Page 148 - Understanding Automotive Electronics

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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

particular automotive system. Standard commercial I/O devices are available
from the manufacturers of each microprocessor that are specifically configured
to work with that processor. These I/O devices are implemented as an IC chip
and are very versatile in application. A typical such I/O device has two 8-bit
data ports for connecting to peripheral devices, and an 8-bit port that is
connected to the data bus of the computer.
Figure 4.22 is a block diagram of a typical commercial I/O device. In this
device there are two ports labeled A and B, respectively. These ports can be
configured to act as either input or output, depending on the data in the data
direction register. Normally the correct code for determining direction is
transferred to the I/O device from the microprocessor via the system data bus.
Whenever the microprocessor is either to transfer data to the I/O device
or receive data from it, a specific address is generated by the processor. This
address is decoded, using standard logic, to form an electrical signal that
activates the chip select inputs to the I/O. In addition, the read/write (R/W)
output of the microprocessor is activated, causing data to be received (read)
from a peripheral device, or transmitted (write) to a peripheral device.
This use of address lines to activate the I/O is known as memory-mapped I/O.
In memory-mapped I/O, input or output of data is selected by reading from the I/O
input address or writing to the I/O output address.

INSTRUMENTATION APPLICATIONS OF MICROCOMPUTERS
Microcomputers can In instrumentation applications of microcomputers, the signal processing
convert the nonlinear operations are performed numerically under program control. The block
output voltage of some diagram of a typical computer-based instrument is depicted in Figure 4.23. In
sensors into a linear volt- this example instrument, an analog sensor provides a continuous-time voltage,
age representation. The V , that is proportional to the quantity being measured. The continuous-time
o
sensor output voltage is voltage is sampled at times determined by the computer. The sampled analog
used to look up the cor- voltage is then converted to digital format (e.g., 8 to 16 bits) using an ADC.
responding linear value The digital data is connected to port A of the I/O device of the computer to be
stored in a table. read into memory.
The ADC generates a signal when the conversion from analog to
digital is completed. This signal is normally termed end of conversion (EOC).
The EOC signal provides an interrupt signaling the computer that data is
ready.
The signal processing to be performed is expressed as a set of operations
that is to be performed by the microprocessor on the data. These operations
are termed the algorithm for the signal processing operation. The algorithm is
converted to a set of specific computer operations that becomes the program
for the signal processing. After the signal processing is completed, the result is
ready to be sent to the display device. The digital data is sent through I/O to
port B to the DAC. There it is converted back to sampled analog. The

UNDERSTANDING AUTOMOTIVE ELECTRONICS 135