5-interfacing Io Devices - Student Version

INTERFACING I/O DEVICES 1

Iskandar Yahya [email protected] 03-89216591

INTRODUCTION 

Objectives Parallel and Serial Mode  Identify device/port address of peripheral-mapped I/O and memory mapped I/O  OUT & IN instructions.  Memory related instructions  Differentiate peripheral-mapped and memorymapped I/O  To interface 8085 with other devices 

Parallel Vs Serial  Parallel I/O - All 8-bits of data are transferred between 8085 and external device using the entire data bus 

Serial I/O - Bits are transferred one at a time, along one data line

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INTRODUCTION I/O Port Identification and Addressing 

Memory Mapped I/O  Access and identify as memory registers using memory space  Memory related control signal  16-bit address i.e. from 0000H to FFFFH  8085 treats I/O ports as if it was communicating with a memory location

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Peripheral Mapped I/O  Separate address scheme (8-bit i.e from 00H to FFH)  Enabled and identified by I/O related control signals 3

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Data transfer process is identical for both method Need to understand the foundation I/O operations in

BASIC INTERFACING CONCEPTS 

8085’s I/O Data Transfer Process: -

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Things to understand:  How 8085 selects an I/O device?  What hardware chips are necessary?  What software instructions are used?  How data are transferred?

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Peripheral-mapped and memory-mapped I/O have the same basic concept in interfacing. The difference is the Instructions used for each.

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PERIPHERAL I/O INSTRUCTIONS Two instructions: IN (code DB) and OUT(code D3)  IN - data input into the accumulator from device eg. Keyboard  OUT - data output from accumulator to device e.g printer, display  Each instructions are 2-byte - first byte denotes operation and second byte specifies Memory Machine Mnemonic Memory Contents the address. 

Address

Code

s

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PERIPHERAL I/O INSTRUCTIONS OUT INSTRUCTION



This instruction above is used to channel out the content of accumulator to an output device with the address 10H. (address is also called port number of device)

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Address of output device is 8-bit long, so 8085 can communicate with 256 output devices, with address from 00H to FFH.

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We can simply choose any address for our device, depending on our application. 6

PERIPHERAL I/O INSTRUCTIONS OUT INSTRUCTION

Instruction OUT M1 (Opcode Fetch) T1

T2

T3

20H

50H

Opcode D3H

M2 (Memory Read) T4

T1

unspecified

T2

M3 (I/O Write) T3

20H 51H

T1

T2

T3

Port Address 01H 01H

Port Address 01H

Accumulator Content

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PERIPHERAL I/O INSTRUCTIONS OUT INSTRUCTION 

The 8085 executes the OUT instruction in three machine cycles and it takes 10 T-states (clock periods) to complete.

The process:  At First machine cycle M1  Place high order memory address 20H on A15-A8  Place low order memory address 50H on AD7-AD0  ALE goes high and IO/M' goes low.  ALE indicates the availability of AD7-AD0 and used to demultiplex the bus  IO/M' low indicates a memory related operation  At T2, (RD)' (active low) signal is sent and combined with IO/M' signal to activate the (MEMR)' signal (active low)  Fetch the instruction code D3 using data bus, decodes it, finds out that it suppose to be 2-byte, so must read the second byte.

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PERIPHERAL I/O INSTRUCTIONS OUT INSTRUCTION 

At the second machine cycle M2  Same process as in M1, but this time with the memory address 2051H  Gets the device port address 01H, second byte of the OUT instruction

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At the third machine cycle M3  01H is placed on the low-order (AD7-AD0) and high-order (A15-A8) address bus  IO/M' goes high to indicate I/O operation.  At T2, the content of accumulator is placed on data bus (AD7-AD0), followed by control signal (WR)’.  ANDing the IO/M' and (WR)' signal, the (IOW)' is generated to enable the output device

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The data remains on the data bus (AD7-AD0) for two T-states (T2 & T3), before the processor executes the next instruction.9 Therefore we must latch the data bus within the two T-states before it is lost.

PERIPHERAL I/O INSTRUCTIONS IN INSTRUCTION Memory Address

Machine Code

Mnemonic Memory Contents s



IN 8-bit is a two-byte instruction with hex opcode DB followed by the port address of an input device.

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8085 will read the contents of the addresses 2065H and 2066H, and read the switch positions (input data) at port 84H by enabling the interfacing device of the port.

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Input data byte containing the switch positions from the input port will be placed in the accumulator.

