Real-Time Systems Development

Introduction to real-time systems 15

paid to the final executable code. There are circumstances where this opti-

mistic disregard may lead to difficulties. A commonly used environmental

monitoring arrangement involves a transducer being interfaced to a voltage-

to-frequency converter (thanks to Laurence O’Brien for sharing this lop-sided

bug with me). The cost advantage of not using an ADC interfaced to a serial

transmission link is the prime motivation. With a V2F unit, the transducer

analogue voltage is converted to a pulse frequency code: the larger the volt-

age, the higher the frequency; the lower the voltage, the lower the frequency.

The computer only has to dedicate a single bit input port to accept the infor-

mation in serial mode. However, there remains the problem of converting this

pulse frequency code into normal integer format. For an HLL programmer

the following code might appear attractive. It runs and offers the beguiling

appearance of success, but it entails an interesting bug related to the code

generated by the compiler. Unfortunately, the time spent in the two oppos-

ing arms of the

IF/ELSE structure is not matched. So with an actual 50/50

situation, the results would not come out as 50/50, because of the dwell time

bias. This can be checked by reversing the code and running both versions

back to back. Inspecting assembler code listings from the compiler will also

reveal the discrepancy.

loop for 100 msec { loop for 100 msec {

if (input

–

bit) ←−−−−−−−−−−−−−−−−−−−−−−−−→ if (!input

–

bit)

hcount++; hcount++;

else else

lcount++; lcount++;

}}

temp1 = tempX*hcount/ temp2 = tempX*hcount/

(lcount+hcount) (lcount+hcount)

1.10 High speed timing

Perhaps an example would now be useful of the opposite situation, when pro-

cessors simply cannot run fast enough. Consider a laser range-finder, intended

for use in civil surveying, or more ominously for battlefield targeting. It works

by preparing a pulse laser for firing, emitting a pulse of light, waiting for the

reflected echo to return, and, by timing the duration of the flight, calculating

the distance travelled.

The speed of light is 3

×

10

8

m/sec.

For a target 20 km away, the pulse of light will travel 40 km (4

× 10

4

m).

So time taken

=

distance

speed

=

4 × 10

4

3 × 10

8

= 1. 3 × 10

−4

s = 130 µs

If the item being surveyed is only 50 m distant, the time of flight will be

reduced to 325 ns.