Page 155 - Understanding Automotive Electronics

2735 | CH 4 Page 142 Tuesday, March 10, 1998 11:06 AM

4 MICROCOMPUTER INSTRUMENTATION AND CONTROL

Figure 4.25
Illustration of Table
Lookup and
Interpolation

FPO

tabulated values, the value for X corresponding to V is found by linear
o
interpolation:

(
X = X 1 + ( X 2 – X 1 ) V o – V 2 )/ V 1 – V 2 )
(

A microcomputer can perform the same operation using tabulated values
for the relationship between V and X (i.e., V (X ) in correct mathematical
o
o
notation). This method is illustrated using a specific example of the
measurement of a variable X using a sensor output voltage, and variable X is
assumed to be that which is illustrated in Figure 4.24. A microcomputer is to
obtain the value for X using a table lookup operation.
The portion of the microcomputer that is involved in the table lookup
process is illustrated in Figure 4.26. The relationship V (X ) is stored in ROM
o
for representative points along the curve. This data is stored using V values as
o
addresses, and corresponding values of X as data. For example, consider a point
(V , X ). The data X is stored at memory location V in binary format.
1
1
1
1
The operation of the table lookup is as follows. The sensor S has output
voltage V . The computer reads the values of V (using an ADC to convert to
o
o
digital format) through the I/O device. Then the MPU under program control
(program ROM) calculates the addresses for the two nearest values toV , which
o
are V and V (V < V < V ). The computer, under program control, reads values
1
1
2
2
o
X and X and then calculates X using the preceding formula.
2
1
142 UNDERSTANDING AUTOMOTIVE ELECTRONICS