As with any display, the more segments that need to be controlled in an LED display assembly means more lines will be wanted to control it. With the big number of segments found in multiple-digit alphanumeric LED packages, it might seem sensible to consider a shift register scheme with a serial input of display data, and use this rather than the standard binary coded decimal to 7-segment decoder chips or other kinds of decoders utilized in the older displays.
An LED driver circuit that uses shift registers would have multiple registers connected in a daisy chain fashion so the output of one becomes the input of the next. There would only be one data line inputting data to the first register with clock pulses. The data may be sent according to protocols like SPI or I2C, but another easy way would be just to set up a clock signal on one of the microcontroller's output pins and program another output pin to transfer the display information according to the timing on the clock pin. This also has the advantage that it does not need to be a certain frequency, and the frequency could even be varied while info is transferred and may be paused any time.
This would not cause the display to appear unstable or anything like that, since the bits on the shift register aren't connected directly to the LED segment drivers. Instead, they are connected to a latch register in between, which is in parallel with the shift register, and it'll accept information only when the shift register is completely filled with new data.
At that point, a third output pin on the MCU is used to supply the timing pulse (or latch signal) which copies the shift register info into the latch register, which is used to drive the individual LED segments immediately through driver circuits. The drivers are generally built right into the same IC package along with the registers, and a number of them can drive up to twenty-four mA of current, which is way more than an output pin on a microcontroller can handle.
So by employing only 3 additional output pins on an MCU, a huge multiple-digit LED display can be controlled, where the display itself could either be in the form of a 7 segment display, a 14 or 16 segment alphanumeric LED display, or maybe even a dot matrix display.
An LED driver circuit that uses shift registers would have multiple registers connected in a daisy chain fashion so the output of one becomes the input of the next. There would only be one data line inputting data to the first register with clock pulses. The data may be sent according to protocols like SPI or I2C, but another easy way would be just to set up a clock signal on one of the microcontroller's output pins and program another output pin to transfer the display information according to the timing on the clock pin. This also has the advantage that it does not need to be a certain frequency, and the frequency could even be varied while info is transferred and may be paused any time.
This would not cause the display to appear unstable or anything like that, since the bits on the shift register aren't connected directly to the LED segment drivers. Instead, they are connected to a latch register in between, which is in parallel with the shift register, and it'll accept information only when the shift register is completely filled with new data.
At that point, a third output pin on the MCU is used to supply the timing pulse (or latch signal) which copies the shift register info into the latch register, which is used to drive the individual LED segments immediately through driver circuits. The drivers are generally built right into the same IC package along with the registers, and a number of them can drive up to twenty-four mA of current, which is way more than an output pin on a microcontroller can handle.
So by employing only 3 additional output pins on an MCU, a huge multiple-digit LED display can be controlled, where the display itself could either be in the form of a 7 segment display, a 14 or 16 segment alphanumeric LED display, or maybe even a dot matrix display.
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