June 19, 1997

µC-based one-shot has wide range

Yongping Xia, International Teldata Corp, Los Angeles, CA

The µC-based, digitally programmable, monostable multivibrator in Figure 1 has more accurate timing than a conventional RC-based device because a 20-MHz crystal controls the µC. The PIC16C54 is a low-cost, high-speed, 18-pin µC. At a full speed of 20-MHz and using the accompanying program, this µC can generate programmable pulses of 10 µsec to 100 sec per step. (Click here to download the file from DI-SIG, #2050.)

Inputs S₀ through S₆ control the step size (Table 1), and S₇ through S₉ determine the range (Table 2). When S₇=S₈=S₉=logic 0, S₀ through S₆ determine the pulse width, which can range from 10 µsec to 1.27 msec with 10 µsec per step. When S₇=S₈=S₉=logic 1, each step is equivalent to 100 sec. Thus, the maximum pulse width is 3 hours, 31 minutes, and 40 sec. (DI #2050)

Figure 1

A µC- and crystal-based monostable multivibrator exhibits much more accurate timing than its RC-based counterpart.

Table 1--Timing step sizes (when S₇=S₈=S₉=0)
S6	S5	S4	S3	S2	S1	S0	Time (msec)
0	0	0	0	0	0	1	0.01
0	0	0	0	0	1	0	0.02
			.
			.
			.
1	1	0	0	1	0	0	1.00
			.
			.
			.
1	1	1	1	1	1	0	1.26
1	1	1	1	1	1	1	1.27

Table 2--Range-control settings
S9	S8	S7	Factor (×)
0	0	0	1
0	0	1	10
0	1	0	100
0	1	1	1000
1	0	0	10,000
1	0	1	100,000
1	1	0	1 million
1	1	1	10 million

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