3. 3
Introduction
Processor process data.
memory storage
buses communication
• Communication: transfer of data among
processors and memories.
• This communication is known as interfacing.
4. 4
Basic Terminology
unidirectional (rd’/wr , enable)
Wires
bidirectional (data)
• A set of wires with the same function :
• Bus
• Protocol: rules for communicating over the wires. (low level HW protocols)
bus structure
Processor Memory
rd'/wr
enable
addr[0-11]
data[0-7]
bus
5. The primary purpose of memory interfacing is to
facilitate the transfer of data between the CPU
and memory modules effectively.
It involves managing data flow and addressing,
and timing requirements to ensure seamless
communication and reliable data storage.
The CPU uses memory addresses to access
specific locations in memory, read data, and write
data as needed.
5
6. Real-time Embedded Systems/Interfacing 6
• Memory is device used to store the information or data
• Most of the memories used are semiconductor memories because of their
small size, high speed, reliability and low cost
• It consist of mostly flip flops and some other additional circuits for
buffers
Memory capacity
The no of bits that a memory can store is called its capacity
Memory organization
• Each memory chip contains 2N memory locations, where N is the no
of address pin on the chip
• Each location contains M bits, where M is the no of data pins on the
chip
• The entire chip will contain 2N X M bits
7. Real-time Embedded Systems/Interfacing` 7
Address pins
• Address Lines: The input lines that select a memory location within the
memory device.
• Decoders are used, inside the memory chip, to select a specific location
• The number of address pins on a memory chip specifies the number of
memory locations.
8. Key concepts in memory interfacing
There are some key concepts in memory
interfacing:
Address Bus: The address bus is a set of wires
that carry memory addresses from the CPU to
the memory module.
The number of address lines determines the
maximum memory capacity that can be
addressed.
For example, a 16-bit address bus can address
up to 64KB (2^16) of memory locations.
8
9. Data Bus: The data bus is a set of wires that
enable the bidirectional transfer of data
between the CPU and memory module.
The width of the data bus determines the
amount of data that can be transferred in a
single operation.
For instance, a 32-bit data bus can transfer 32
bits (4 bytes) of data at a time.
The number of data lines (m-bits)
determines the size of each location in the
memory.
9
10. Control Signals: These signals manage the
flow of data between the CPU and memory,
indicating whether it is a read operation, write
operation or other control actions like enabling
memory access.
Memory Decoding: Memory decoding is the
process of interpreting the address lines to
select a specific memory location.
It ensures that the CPU can access the intended
memory device accurately.
10
12. Memory Types: Different memory
technologies, such as SRAM (Static RAM)
and DRAM (Dynamic RAM), have distinct
characteristics, including speed, cost, and
volatility.
The memory interfacing design should
consider these aspects based on the system’s
requirements
12
13. Memory Hierarchy: Modern computer systems
often use a memory hierarchy to optimize
performance.
This hierarchy includes multiple levels of
memory, such as cache, main memory, and
secondary storage, with varying access speeds
and capacities
Interfacing with I/O Devices: Memory
interfacing also extends to communication
between the CPU and input/output (I/O)
devices, such as hard drives, solid-state drives,
and peripherals
13
14. What is I/O Interfacing?
Input/Output (I/O) interfacing is a critical aspect
of computer system design that deals with the
communication between the central processing
unit (CPU) and various peripheral devices.
I/O devices include external devices such as
keyboards, mice, monitors, printers, hard
drives, USB drives, network interfaces, and
more.
The I/O interface facilitates the exchange of
data between the CPU and these devices,
enabling users to interact with the computer
and transfer data to and from the system.
14
16. 16
Microprocessor interfacing: I/O addressing
The microprocessor's pins used to communicate data to
and from it, are called I/O pins.
We normally consider the access to peripherals (not
memory), as I/O.
Two common methods for using pins to support I/O : Port-
based I/O (Parallel I/O), and Bus-based I/O.
In parallel I/O , a port can be directly read and written by
processor instructions, like any register.
Ex.P0=255, g=P2 .
Ports are often bit-addressable.
In bus-based I/O, the microprocessor has a set of address,
data, and control ports corresponding to bus lines, and uses
the bus to access memory and peripherals.
17. 17
Extensions
Parallel I/O peripheral.
When processor only supports bus-based I/O
but parallel I/O needed.
Each port on peripheral connected to a register within
peripheral. The microprocessor can read/write those
registers.
Extended parallel I/O.
When processor supports port-based I/O
but more ports needed.
One or more processor ports interface with
parallel I/O peripheral extending total number
of ports available for I/O.
e.g., extending 4 ports to 6 ports in figure.
Processor Memory
Parallel I/O peripheral
Port A
System bus
Port C
Port B
Adding parallel I/O to a bus-
based I/O processor
Processor
Parallel I/O peripheral
Port A Port B Port C
Port 0
Port 1
Port 2
Port 3
Extended parallel I/O
18. 18
Memory-Mapped I/O and Standard I/O
They are two bus-based methods for microprocessor to communicate with
peripherals.
