Microcontroller Serial Port Measures Pulse Width

Many industrial and instrumentation systems need to measure the duration of pulse inputs, such as frequency from rotational-speed sensors, gating and strobe pulses from external systems, and PWM (pulse-width-modulated) inputs. Designers generally use on-chip timers and edge-driven interrupts for this purpose. If one of these components is unavailable, however, you can employ an unused on-chip serial- synchronous receiver to make those measurements.

You can set the baud rate of the serial port receiver for the necessary timing accuracy. The receiver interrupts the microcontroller after every 8 bits. You can embed the pulse-width acquisition routine, which resides in your application program, to read the byte that the ISR (interrupt-service routine) receives. It counts and accumulates the number of ones and zeros the bytes receive to measure the duration of an incoming pulse (Figure 1).


Figure 1

The algorithm measures the duration between two consecutive rising edges. The microcontroller detects a rising edge or a falling edge when a received byte is neither 0xff nor 0x00. If a byte is less than 0x80H (100 000b), then the byte marks a rising edge. If the byte is equal to or greater than this value, then the byte marks a falling edge.

The leading rising edge resets the bit counter to the number of trailing ones in the rising-edge byte by shifting the bits to the right. The bit counter increments by eight at the arrival of every byte, including the one that marks the falling edge. When the counter receives the trailing rising edge, marked by the next rising-edge byte, which is greater than 0x80h but less than 00H, it again counts the number of leading zeros in this byte and adds them to the accumulated-bit counter. The accumulated-bit count at this point directly relates to the time period of the pulse train by a factor equaling the baud rate.

Figure 2 depicts a 200Hz pulse train, which has a 5-msec period between two consecutive rising edges. The baud rate is 256kbaud. During a measurement cycle, assume that the leading rising edge is marked as 0011 1111b. The microcontroller counts the number of trailing ones by shifting them right and initializing the bit counter as six. This count corresponds to approximately 23.43μs.


Figure 2

Next, every byte before the rising-edge byte increments the bit counter by eight. Simple calculation shows that the sum is 159 bytes, or 1272 bits. At this point, the total bit count is 1278, including six one bits received in the first rising-edge byte.

The pulse train now encounters its trailing rising-edge byte as 0011 1111b. When this encounter occurs, you need to shift the zeros left to count two bits. The total bit count between the rising edge now is 1280. At a 256kbaud rate, this figure corresponds exactly to 5ms, or 200Hz.

Figure 3 explains how you can use this concept to measure frequencies in hundreds of hertz.


Figure 3

You can tailor this bit-counting concept to your application’s requirements. For measuring only a low period of a pulse, you need to detect a falling edge and count the bits until you encounter a rising edge. You can use this concept to read an incoming PWM signal by reading high periods of a known incoming pulse frequency.