Книга: Introduction to Microprocessors and Microcontrollers

Note

The smaller number of pins on the d.i.l. layout has limited the number of address pins to nineteen and therefore the maximum memory that can be accessed is now 2¹⁹ or 512k (really 524288 since a ‘k’ in this case is taken as 1024 as we met in Chapter 2).

Address buffers

These are tri-state buffers just like the data buffers except they are oneway devices. The microprocessor sends addresses out along the address bus but no address information can come in this way. If we need to send an address into the microprocessor then we are loading information and all information whatever it is goes into the data bus which, as we have seen, can send any data out to the external circuits or accept any data into the microprocessor.

Clock generator

As we saw in Chapter 7, a microprocessor needs a square wave signal to keep all the internal operations in step otherwise it will all end in chaos with data and instructions moving at the wrong times and getting jumbled up.

In this particular microprocessor the clock speed can be set to values between 6 MHz and 33 MHz. The clock signal originates from either a crystal or an external signal source – but never at the same time. Using an external signal can be useful where the microprocessor is used as part of a larger installation and this would allow the microprocessor timing to be governed by the surrounding circuitry. There are three nice things about increasing the frequency of a crystal: they get smaller, lighter and cheaper. The clock circuitry includes a divide-by-two circuit to allow the use of a double-frequency crystal with the above-mentioned benefits.

Оглавление книги