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PERIPHERAL I/O INSTRUCTIONS IN INSTRUCTION

Instruction IN M1 (Opcode Fetch) T1

T2

T3

20H

65H

Opcode DBH

M2 (Memory Read) T4

T1

unspecified

T2

M3 (I/O Write) T3

20H 66H

T1

T2

T3

Port Address 84H 84H

Port Address 84H

Data From Input Port

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PERIPHERAL I/O INSTRUCTIONS IN INSTRUCTION

The process:  M1 and M2 are identical to OUT instruction.  At third machine cycle M3  8085 places address of input port (84H) on both high-order AD7-AD0 and low order A15-A8 address bus  Sends (RD)' signal, which will prompt I/O Read signal, (IOR)'  The (IOR)' enables input port, data obtained from port onto the bus and placed into the accumulator.  M3 for IN and OUT are essentially the same, the differences are  

IN use (RD)', OUT use (WR)' Data flow from input port to accumulator for IN, from

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT)

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We saw that, for OUT instruction, the content of the accumulator will be on the data bus for a period of two T-periods only.

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To catch the data, we need a data latch so that we can display or print the data, or even transfer it to an external device.

Questions: 1. When should we enable the latch? 2. What is the address of the latch?

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT)

The answer to both is at M3: -

Need to create:  Pulse to indicate the presence of address on bus  Generate timing pulse to indicate data byte is on the bus  Use both pulse to enable the latch to catch the data

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT) A7 Address Lines A7 – Ao A0

Data Bus (D7 – D0)

Decod er

Latch or Buffer

AND Control Signal or

Device Select Pulse

To Peripherals

Enabl e

Figure shows the basic blocks of a decoding circuit. The process are summarized as follows:  Decode address bus to generate a unique pulse corresponding to the device address on the bus; this is called the device address pulse or I/O address pulse.  Combine (AND) the device address pulse with the control signal to generate a device select (I/O select) pulse that is generated only when both signals are asserted.  Use the I/O select pulse to activate the interfacing

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT)

Example of a practical decoding circuit (Device address 01H)

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER

Figure above is a practical decoding circuit: 

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



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A7-A0 address line connected to 8-input NAND gate that functions as decoder Address 01H (00000001), A0 directly connected, A7 inverted to produce 01H G1 generate low pulse (active low)and combines with G2, which is low too, to select (IOSEL) or enable the data latch At this time the content of the accumulator is on the data bus IOSEL will clock the latch to catch the data, and send the data to the output device.

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER

Absolute Decoding  All eight address lines are decoded to generate unique pulse  Figure shows unique pulse will be generated if and only if address 01H (ooooooo1) is on the address bus  Good design practice  Costly (uses many extra devices/gates) Our previous decoding circuit employs this technique

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT)

Partial Decoding  Only some of the address lines are decoded, device has multiple addresses  Figure below shows A1 and A0 are omitted (don’t care states)and replaced by IO/M' and (WR)',  So device has the addresses 00H, 01H, 02H, and 03H  Used in small system  Less components A7 A6 A5 A4 A3 A2 A1 A0 (Don’t care) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 1

Puls e

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (OUT)

Decoding circuit implemented using Partial Decoding technique

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PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (IN)

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Input Interfacing: Example circuit

PERIPHERAL I/O INSTRUCTIONS DEVICE SELECTION & DATA TRANSFER (IN)

Input Interfacing Example of 8-Key input port. Basic concept is the same as output The process:  Address line connected to 8-inout NAND gate.  When address is FFH, NAND out put goes low and combined with (IOR)' in gate G2  G2 generate device select pulse used to enable tristate buffer  Data from keys are put on the data bus D7-D0 and loaded onto the accumulator The difference of this input circuit to the previous output circuit:     -

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PERIPHERAL I/O INSTRUCTIONS INTERFACING I/OS USING DECODERS



3-to-8 decoder/Demux

I0 I1 I2

Another scheme of address decoding is to use 3-to-8 demux (decoder) and4-input NAND to decode address busO O O O O I2 I1 I0

O0 O1 O2 O3 O4 O5 O6 O7

7

6

5

4

3

O 2

1

1

1

1

1

1

O 1

O 0

0

0

0

1

0

0

0

1

1

1

1

1

1

1

0

1

0

1

0

1

1

1

1

1

0

1

1

0

1

1

1

1

1

1

0

1

1

1

1

0

0

1

1

1

0

1

1

1

1

1

0

1

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1

1

1

1

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1

1

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1

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1

1

1

1

1

1

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1

1

0

1

1

1

1

1

1

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PERIPHERAL I/O INSTRUCTIONS INTERFACING I/OS USING DECODERS

Another scheme of address decoding. Use 3-to-8 demux (decoder) and4-input NAND to decode address bus

