Книга: Introduction to Microprocessors and Microcontrollers

Power saving

The power consumed by a microprocessor is mainly the result of internal activity so the more we make it do, the more power it uses. Changing the clock speed from 6 MHz to 33 MHz nearly doubles the power requirement.

We can add the ‘sleep’ instruction to the program and this has the effect of stopping the CPU clock and the data and address buses are disconnected. To wake the CPU up, we apply a signal to one of the interrupt inputs or activate the reset circuit by holding the voltage on the reset pin low.

We can also switch off the input/output circuitry on the microprocessor putting it into a ‘system stop’ mode. The Z8X180 family has three members: Z80180, Z8S180 and the Z8L180. The S version operates with a power supply of 5 volts and the L version supply is reduced to 3.3 volts.

The effect on the power consumption of switching between normal operation and ‘system stop’ mode is shown in Table 8.1.

Table 8.1

Bus state controller

In a system, the microprocessor is connected to all the surrounding circuits by a series of connections called the system bus. Sometimes these devices may wish to send data along the system bus and the bus state controller ensures that we don’t have multiple devices trying to send data along the same connections at the same time.

Direct memory access (DMA) controller

As the name may suggest, this circuit provides direct access to the memory without using the central processing unit. The DMA can provide high-speed transfer of data from one part of the memory to another. It can also pass data directly to and from the memory and external devices. This provides significant improvements in operating speed of the whole system.

Dynamic memory refresh

Do you remember that the problem with dynamic memories was the constant refreshing that was needed to keep the RAM data intact? Well, an internal register is used to handle the refreshing process for us. It fits in the job between instructions and does not slow the microprocessor in any way and neither the programmer nor the user is aware that it is continuing in the background. Some microprocessors do not have this register and an external chip is added to the system to perform the refreshing. Either way, it has no obvious effect on the operation of the system.

Wait states

To make it easier to send information to relatively slow external devices the microprocessor is able to insert ‘wait states’ which can insert more time into its bus cycle timing. They can be programmed in by the software being used or internally by a wait state generator.

Serial inputs and outputs

To move data in or out in serial mode in which the stream of data is applied to a single input connection one bit after another we are given a choice of two options.

First is a clocked serial input/output (CSIO). This is a simple high-speed data connection to another microprocessor that is capable of sending and receiving data, though not at the same time. The transmission is synchronized to the microprocessor clock.

There are also two asynchronous serial communications interfaces (ASCI). These provide two other data connections that can be programmed to select the required speed of transmission and provide two-way transmission.

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