Question 9.9: Frequency measurements are made on the truncated counter in ...
Frequency measurements are made on the truncated counter in Figure 9–58 as indicated. Determine if the counter is working properly, and if not, isolate the fault.
Check to see if the frequency measured at TC 4 is correct. If it is, the counter is working properly.
truncated modulus = full modulus – preset count
= 16^{4} – 82C0_{16}
= 65,536 – 33,472 = 32,064
The correct frequency at TC 4 is
f_{4} = \frac{10 MHz}{32,064} \cong 312 Hz
There is a problem. The measured frequency of 637.8 Hz does not agree with the correct calculated frequency of 312 Hz.
To find the faulty counter, determine the actual truncated modulus as follows:
modulus = \frac{f_{in}}{f_{out}} = \frac{10 MHz}{637.8 Hz} = 15,679
Because the truncated modulus should be 32,064, most likely the counter is being preset to the wrong count when it recycles. The actual preset count is determined as follows:
truncated modulus = full modulus – preset count
preset count = full modulus – truncated modulus
= 65,536 – 15,679
= 49,857
= C2C0_{16}
This shows that the counter is being preset to C2C0_{16} instead of 82C0_{16} each time it recycles.
Counters 1, 2, and 3 are being preset properly but counter 4 is not. Since C_{16} = 1100_{2}, the D_{2} input to counter 4 is HIGH
when it should be LOW. This is most likely caused by an open input. Check for an external open caused by a bad solder connection, a broken conductor, or a bent pin on the IC. If none can be found, replace the IC and the counter should work properly.