Seven-segment LED interfacing
The 7-seg LED can have common anode or common cathode. With common anode, the anode of the LED is driven by the positive supply voltage and the microcontroller drives the individual cathodes LOW for current to flow through LEDs to light up. In this configuration, the sink current capability of the microcontroller is critical. With common cathode, the cathode of the LED is grounded and microcontroller drives the individual anodes HIGH to light up the LED. In this configuration, the microcontroller pins must provide sufficient source current for each LED segment. In either configurations, if the microcontroller does not have sufficient drive or sink current capacity, we must add a buffer between the 7-seg LED and the microcontroller. The buffer for the 7-seg LED can be an IC chip or transistors.
The seven segments of LED are designated as a, b, c, d, e, f, and g as shown in the following figure:
A byte of data should be sufficient to drive all of the segments. In the example below, segment a is assigned to bit D0, segment b is assigned to bit D1, and so on as shown below:
|Assignments of port pins to each segments of a 7-seg LED|
The D7 bit is assigned to decimal point. One can create the following patterns for numbers 0 to 9 for the common cathode configuration:
|Segment patterns for the 10 decimal digits for a common cathode 7-seg LED|
Since the same segment for both digit 1 and digit 2 are connected to the same I/O port pin, the common cathode of each digit must be driven separately so that only one digit is on at a time. The two digits are turned on alternatively. For example, if we want to display number 14 on the 7-seg LED, the following steps should be used:
- Configure Port B as output port to drive the segments,
- Configure Port A as output port to select the digit,
- Write the pattern of numeral 1 from the above table to Port B
- Write one bit of Port A to activate the tens digit,
- Delay for some time,
- Write the pattern of numeral 2 from the above table to Port B,
- Write one bit of Port A to activate the ones digit,
- Delay for some time,
- Repeat from step 3 to 8.
At low frequency of alternating digits, the display will appear to be flickering. To eliminate the flickering display, each digit should be turned on and off at least 60 times each second.
Notice in the above figure, a single pin is used to select each digit. That means if we want 4 digits we must use a total of 12 pins. That is 8 pins for the segments a through g, decimal point, and 4 pins to select each digit.