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PERIPHERAL I/O INSTRUCTIONS INTERFACING I/OS USING DECODERS

This is how the circuit work:  3-to-8 decoder has 3 inputs, 3 enable pins and 8 outputs  For the decoder to be functional, all the enable switches must be active, i.e. E1' = 0 (active low), E'2 = 0 (active low) and E'3 = 1 (active high)  Addresses lines A2, A1 and A0 are used as inputs, therefore we can have 8 different output device or port addresses. (2^3 = 8)  A7-A3 are used to enable the encoder  Combine the decoded signal with the appropriate control signal to generate I/O select pulse  Here, O0 is logically ANDed with (IOW)' signal to select the output port for output operation  O2 is logically NAND with (IOR)' to select the tristate buffer for input operation

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PERIPHERAL I/O INSTRUCTIONS INTERFACING I/OS USING DECODERS Decoder Enable A7 A6 A5 A4 A3

Input A2 A1 A0

Address  

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Based on the circuit, select pulse will be generated if input address is F8H, where O0 will be active and LED will display the content of accumulator

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Select pulse will also be generated if input address is FAH, where O2 will be active in this case and the buffer will channel the keyed data into the accumulator

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PERIPHERAL I/O INSTRUCTIONS BASIC INTERFACING CONCEPT SUMMARY

Basic concepts and steps for peripheral I/O interfacing:  The device address or port number is placed on the demultiplexed low order as well as high order address bus  Either the high order bus (A15-A8) or the demultiplexed low order bus (A7-A0) can be decoded to generate the pulse that correspond to the device address  Device address is AND with either the (IOW)' for output or (IOR)' for input control signal. When both is active, the device I/O device will be selected  Output device, Latch is used  Input device, tri-state buffer is used  Address bus can be decoded by Absolute or Partial (liner-select)decoding. Partial technique reduces

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PERIPHERAL I/O INSTRUCTIONS BASIC INTERFACING CONCEPT SUMMARY



8085’s I/O Data Transfer Process:  Places appropriate address on the address bus  Sends the control signals  Enables the interfacing device  Transfer data Interesting Question: Can an input port and an output port have the same port address? Answer:

Another Interesting Question: How will the port number be affected if we decode the high-order address lines A15-A8 rather than A7-A0? Answer:

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INTERFACING OUTPUT DISPLAY EXAMPLE: SEVEN SEGMENT OUTPUT DISPLAY AS AN OUTPUT DEVICE

Problem: 

Design a seven-segment LED output port with the device address F5H, using a 74LS138 (3-to-8) decoder, a 74LS20 4-input NAND gate, a 74LS02 NOR gate, and a common-anode seven-segment LED

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Write instructions to display digit 7 at the port

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INTERFACING OUTPUT DISPLAY EXAMPLE: SEVEN SEGMENT OUTPUT DISPLAY AS AN OUTPUT DEVICE Solution: Seven-Segment LED

To display digit "7" at the LED as in figure, the requirements are:  Logic "0" is required to turn on a segment because it is a common-anode seven-segment LED  To display "7", segments A, B, and C should be turned on Data Lines D D D D D D D D    The binary code should be: 7 6 5 4 3 2 1 0 30

INTERFACING OUTPUT DISPLAY EXAMPLE: SEVEN SEGMENT OUTPUT DISPLAY AS AN OUTPUT DEVICE

Interfacing circuit:  For output port with address F5H, the address lines A7-A0 should have the following logic: A7

A6

A5

A4

A3

A2

A1

A0

 

We need to use 74LS138, which has only 3 input pins. Therefore,  Use A2, A1, A0 as input lines to the decoder.  Connect A3 to active low enable (E1)'  The remaining lines connect to (E2)' through 4-input NAND gate Output O5 is ANDed with control signal (IOW)' using the NOR gate (negative input AND) to generate pulse to enable the latch.

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INTERFACING OUTPUT DISPLAY EXAMPLE: SEVEN SEGMENT OUTPUT DISPLAY AS AN OUTPUT DEVICE

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INTERFACING OUTPUT DISPLAY EXAMPLE: SEVEN SEGMENT OUTPUT DISPLAY AS AN OUTPUT DEVICE

Instructions: ;Load seven-segment code in the accumulator ;Display digit 7 at port F5H ;End

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First instruction loads 78H in the accumulator, which is the code to display digit "7“

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Second instruction sends contents of the accumulator (78H) to the output port F5H

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INTERFACING INPUT DEVICES EXAMPLE: DATA INPUT FROM DIP SWITCH 

Interfacing inputs devices is similar to that of output devices. The differences are in the bus signals and circuit components.

