51 MCU Series Knowledge 9--Timer, Counter (2)

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1. When we use a single-chip microcomputer to count a pulse, how do we determine the upper limit of the frequency allowed for the pulse?

When set to counting mode, the external pulse signal is counted through pin T0 (P3, 4) or T1 (P3-5). When the input pulse signal has a negative jump (or positive jump, the specific situation depends on your register settings), the timer value is increased by 1. During S5P2 of each machine cycle, the external input is sampled. If the value sampled in the first cycle is 1, and the value sampled in the next cycle is 0, then during S3P1 of the next cycle, that is, 24 oscillation cycles, the highest frequency of the external input count pulse is 1/24 of the oscillation frequency.

There is no limit on the duty cycle of the external input signal, but in order to ensure that a given level is sampled at least once before changing, this level must be maintained for at least one machine cycle. (Tcy is the machine cycle)

△ Conclusion: The maximum frequency of the counting pulse is 1/24 of the oscillation frequency, and the level (high, low) of the input pulse is maintained for more than one machine cycle.

The MCU divides a machine cycle into six state cycles and twelve beats. In each state cycle, the beat completes different actions, realizing time-sharing multiplexing on the hardware. For example, when the input pulse is sampled in S5P2, the count value is increased by 1 in the S3P2 MCU. These two actions are completed by different hardware in different state cycles in the same machine cycle.