SINGLE CHIP MICROCOMPUTER

So far we have studied LSI/VLSI chips that provide a CPU, memory, general purpose interfaces. We can build a microcomputer using these chips with the help of additional SSI/MSI components that include latches, drivers, decoders, etc. which is normally termed as Glue logic. This is a microcomputer built using separate components (CPU, Memory, etc.). For some specific applications, we also have single chip computers in a VLSI chip. This single chip microcomputer will have a CPU, memory and I/O interfaces, timers, ADC/DACs etc. on a single chip itself. This component is called a Single Chip Microcontroller. Nowadays it is also being referred to as an Embedded System or to some extent it could also be called at System-on-Chip (SoC).  The term SoC is generally at the high-end of the single chip microcontroller. This could be used for specific purposes. Since memory and I/O interfaces are available on the chip alongwith the CPU itself, there will be some constraints on the amount of memory and interfaces that could be provided on the chip itself. As we will see later, the currently available microcontrollers will have limited amount of memory in terms of ROM, EPROM, Flash ROM/RAM and I/O ports.  If this chip could be used as it is for a specific application it will be highly beneficial and system built around a single chip microcontroller would be highly reliable.  One can go to a ROM based microcontrollers if the production volume is large. For prototyping and limited applications, one can go for EPROM or Flash ROM based single chip microcontrollers.  

The manufacturers of Single chip  microcontrollers will provide facility to have external memory, if required but one may have to sacrifice some I/O ports to have this extra memory requirement. Now we will study the Single chip microcontrollers in details.

Table : Examples of Typical microcomputer applications

These applications require special electronic devices that performs functions which are very dedicated for their purpose. It doesn't require operating system, only application programme which one can write and put in the memory. Nowadays we are having another word called Embedded where the hardware part is a microcontroller or it can be a Application Specific Integrated Circuit (ASIC).

Microcontrollers (Embedded Systems)

CPU + Interface

CPU + ROM + RAM + Interfaces + Timers + A/D, D/A Converter

For prototyping ROM is replaced by EPROM. ROM is essentially used for programing code.

CPU + EPROM + RAM + Interfaces + Timers + A/D, D/A Converters

CPU + FLASH ROM + RAM + Interfaces + Timers + A/D, D/A Converters

Examples : 8048, 8748, 8051, 8751, i960, 6801, 68701, 68HC11, etc.

On chip ROM, RAM (Code and data memory), interfaces are very limited. (Examples - ROM, EPROM, 4 KB, RAM 256 B)

Reliability + Suitable for specific application without additional external hardware.

Also provision is available to expand memory outside the chip.

For Examples of Motorola and Intel's Microcontroller chips, see table

AT91 MCU Products

PDC - Peripheral Data Controller
MPI - Multi Processor Interface
WD - Watchdog
PMC - Power Management Controller

Now we will take a small case study of Motorola's 6801. Following are the essential features of this Microcontroller.

• Expanded M6800 instruction set
• 8 x 8 Multiply instruction
• Serial communication interface
• On-Board UART
• upward source and Object code compatible with MC6800
• 16-Bit Three function programmable Timer
• Single chip or expanded operation to 64K address space
• Buse compatibilyt with MC6800 family
• 2048 bytes of ROM
• 128 bytes of RAM (64 bytes retainable during powerdown mode)
• 31 parallel I/O lines and two handshake control lines
• Internal Clock/Divided-by-Four mask option (MC6801)
• External Clock/Divided-by-One mask option (MC6801E)
• TTL compatible inputs and outputs
• Interrupt capability

Pin Assignment Diagram

31 - Port lines

1 - Vcc

1 - Ground

1 - Standby

1 - Clock In and Clock Out

2 - Crystals

2 - Interrupts

1 - Reset

2 - External

Internal Organisation of the Chip is given in the Figure below:

MC6801 - Single chip Microcomputer

It is recommended to be used in the Single chip mode and connection for the single chip mode is given in the diagram.

Fig - Single Chip Mode

To use in the expanded mode, it is given in the diagram given below:

Fig - MC6801 Microcomputer Family Block Diagram

If used in the multiplexed mode, ports will be used as address lines. So you lose two ports and left with one port only. If you used the non-multiplexed mode, you loose something and port are used as address lines. This is the mode where you loose the flexibility of the ports.

Table

* The MC 6803 operated only in modes 2 and 3

Fig - Typical mode programming circuits.

Different types of modes are:

1. Main mode is single chip mode : In single chip mode, all the ports are available and you can use them freely as you want.
2. Non-Multiplexing
3. Multiplexed Add : In this mode, Port 3 gives lower order address and Port 4 gives higher order.

In single chip mode, port 2 has two more signals - SC1 and SC2 and they are used for normal handshake transfer.

Port 3 Control & Status Register

Latch Enable	Controls the input Latch of Port 3 when set, /IS3 negative edge latches the data. Cleared during reset.
OSS	Output strobe select. When clear strobe is generated by READ and when set strobe is generated by a write. Cleared on reset.
IS3 IRQE	Enables interrupt when IS3 flag is set.
IS3 Flag	Only Read flag. Set by of IS3. Cleared by READ of this register.

SC1 functions as /IS3 and can be used to indicate that Port 2 input data is ready or output data has been accepted.

SC2 is configured as /OS3 and can be used to strobe output data or acknowledge input data.

SC1, SC2 in Single chip mode:

SC1 and SC2 is Expanded non-multiplexed mode:

SC1 is IOS input/output select. It can be used as CS for the external memory in the address range 0100-01FF.

SC2 is R/W

Expanded Multiplexed mode:

SC1 - Address Strobe (AS)

SC2 - R/W

In Multiplexed mode, full 16 bit address can be given where as in Non-multiplexed mode, external memory has register of 8 bits because only port 4 provides the address.

We will now take an application where Motorola 6801 can be used. (See figure below)

Figure  - Block Diagram of Programmable Time

We have a free running counter, output compare register and input capture register.