peripheral controller chip:
– PIC can deal with up to 64 interrupt inputs
– interrupts can be masked
– various priority schemes can also programmed.

• originally (in PC ) it is available as a separate IC

• Later the functionality of (two PICs) is in the
motherboards chipset.

• In some of the modern processors, the functionality of
the PIC is built in.

• A Programmable Interrupt Controller (PIC) is a device which
allows priority levels to be assigned to its interrupt outputs. When
the device has multiple interrupt outputs to assert, it will assert
them in the order of their relative priority. Common modes of a
PIC include hard priorities, rotating priorities, and cascading
priorities. PIC often allow the cascading of their outputs to inputs
between each other.

• PIC typically have a common set of registers:
• Interrupt Request Register (IRR), In-Service Register (ISR),
Interrupt Mask Register (IMR).

– The IRR specifies which interrupts are pending
acknowledgement, and is typically a symbolic register which
can not be directly accessed.
– The ISR register specifies which interrupts have been
acknowledged, but are still waiting for an End Of Interrupt
(EOI).
– The IMR specifies which interrupts are to be ignored and not
acknowledged.

All interrupt requests must pass through the PIC’s interrupt request register (IRR) and interrupt mask register (IMR).
If put in service, the appropriate bit of the in-service (IS) register is set.

Basic Interrupt Event
• External device sends an interrupt signal, to one of the Interrupt Request (IR) pin,
or an internal interrupt occurs.

• The 8259 Chip signal to the CPU the interrupt via the INT pin.

• The CPU finishes the present instruction and sends Interrupt Acknowledge (INTA).

• The interrupt type is sent to the CPU via the Data bus.

• The contents of the flag registers are pushed onto the stack.

• Both the interrupt and flags are cleared. This disables the IR pin.

• The contents of the code segment register (CS) are pushed onto the Stack.

• The contents of the instruction pointer (IP) are pushed onto the Stack.

• The interrupt vector contents are fetched, from and then placed into the IP and into
the CS so that the next instruction executes at the Interrupt Service Routine (ISR)
addressed by the interrupt vector.

• While returning from the interrupt-service routine by the Interrupt Return (IRET)
instruction, the IP, CS and Flag registers are popped from the Stack and return to
their state prior to the interrupt.

Modes
• Fully Nested mode
• Special Fully Nested mode
• Nonspecific Rotating
• Specific Rotating
• Special Mask
• Polling

Operation Command Words
• After the Initialization Command Words
(ICWs) are programmed into the 8259,
the chip is ready to accept interrupt
requests at its input lines. However,
during the 8259 operation, a selection
of algorithms can command the 8259 to
operate in various modes through the
Operation Command Words.

OPERATION
• PIC is to be initialized and programmed to control its operation.

• The operation in simple words:
when an interrupt occurs , the PIC determines the highest priority,
activates the processor via its INTR input, and sends the type
number onto the data bus when the processor acknowledges the
interrupt.

•Priority:
What is used in PC is fully nested mode. That is the lowest numbered
IRQ input has highest priority. Lower priority interrupts will not
be forwarded to the processor until the higher priority interrupts
have been serviced.

8259 in PC XT

ICW1: 13H
ICW2: 08H
ICW3: 09H

Why ADC ?
• Digital Signal Processing is more
popular
– Easy to implement, modify
– Low cost

• Data from real world are typically Analog

• Needs conversion system
– from raw measurements to digital data
– Consists of
• Amplifier, Filters
• Sample and Hold Circuit, Multiplexer

ADC
• Analog-to-Digital converter (ADC):
A circuit that converts an analog voltage to
digital.

Analog to Digital Conversion ADC

V+ref
•
Input voltage = V) output code =
N (MAX) bit n
ADC 0110001
0100010
0100100
0101011
:
:
V-ref :

ADC Major characteristics

• n=converted code, V=input voltage,
 V − V− ref 1  V+ ref − V− ref
n= +  , where ∆V = ,
 ∆V 2 −1
N
2  integer
e.g V− ref = 0, ∆V = 10mV , see the figure on next page.

• The linearity measures how well the transition voltages
lie on a straight line.
• The differential linearity measures the equality of the
step size.

Analog to digital converter example

• Convert an analog level to digital output
• e.g. V-ref=0V, ∆ V=10mV.

ADC

• Conversion Time: The time required to
convert an analog voltage to digital.

• For an 8-bit ADC:
Output = Vin x 255
Vref

Conversion rate=inverse of conversion time

Converter Errors
• Offset Error • Integral Linearity Error

• Gain Error • Differential Linearity Error

• Can be eliminated by initial • Nonlinear Error
adjustments – Hard to remove

Terminologies
• Converter Resolution • Conversion Time
– The smallest change – Required time (tc) before
required in the analog the converter can
input of an ADC to provide valid output data
change its output code
by one level

• Converter Throughput Rate
• Converter Accuracy
– The number of times the
– The difference between
input signal can be
the actual input voltage
sampled maintaining full
and the full-scale
accuracy
weighted equivalent of
the binary output code – Inverse of the total time
– Maximum sum of all required for one
successful conversion
converter errors
including quantization – Inverse of Conversion
error time if No S/H(Sample
and Hold) circuit is used

S/H increase Performance
• S/H (Sample and Hold)

– Analog circuits that quickly samples the input signal
on command and then holds it relatively constant
while the ADC performs conversion

– Aperture time (ta)
• Time delay occurs in S/H circuits between the time the hold command
is received and the instant the actual transition to the hold mode takes
place
• Typically, few nsec

Converting bipolar to unipolar
• Using unipolar converter
when input signal is bipolar • Input signal is scaled and an offset
is added
– Scaling down the Add
offset
input
– Adding an offset scaled
• Bipolar Converter
– If polarity
information in
output is desired
– Bipolar input range
• Typically, 0 ~ ± 5V
– Bipolar Output
• 2’s Complement
• Offset Binary
• Sign Magnitude

ADC Various Approaches
• 3 Basic Types

• Digital-Ramp ADC
• Successive Approximation ADC
• Flash ADC

Digital-Ramp ADC
• The output of the DAC is applied to the
other terminal of the comparator.
• Since the output of the DAC is increasing
with the counter, it will trigger the
comparator at some point when its voltage
exceeds the analog input.
• The transition of the comparator stops the
binary counter, which at that point holds the
digital value corresponding to the analog
voltage.

Digital-Ramp ADC
• Conversion from analog to digital form
inherently involves comparator action
where the value of the analog voltage at
some point in time is compared with some
standard.
• A common way to do that is to apply the
analog voltage to one terminal of a
comparator and trigger a binary counter
which drives a DAC.

Counter Type ADC
• Block diagram • Operation
– Reset and Start Counter
– DAC convert Digital output
of Counter to Analog signal
– Compare Analog input and
Output of DAC
• Vi < VDAC
– Continue counting
• Vi = VDAC
– Stop counting
• Waveform
– Digital Output = Output of
Counter
• Disadvantage
– Conversion time is varied
• 2n Clock Period for Full
Scale input

Tracking Type ADC
• Tracking or Servo Type • Can be used as S/H circuit
– Using Up/Down – By stopping desired
Counter to track input instant
signal continuously – Digital Output
• For slow varying – Long Hold Time
input • Disabling UP (Down)
control, Converter generate
– Minimum (Maximum)
value reached by input
signal over a given
period

Successive Approximation ADC
• Most Commonly used in • Block Diagram
medium to high speed
Converters
• Based on approximating the
input signal with binary
code and then successively
revising this approximation
until best approximation is
achieved
• SAR(Successive
Approximation Register)
holds the current binary
value

Successive Approximation ADC
• Circuit waveform
• Conversion Time
– n clock for n-bit ADC
– Fixed conversion time
• Serial Output is easily
generated
– Bit decision are made
in serial order

Dual Slope Integrating ADC
• Operation
– Integrate • Excellent Noise Rejection
– Reset and integrate – High frequency noise cancelled
out by integration
• Applications – Proper T1 eliminates line noise
– DPM(Digital Panel Meter), – Easy to obtain good resolution
DMM(Digital Multimeter)

Voltage to Frequency ADC
• VFC (Voltage to Frequency • Low Speed
Converter) • Good Noise Immunity
– Convert analog input • High resolution
voltage to train of pulses – For slow varying
• Counter signal
– Generates Digital output – With long conversion
by counting pulses over time
a fixed interval of time
• Applicable to remote data
sensing in noisy
environments
– Digital transmission
over a long distance

Flash ADC
• The resistor net and comparators
provide an input to the combinational
logic circuit, so the conversion time is
just the propagation delay through the
network - it is not limited by the clock
rate or some convergence sequence.

Parallel or Flash ADC

• Very High speed conversion
– Up to 100MHz for 8 bit
resolution
– Video, Radar, Digital
Oscilloscope
• Single Step Conversion
– 2n –1 comparator
– Precision Resistive
Network
– Encoder
• Resolution is limited
– Large number of
comparator in IC

ADC080x, 8-Bit µP Compatible A/D
Converters
• CMOS 8-bit successive approximation A/D converters that
use a differential potentiometer ladder—similar to the
256R products.

• These A/Ds appear like memory locations or I/O ports to
the microprocessor and no interfacing logic is needed.

• Differential analog voltage inputs allow increasing the
common-mode rejection and offsetting the analog zero
input voltage value.

• In addition, the voltage reference input can be adjusted to
allow encoding any smaller analog voltage span to the full
8 bits of resolution.

• Compatible with 8080 µP derivatives
—no interfacing logic needed - access
ADC080x Features
time - 135 ns

• Easy interface to all microprocessors,
or operates “stand alone”

• Differential analog voltage inputs

• Logic inputs and outputs meet both
MOS and TTL voltage level
specifications

• Works with 2.5V (LM336) voltage
reference

• On-chip clock generator

• 0V to 5V analog input voltage range
with single 5V supply

• No zero adjust required

Digital-to-Analog Conversion
• When data is in binary form, the 0's and
1's may be of several forms such as the
TTL form where the logic zero may be a
value up to 0.8 volts and the 1 may be a
voltage from 2 to 5 volts.
• The data can be converted to clean
digital form using gates which are
designed to be on or off depending on
the value of the incoming signal.


• Data in clean binary digital form can be
converted to an analog form by using a
summing amplifier.
• For example, a simple 4-bit D/A
converter can be made with a four-input
summing amplifier.

DAC
• Parallel Interface: Transfers 8-bits (or
more) at once.
• Digital-to-Analog Converter (DAC) converts
8-digital data to analog.

Digital to analog converter (DAC)

V+ref ( High Reference Voltage)
• Output voltage = Vout(n)
Input code n
(NMAX bit Binary code)
0110001 NMAX
0100010 (bit length)
0100100 DAC
0101011
:
:

V-ref (Low Reference Voltage)
AD/DA (v.9a) 116

DAC Formula & Resolution
• Vout = Input x Vref
256 (for 8-bit)
• Vout = DAC output analog voltage
• Input = Decimal value of binary input
• Vref = Reference DC voltage
Resolution
The worst case error introduced when
converting. In an 8-bit DAC, there are 255
possible steps. The resolution is the smallest
step size, or 1/255, 0.39%.

Digital-to-analog conversion

 x0 20 + x1 21 + x2 22 + K + xn −1 2 n −1 
Vout =k n ÷
 2 

reference voltage in “multiplying” DAC
i.e., 00...0 => 0 volts; 11....1 => k volts (slightly less)
k / 2n = “step size”

DAC: basic equation
DAC output
V
+ ref
−V
−ref
 V+ref
Vout (n) = V +n 
−ref  NMAX 
 2 
=V + n∆V ∆V
−ref

V-ref
• At n=0, Vout(0) = V-ref

• At max. n= 2NMAX -1,
– Vout cannot reach V+ref , a kind of tree-planting problem
– E.g. NMAX=4, n=0, 1, 2, … 15
• Some DACs have internal reference voltage settings, some
can be set externally.

DAC: characteristics
• Glitch: A transient spike in the output of a
DAC that occurs when more than one bit
changes in the input code.
– Use a low pass filter to reduce the magnitude
– Use sample and hold circuit is a better
solution
• Settling time: Time for the output to settle
to typically 1/4 LSB after a change in DA
output.


• 2 Basic Approaches

– Weighted Summing Amplifier
– R-2R Network Approach

Weighted Sum DAC
• One way to achieve D/A conversion is to
use a summing amplifier.
• This approach is not satisfactory for a
large number of bits because it requires
too much precision in the summing
resistors.
• This problem is overcome in the R-2R
network DAC.

Interfacing ics for microprocessor

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