In memory-mapped I/O, peripherals occupy specific addresses in the existing
address space.
e.g., Bus has 16-bit address, lower 32K addresses may correspond to
memory, and upper 32k addresses may correspond to peripherals.
In standard I/O (I/O-mapped I/O), the bus includes an additional pin (M`/IO),
to include whether the access is to memory or peripheral.
e.g., Bus has 16-bit address, all of them for memory addressing if
M`/IO=0, and all of them for I/O addressing if M`/IO=1.
19. 19
Memory-Mapped I/O Vs. Standard I/O
Memory-mapped I/O
Requires no special instructions
Assembly instructions involving memory like MOV and ADD
work with peripherals as well.
Standard I/O requires special instructions (e.g., IN, OUT) to
move data between peripheral registers and memory.
Standard I/O
No loss of memory addresses to peripherals.
Simpler address decoding logic in peripherals possible.
When number of peripherals much smaller than address
space then high-order address bits can be ignored
smaller and/or faster comparators.
20. 20
A basic memory protocol
Interfacing an 8051 to external memory
Ports P0 and P2 support port-based I/O when 8051 internal
memory being used.
Those ports serve as data/address buses when external memory
is being used.
16-bit address and 8-bit data are time multiplexed; low 8-bits of
address must therefore be latched with aid of ALE signal.
8051
74373
P0
HM6264
D Q
8
P2
ALE G
A<0...15>
D<0...7>
/OE
/WE
/CS
/WR
/RD
/CS1
/PSEN
CS2
27C256
/CS
A<0...14>
D<0...7>
/OE
P0
P2
Q
ALE
/RD
Adr. 7..0
Adr. 15…8
Adr. 7…0
Data
21. Interrupts (interrupt driven I/O)
Servicing: read & process data from peripheral
whenever it has new data.
Unpredictable
Polling: MP repeatedly check for data
Simple to implement
Waste many clock cycles
External interrupts
Repeatedly MP checks Int pin after executing
instruction, if asserted => jump to ISR.
Maskable vs Nonmaskable Interrupt
Internal Interrupt (divide by 0,…)
Software Interrupt .
21
22. Interrupt Addressing
Fixed Int.: ISR address built in MP
Vectored Int.:
ISR address requested from peripherals by Interrupt
pin asserted by MP.
The address stored in peripheral by extra register.
Interrupt Address Table (compromise between
fixed & vectored)
Table holds ISR addresses
Peripherals provide entry number instead.
22
23. Watch dog timer
A watchdog timer can be thought of as having the
inverse functionality than that of a regular timer. We
configure a watchdog timer with a real-time value, just as
with a regular timer.
However, instead of the timer generating a signal for us
every X time units, we must generate a signal for the
timer every X time units.
If we fail to generate this signal in time, then the timer
generates a signal indicating that we failed. We often
connect this signal to the reset or interrupt signal of a
general-purpose processor.
23
24. Watch dog timer
A watchdog timer provides a mechanism of ensuring that our
software is working properly; every so often in the software, we
include a statement that generates a signal to the watchdog timer (in
particular, that resets the timer).
If something undesired happens in the software (e.g., we enter an
undesired infinite loop, we wait for an input signal that never arrives,
a part fails, etc.), the watchdog generates a signal that we can use
to restart or test parts of the system.
Using an interrupt service routine, we may record information as
to the number of failures and the causes of each, so that a service
technician may later evaluate this information to determine if a
particular part requires replacement.
Note that an embedded system often must recover from failures
whenever possible, as the user may not have the means to reboot
the system in the same manner that he/she might reboot a desktop
system. 24
26. UART
A UART (Universal Asynchronous Receiver/Transmitter) receives
serial data and stores it as parallel data (usually one byte), and
takes parallel data and transmits it as serial data.
Need to communicate bytes of data between devices separated by
long distances.
A set the transmission and reception rate, called the baud rate,
which indicates the frequency that the signal changes.
Common rates include 2400, 4800, 9600, and 19.2k.
An extra bit may be added to each data word, called parity, to detect
transmission errors -- the parity bit is set to high or low to indicate if
the word has an even or odd number of bits.
26
28. UART
Internally, a simple UART may possess
a baud-rate configuration register, and
two independently operating processors,
one for receiving and the other for transmitting.
The transmitter may possess a register, often called a transmit
buffer, that holds data to be sent. This register is a shift register, so
the data can be transmitted one bit at a time by shifting at the
appropriate rate. Likewise, the receiver receives data into a shift
register, and then this data can be read in parallel.
28
29. UART
To use a UART, we must configure its baud rate by writing to the
configuration register, and then we must write data to the transmit
register and/or read data from the received register.
Unfortunately, configuring the baud rate is usually not as simple as
writing the desired rate (e.g., 4800) to a register.
