The popular TC1602A-01T LCD features an LED backlight. The backlight is nominally 5 V but reading the datasheet gives some warnings.
1602 LCD backlight specification. Note the caution!
It appears that the LED has a forward voltage, \( V_f \) of 4.1 V at 100 mA but that there is no current limiting built in. If connected directly to a 5 V supply then excessive current would flow (as predicted by the IV curves), the backlight would be destroyed and the display ruined.
A simple LED current limiter will suffice.
1602 LCD backlight current limit and switch.
We can calculate the value of R1 from the voltage drop across it and the current through it:
\( R_1 = \frac {V_{SS} – 4.1}{0.1} \)
For a 5 V supply this would give R1 = 9 Ω. The nearest standard value is 10 Ω and this would be fine.
Q1 is a standard NPN switch. If used it will drop about 0.2 V when on and this could be subtracted from the R1 calculation above. If switching is not required then it can be omitted and the LED cathode connected to ground.
Figure 1. Standard 7-segment display layout and segment identification. The decimal, if supplied, point requires an eight LED.
7-segment (or “seven-segment) LED displays are commonly used to display digital information. Each segment consists of one or more LEDs and can be lit independently of all the others to form the digits 0 to 9. A limited range of alphabetic characters can also be formed but these are of mixed case and can be difficult to read.
The standard segment identification is shown in Figure 1 and segments are named A to G starting at the top and going clockwise with G being the centre segment.
Typically the displays have a common pin for all the LED anodes or cathodes. Eight pins will suffice for a true seven-segment display whereas nine are required for one with a decimal point.
Figure 2. As might be expected, package pin-count is reduced by using a common pin for one side of all the LEDs. In this case a common anode is used.Figure 3. A common cathode display. These have the advantage that, for multiplexed displays, the digit can be strobed using an NPN transistor in the cathode connection to ground.
By multiplexing seven-segment displays the number of pins required to drive the displays can be reduced. In the scheme of Figure 4 the segments are driven high by the micro-controller. The required segments for digit 1 are set on lines A to G and Q1 strobed (turned on briefly). Then the required segments for digit 2 are turned on and Q2 is strobed, etc. The sequence is run quickly enough that all digits appear to be continuously lit due to persistance of vision.
When multiplexing displays in this manner it is normal to run at a current higher than nominal. This is acceptable because the segments will be powered for a maximum of 25% of the time – 25% duty-cycle.
Note that 32 segments can be controlled using only 12 outputs from the controller. This efficiency in hardware and PCB layout is what makes multiplexing so popular.
Reducing controller pins
Figure 5. The 4511 BCD to 7-segment latch / decoder / driver.
By using a latch / decoder / driver such as the 4511 decoder (datasheet) the designer has many more options for display. By using the BCD decoder only four lines are required to drive the seven segments.
Figure 6. 4511 function diagram.
BL: blanking. Pull low to turn all segments off. Display: blank.
LT: lamp-test. Pull low to turn all segments on. