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Microcontrollers

Unit 1 - Introduction to Microcontrollers

Unit Information

• 1.1 Introduction to the microcontroller part of the course
• 1.2 What is a microprocessor?
• A Brief History of the Microprocessor (via link)
• History of the Microchip PIC (via link)
  • 1.2.1 Types of memory
  • 1.2.2 RandomAccess Memory (RAM)
  • 1.2.3 The Chip Select Line
• 1.3 What is I/O?
• 1.3.1 Digital I/O
• 1.3.2 Output Port
• 1.3.3 Input Port
• 1.4 What is a microcontroller?
• Summary of the first 20 years of Microprocessor/controller Evolution (via link)
• List of current popular microcontroller manufacturers (via link)
• Study task 1
• 1.4.1 Memory in a microcontroller
• 1.4.2  The I/O Ports
• Study task 2
• Self Assessment
• 1.5 Conclusion

1.1 Introduction to the microcontroller part of the course

The microcontroller is a very common component in modern electronic systems. Its use is so widespread that it is almost impossible to work in electronics without coming across it.

Microcontrollers are used in a wide number of electronic systems such as:

• Engine management systems in automobiles.
• Keyboard of a PC.
• Electronic measurement instruments (such as digital multimeters, frequency synthesisers, and oscilloscopes)
• Printers.
• Mobile phones.
• Televisions, radios, CD players, tape recording equipment.
• Hearing aids.
• Security alarm systems, fire alarm systems, and building services systems.

One could go on adding to this list, but the reader should now be getting the impression that there is a very wide scope for use of microcontrollers.

The next few sections will attempt to answer the following questions:

1. What is a microprocessor?
2. What is a microcontroller?
3. What is the difference between a microprocessor and a microcontroller?

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1.2 What is a microprocessor?

A microprocessor is a programmable digital electronic component that incorporates the functions of a central processing unit (CPU) into a single IC package. These functions are:

• The ability to execute a stored set of instructions to carry out user defined tasks.
• The ability to be able to access external memory chips to both read and write data from and to the memory.

Microprocessors have been around since the start of the 1970's.  For a brief history about the initial development of the microprocessor :- please click here.

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1.2.1 Types of Memory can be obtained as either:

• Read Only Memory (ROM). This is memory that can only be read, the data being stored in the memory device during its manufacture. Once data has been written onto ROM memory, it cannot be easily removed and is designed for 'read only' use. ROM is referred to as being non-volatile as it retains its contents even when the power is turned off.
• Erasable Programmable Read Only Memory (EPROM). This is similar to ROM type memory but the user can program it. The contents of the memory can be erased from the memory by exposing the memory chip to ultraviolet radiation for a short period of time. It can therefore be used many times over.
• Electrically Erasable Programmable Read Only Memory (EEPROM). Similar to EPROM but has part or all of the memory contents erased by the microprocessor.

Both ROM and EPROM memory are used to hold the program code of a microprocessor used in an embedded system, ie. a microprocessor used in an application where the program code is always the same and is designed to execute every time the system is switched on. Most development work is done using EPROM or EEPROM type memory, ROM memory being used in the final production version (when all the program code has been fully tested).

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1.2.2 Random Access Memory (RAM)

All microprocessor systems need memory that can be both read from and written to - such memory is RAM. RAM got its name because early read-write memories were sequential, and did not allow random access. RAM memory is used to store dynamic data (that will change during the operation of the program). RAM takes the form of integrated circuits that allow the stored data to be accessed in ANY order — that is, at random and without the physical movement of the storage medium or a physical reading head. The word "random" infers that any piece of data can be returned quickly, and in a constant time, regardless of its actual physical location, in relation to the previous data storage location.
The key benefit of RAM is that retrieval times are short and consistent. The disadvantages of RAM are cost, and the loss of data when power is turned off (volatile).

So a typical microprocessor system will contain both ROM (could be EPROM, EEPROM, or ROM) to store the program code, and RAM to store dynamic data.

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1.2.3 The Chip Select Line

When several devices are connected to the same set of input wires (e.g, via a bus), and it is required to send data or commands to each device independently of the others on the bus, we use a chip select line.

When the chip select pin is held in the inactive state, the the chip or device pays no heed to changes in the state of its input pins. When the chip select pin is held in the active state, the chip or device assumes that any input changes it "hears" are meant for it, and responds as if it is the only device on the bus.

In summary, the chip select is an input-enable switch. "ON" means the chip or device responds to changes on its input pins, while "OFF" tells the chip or device to ignore everything.

Figure 1, below, illustrates the basic connections between the microprocessor and external memory.

Fig 1 : Basic connections between the microprocessor and its memory devices

In figure 2, we see the addition of address decoding logic to identify a particular range of address values to activate the chip select signal. This way, the memory device can only be selected within a particular range of addresses. The chip select line allows the memory to be either selected (/CS = 0, sometimes referred to as enabled) or not selected (/CS = 1, sometimes referred to as disabled).

Fig 2 Address decoding logic is added to generate a "Chip Select" signal for the memory device

The address decoding logic is simply a decoder that can be used to decode the particular combination of address inputs and activate one (of a number of) chip select outputs. See TTL data book for possible devices such as the 74138 and the 74139.

Example of Address Decoding Logic

Derive logic to select any addresses in the range A000 hex to AFFF hex.  The bit pattern is illustrated here:

In the example shown in figure 3, the chip select signal is active low when the input signal (A15, A14, A13, A12) is 1010. At all other times the chip select signal is logic 1 (disabled).

Fig 3  Example of chip select signal

The address decoding logic can be used to generate a chip select signal for a number of memory devices. Each chip select output signal responds to a different address value, as in figure 4.

Fig 4 Three different Chip Select signals

The address decoding logic is usually used to select more than one memory chip since most microprocessor systems use a number of memory chips to make up a complete system. Only one chip select line will be enabled at any one time. This means that the microprocessor sees only one memory device connected to the data bus at any one time, i.e. when the particular memory device is selected.

When no memory device is selected (as will be the case when the microprocessor is performing internal operations when memory access is not required), the data bus is placed into a mode called tri-state. This tri-state mode places an open circuit condition between the microprocessor and memory, thus disconnecting the data bus between memory and microprocessor. In fact, when the address decoding logic selects a particular memory device to be connected to the microprocessor, the other memory devices in the system are disconnected from the microprocessor. In this way, only one memory device can be connected to the microprocessor at any one time, the one selected by the address decoding logic.
 

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1.3 What is I/O? I/O is input or output (Input/Output)

I/O (input/output) is the collection of interfaces that different functional devices, of any information processing system, use to communicate with each other. Every information transfer is an output from one device and an input into another. For instance, on a computer, a keyboard and mouse are considered input devices while monitors and printers are considered output devices. Typical devices for communication between computers, such as modems and network cards, operate as both input and output devices.

I/O can be:

• A number of digital bits formed into a number of digital inputs or outputs called a port. These are usually eight bits wide and thus referred to as a BYTE wide port. ie. byte wide input port, byte wide output port.
• A serial line from the microprocessor (Transmit or TX) and a serial line to the microprocessor (Receive or RX) allowing serial data in the form of a bit stream to be transmitted or received via a two wire interface.
• Other I/O devices such as Analogue to Digital Converters (ADC) and Digital to Analogue Converters (DAC), Timer modules, Interrupt controllers etc. (which will be discussed later in the context of microcontrollers).

These are relatively complex sub systems that can be obtained as separate ICs. They are connected to the microprocessor in a similar manner to that of the memory devices.  Indeed, they often contain their own memory to support internal operations (ie. registers).

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1.3.1 Digital I/O

A digital I/O port can be realised using a number of D type Flip-Flops.

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1.3.2 Output Port

Figure 5 illustrates a four bit output port. The inputs are connected to the data bus whilst the outputs are connected to whatever output interface is to be controlled.

Fig 5 A 4-bit output port

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1.3.3 Input Port

The input port, shown in figure 6, allows outside world inputs (left hand side of figure 6) to be stored in the data latches so they can be read by the microprocessor via the data bus (right hand side of figure 6).

Fig 6 A 4-bit input port

The data bus connections must be via tri-state buffers so that the input port is only connected to the data bus when the input port is selected. This is achieved by connecting a chip select signal to the enable input signal line. Note that the tri-state enable is active low. We have covered some of the basic concepts of the microprocessor.  There is of course much more but we can pick up on these later.  We now need to turn our attention to the question what is a microcontroller?
 

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1.4 What is a microcontroller?

Basically, a microcontroller is a device which integrates a number of the components of a microprocessor system onto a single chip (IC), with the following common features:

• The CPU core - ranging from simple 4-bit processors to sophisticated 32/64-bit processors
• Memory (both ROM and RAM)
• Some parallel digital I/O

Figure 7 shows an un-expanded microcontroller, which contains a number of commonly used sub-units.

Fig 7 Main components of a microcontroller

Most microcontrollers will also combine other devices such as:

• A Timer module to allow the microcontroller to perform tasks for certain time periods.
• A serial I/O port to allow data to flow between the microcontroller and other devices such as a PC or another microcontroller.
• An ADC to allow the microcontroller to accept analogue input data for processing.

Figure 8 shows a single chip microcontroller and CPU. The CPU is the processing module of the microcontroller.

Fig 8 A single chip microcontroller

The above figure illustrates a typical microcontroller device and the different sub units integrated onto the microcontroller microchip.

Microcontrollers are nowadays brimming with added functionality on this single IC and often include:

• Communications Peripherals: SPI, I2C™, UART, CAN, USB, Ethernet, IrDA®, LIN.
• Control Peripherals: capture/compare, counters, real-time clock & calendar, motor control & PWM
• Integrated Display Drivers: LED, LCD.
• On-chip internal oscillators and Phase-Locked Loop (closed loop feedback control).
• Analog Peripherals: A/D Converters, comparators, op amps, brown-out detection & reset, low voltage detection, temperature sensors, D/A Converters and voltage regulators.

Before we dive into modern day complexities, lets look at the heart of the microcontroller - the CPU core.  In the past this has traditionally been based on an 8-bit microprocessor unit.  For example, Motorola uses a basic 6800 microprocessor core in their 6805/6808 microcontroller devices.

[link to Summary of the first 20 years of Microprocessor/controller Evolution]

In recent years, microcontrollers have been developed around specifically designed CPU cores for example, the Microchip PIC range of microcontrollers.

[link of current popular microcontroller manufacturers]

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1.4.1 Memory in a microcontroller

The amount of memory contained within a microcontroller varies between different microcontrollers.  Some may not even have any integrated memory (eg. Hitachi 6503, now discontinued).  However, most modern microcontrollers will have integrated memory. The memory will be divided up into ROM and RAM, with typically more ROM than RAM.

Figure 9 shows the ROM of a microcontroller which is used to store the application program.

Fig 9 Microcontroller ROM

Typically, the amount of ROM type memory will vary between around 512 bytes and 4096 bytes, although some 16 bit microcontrollers such as the Hitachi H8/3048 can have as much as 128 Kbytes of ROM type memory.

ROM type memory, as has already been mentioned, is used to store the program code. ROM memory can be either ROM (as in One Time Programmable memory), EPROM, EEPROM, or FLASH.

Figure 10 shows the microcontroller RAM which is used to store data.

Fig 10 Microcontroller RAM

The amount of RAM memory is usually somewhat smaller, typically ranging between 1 Kbytes to 64 Kbytes.

RAM is used for data storage and stack management tasks.  It is also used for register stacks (as in the microchip PIC range of microcontrollers).
 

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1.4.2  The I/O Ports

Figure 11 shows microcontroller ports which are used to access the outside world.

Fig 11 Microcontroller ports

The digital I/O ports are the means by which the microcontroller interfaces to the environment.

Digital I/O tends to be grouped into byte wide ports (8 digital bits) that can be configured as either input bits or output bits.  There are some exceptions, such as the microchip PIC 16C54 with one 6-bit RA port and a byte wide RB port.

The number of I/O port bits varies, depending upon the size of the microcontroller.  Some very simple 8 bit microcontroller have as few as 4 bits of I/O, whilst those at the high end range can have as many as 33 bits of I/O (some 16 bit microcontrollers could have around 78 bits of I/O).

A typical interface might look like the one illustrated below in figure 12.

Fig 12 A typical interfacing example

Microcontroller ports can be used to operate LEDs and relays, as well as input the state of switches and logic circuit inputs (not shown in this figure).  Not all microcontroller port outputs are able to drive an LED directly; some need to be interfaced via a buffer such as a 7406 open collector inverting buffer.  In the example above, the relay is driven by such a buffer.

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1.5 Conclusion

From your first browse of the above web sites you should now have a "feel" for the range of microcontroller products currently available.  It is clear that these devices can be applied to many embedded system designs from the simple hardware control applications up to Signal Processing applications. The choice and selection of devices available today is vast.

Note: Intel also produce a range of microcontroller devices.  The Intel 8-bit microcontroller family can be browsed at:   http://intel.com/design/mcs51   You don't need to look at this site in much detail just yet, but simply to be aware of it and keep the website URL handy.

Also see the list of current popular microcontroller manufacturers.
 

[On to the start of Module 2] 

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updated 24.11.10 KS

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