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Data input from DIP switch

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INTERFACING INPUT DEVICES EXAMPLE: DATA INPUT FROM DIP SWITCH

From Figure:  Tri-state octal buffer is used as an interfacing device controlled by active low signals (OE)'  If (OE)' is low, the keyed data shows up at the data bus Interfacing circuit:  All low-order address lines, except A4 and A3, are connected to the decoder  A4 and A3 are don’t care lines  Output pin O4 is used, so it will switch on when the following address presents: A7

A6

A5

A4

|--Enable Lines -------|

A3

A2

A1

A0

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|- Don’t care -| |------- Input --------|

INTERFACING INPUT DEVICES EXAMPLE: DATA INPUT FROM DIP SWITCH  



  

(IOR)' is generated by ANDing IO/M' and (RD)' (IOR)' is ANDed with output of decoder to produce select pulse to enable the tri-state buffer Once the tri-state buffer is enabled, the logic levels of the switches i.e. keyed data is placed on the data bus, before placed into the accumulator Closed switch = logic "0" Open switch = logic "1" Input reading is F8H

Instructions to read input at device address 84H:  36

INTERFACING INPUT DEVICES EXAMPLE: DATA INPUT FROM DIP SWITCH



 

Address lines A4 and A3 are not used, therefore the device can have multiple addresses as shown: A7

A6

A5

A4

A3

A2

A1

A0

1

0

0

0

0

1

0

0

1

0

0

0

1

1

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1

0

0

1

0

1

0

0

1

0

0

1

1

1

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0

 

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MEMORY MAPPED I/O In memory-mapped I/O, the devices are assigned and identified by 16-bit addresses  Instructions used:  LDA*  STA* 

-

Control signals used: (MEMR)' and (MEMW)'  This technique is similar to peripheral I/O 

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*-

MEMORY MAPPED I/O Memory Address

Machine Code

Mnemonic Comments s ; Store Contents of accumulator in memory location 8000H    

STA (Store A Direct) is used to store the content of accumulator to the specified memory register.  Here the memory address is 8000H (16-bit)  If we connect an output device with this address (8000H), the accumulator contents will transfer to the output device. 

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MEMORY MAPPED I/O

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For input operation, same principles as output operation

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Use LDA (Load A Direct) instead of STA

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For memory-mapped I/O, the control signals are memory read (MEMR)' and memory write (MEMW)' instead of (IOR)' and (IOW)'

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MEMORY MAPPED I/O DATA TRANSFER INSTRUCTIONS  

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

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Essentially the same as using IN and Out instructions But memory-mapped uses 16-bit addressing, therefore needs 4 machine cycles, instead of 3 as in the time diagram M1 is for reading Opcode (first byte of instruction) M2 is for reading 2nd byte of instruction, which is the low order address M3 is for rading 3rd byte of instruction, which is the remaining high order address M4, data are loaded from the accumulator to the data bus and address 8000H are put on the entire address bus (A15-A0) We can see the difference here. For OUT instruction, the 8-bit address of device is put on both the A15-A8 and A7-A0 address bus, and either one can be decoded to identify the output device

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MEMORY MAPPED I/O DATA TRANSFER INSTRUCTIONS

The steps for device selection and data transfer for memory-mapped I/O: 1. Decode the address bus to generate the device address pulse 2. AND the control signal with the device address pulse to generate the device select (I/O select) pulse 3. Use the device select pulse to enable the I/O port   For interfacing memory-mapped input device, we can use the similar steps, but use the instruction LDA, and the control signal will be (RD)' rather than (WR)'

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MEMORY MAPPED I/O SUMMARY 

Memory-mapped I/O is similar to Peripheral I/O in terms of concept



The differences are as follows:

Characteristics 1.Device address

Memory-mapped I/O

Peripheral I/O

16-bit

8-bit

1.Control signals for (MEMR)'/(MEMW)' Input/Output 1.Instructions available 1.Data transfer

(IOR)'/(IOW)'

STA; LDA; LDAX; STAX; MOV IN and OUT M,R: ADD M; SUB M; ANA M: etc Between any register and I/O Only between I/O and the devices Accumulator

1.Maximum number The memory map (64K) is of I/Os possible shared between I/Os and system memory

The I/O map is independent of the memory map; 256 input devices and 256 output devices can be connected 10 T-states

1.Execution speed

13 T-states (STA,LDA) 7 T-sates (MOV M,R)

1.Hardware requirements

More hardware is needed to decode 16-bit address

Less hardware is needed to decode 8-bit address

1.Other features

Arithmetic or logical operations can be directly performed with I/O DATA

Not available

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