For example, to configure the UART of an 8051, we must use the
following equation:
smod corresponds to 2 bits in a special-function register,
oscfreq is the frequency of the oscillator, and
TH1 is an 8-bit rate register of a built-in timer.
29
33. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
HOW UART WORKS
In UART, data is transmitted in parallel from data bus to
transmitting UART
The transmitting UART adds a start bit, stop bit and a parity bit
to parallel data from data bus
the data packet is transmitted serially from Tx to RX pin
The Rx pin reads the data and converts parallelly by removing
other bits
37. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
UART IN ARDUINO
UART allows the Atmega chip to do serial communication
while working on other tasks, through 64 byte serial buffer.
All Arduino boards have at least one serial port (also known as
a UART or USART): Serial.
Serial is used for communication between the Arduino board
and a computer or other devices.
It communicates on digital pins 0 (RX) and 1 (TX) as well as
with the computer via USB. Thus, if you use these functions,
you cannot also use pins 0 and 1 for digital input or output.
38. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
Arduino functions for USB serial communication
• if (Serial) – indicates whether or not the USB serial connection is open
• Serial.available()-Get the number of bytes available for reading from the serial port.
• Serial.begin()-Sets the data rate in bits per second (baud) for serial data transmission.
• Serial.end()-Disables serial communication, allowing the RX,TX to be used for input & output.
• Serial.find()-reads data from the serial buffer until the target is found.
• Serial.println()-Prints data as ASCII text followed by a carriage return and newline character.
• Serial.read()-Reads incoming serial data.
• Serial.readString() - reads characters from the serial buffer into a String.
• Serial.write() - Writes binary data to serial port. This data is sent as a byte or series of bytes
For other functions refer the URL:
https://www.arduino.cc/en/pmwiki.php?n=Reference/serial#:~:text=Serial%20is%20used%20for%20communication,with%20the%20computer%20
via%20USB.
39. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
EXAMPLE-1: Print data received through serial communication on to the serial monitor
of Arduino
void setup() {
Serial.begin(9600); //set up serial library baud rate to 9600
}
void loop() {
if(Serial.available()) //if number of bytes (characters) available for reading
from serial port
{
Serial.print("I received:"); //print I received
Serial.write(Serial.read()); //send what you read
}
}
40. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
EXAMPLE-1: Arduino code for serial interface to blink switch ON LED when “a” is
received on serial port
int inByte; // Stores incoming command
void setup() {
Serial.begin(9600);
pinMode(13, OUTPUT); // Led pin
Serial.println("Ready"); // Ready to receive commands
}
void loop() {
if(Serial.available() > 0) { // A byte is ready to receive
inByte = Serial.read();
if(inByte == 'a') { // byte is 'a'
digitalWrite(13, HIGH);
Serial.println("LED - On");
}
else { // byte isn't 'a'
digitalWrite(13, LOW);
Serial.println("LED - off");
} } }
This function can be
used
if(Serial.find(“a"))
41. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
Arduino Software Serial Library
The SoftwareSerial library has been developed to allow serial
communication on other digital pins of the Arduino
Uses software to replicate the functionality of the hardwired RX and TX
lines hence the name "SoftwareSerial".
It is possible to have multiple software serial ports with speeds up to
115200 bps.
This can be extremely helpful when the need arises to communicate with
two or more serial enabled devices
Limitations:
maximum RX speed is 57600bps and RX doesn't work on Pin 13
If using multiple software serial ports, only one receive data at a time.
42. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
Arduino functions for UART communication (SoftwareSerial Library)
SoftwareSerial() – Need to enable serial communication
avaialble() – gets the no of bits available for reading from serial port
begin() – sets the speed of serial communication
overflow() – checks the serial buffer overflow has occurred
peek() – returns the character received in the serial port
read() - returns the character that was received on the Rx pin of serial port
Print() – works same as serial.print
Listen() – enables the selected serial port to listen
Write() – print data to transmit pin of software serial
Refer. URL:
https://www.arduino.cc/en/Reference/SoftwareSerial
43. UNIVERSAL ASYNCHRONOUS
RECEIVER/TRANSMITTER
EXAMPLE-3: Arduino code to Receives from the hardware serial, sends to software
serial and Receives from software serial, sends to hardware serial.
#include <SoftwareSerial.h>
SoftwareSerial mySerial(10, 11); // RX, TX
void setup() {
Serial.begin(57600); // Open serial communications and wait for port to open:
while (!Serial) {
; // wait for serial port to connect. Needed for native USB port only
}
mySerial.begin(4800); // set the data rate for the SoftwareSerial port
mySerial.println("Hello, world?");
}
void loop() { // run over and over
if (mySerial.available()) {
Serial.write(mySerial.read());
}
if (Serial.available()) {
mySerial.write(Serial.read());
} }
45. Reference
“Embedded system Design: A unified
Hardware/Software Introduction”, Frank Vahid,
Tony Givarrgis, Wiley, 2002
Real-time Embedded Systems/Interfacing 45
