15. Microcontrollers and how to use them
In the 1960s, electronics started to awake from its slumber that had used thermionic valve technology that was recognizably similar to circuits that had been built for thirty years. The pace of progress was gentle. The first semiconductor material was developed and the transistor came into use in just a few years. The photographic process used to design and produce the transistor quickly led to simple integrated circuits and the microprocessor.
No sooner had the microprocessor and the associated memories arrived in 1971 than it became obvious that the microprocessor was always accompanied by other circuits, like input/output devices, memory and timing circuits so it would be a good move to combine them into a single device.
We had a choice – we could keep everything general and universal and call it a microprocessor or design it for a single purpose and call it a microcontroller.
The multipurpose devices went into computers and even here we had a choice. Computers were either ‘microcomputers’ where price was a significant feature and these microprocessors had some built-in ROM and RAM. Soon, however, speed became the main feature as the prices began to fall and we could afford to equip our homes with computing power which equalled many offices of just a few years previously and the microprocessors became expensive and fast. Speed headlines drive the publicity machines as home and office computers became faster and faster. They sold in their millions. Meanwhile the single-purpose devices, really the descendents of the early microcomputers, were developed further and made really cheaply, sold by the billion and were never mentioned. They power the pocket calculator, video recorders, cameras, microwaves, washing machines and greetings cards that play music – in fact almost anything vaguely electronic.
Just a thought
The microcontrollers outnumber the population of the world many times and as mentioned earlier we are likely to be sharing our homes with, possibly, fifty of them. They are in every essential industry – food production, transport, communications, research, weaponry, power generation, medicine, heating and air conditioning – there is little that we rely on that does not use a microcontroller. If they learn to communicate independently of us, they may develop their own agenda. Now there’s a thought.
Once we have learned to drive, most vehicles are easily recognized as being very similar. We are happy with the general idea and can concentrate on the minor differences. Microcontrollers are much the same. Having already become familiar with the basic building blocks of the simple microprocessor in Chapter 8, we can move very easily into the microcontroller. It is not surprising then to find that all microcontrollers are basically very similar.
To give an overall impression of the range of microcontrollers available we are going to look at three popular ranges. The first is the 8051, probably the most widely used microcontroller, over twenty years old and continuously developed by many different companies and showing no signs of fading away. The next is from the AVR family produced by the Atmel Corporation, one of the leaders in this field. From this range we look at the AT90S/LS2343 one that is small, modern and RISC. The final one will be explored in Chapter 16.
Probably the transition between the microcomputer to the microcontroller occurred with the Intel 8048 as we saw in Chapter 11. The 8048 added on-chip RAM, ROM and a timer so it could be used as a single purpose device such as controlling a keyboard – it was, in fact, a microcontroller.
With the experience gained by using this, it became apparent that there was a significant market for a microcontroller. In 1980, Intel launched the 8051 which, twenty-three years later, is alive and well. In fact very well indeed. It is probably the most popular microcontroller ever. It is made by about 44 suppliers. These suppliers have often added some extra features to make versions or ‘variants’ as they are called particularly suitable for specific jobs. There are at least 92 variants all compatible with the original code. Even within variants, there are a series of options that lifts the total number of members of the 8051 family to several hundred.
Numbering
The device numbering is not very obvious as many microcontrollers are available from several different suppliers with their own product code. They then produce a group of basically similar devices with minor changes like different operating voltages or differing amount of RAM and ROM on-board memory – these groups are referred to as ‘families’.
The family is given a name which often has little connection with the product codes. For example, Intel’s 8051 family has the family name of MCS51. This contains the 803X, 805X, 875X and the low power versions bXC45X and the 8XCX52. As usual the X refers to any figure or letter in that position.
The situation is further confused (or possibly simplified) by referring to all of them as ‘the’ 8051.
The block diagram of the 8051
The block diagram shown in Figure 15.1 is the family portrait of the 8051 family. There are some features that differ between the family members, principally the memory configuration – some versions have less memory and some have none at all. However, we will look at the operation of our ‘middle-of-the-road’ version and worry about the individual differences later.
Figure 15.1 8051 block diagram
The 8051 pinout
As in all micro and digital chips, a line over a pin designation indicates that it is active low or, put more simply, to use this feature we need to apply zero volts.
The pinout shown in Figure 15.2 looks, at first glance to be rather complicated due to the dual use of many pins. This is a common feature of microcontrollers as a method of reducing the number of pins to be used. The more pins, the more expensive and the larger the device. This is bad news for a device often destined to be embedded within another circuit.
Figure 15.2 Pinout of the 8051
There are variants available that provide the increased number of pins so that there is a separate pin for each function.
Reset
Regardless of what program is being run, we must always be able to gain control of the microcontroller just as we must with a microprocessor. The procedure is just the same. The microcontroller has a reset pin which, in the 8051, is taken from the bus control block and, in normal operation must be held to zero volts. When a positive voltage over 2.0 V is applied to it the microcontroller immediately returns to its startup memory location, which in this case is 0000H. We can arrange this to occur automatically when the power is switched on but we should also provide a reset switch to gain control of the system at any time without removing the power. This is the ‘reset’ switch which we use when our computer locks up and ignores us.
When changing microcontrollers, remember to check the polarity of the reset voltage. Compare this circuit in Figure 15.3 with the one shown in Figure 8.7.
Figure 15.3 The reset switch
Clock input
As we have seen in Chapter 7, we are going to need a clock signal. The original design of the 8051 called for a 12 MHz crystal though later versions can run at 33 MHz, 40 MHz or even 44 MHz. As an alternative, we can use a ceramic resonator or an external signal. The clock input is shown in Figure 15.4 using a crystal. To use an external signal throw away the crystal and the capacitors then apply the external signal to the Xtal1 pin and leave Xtal2 disconnected.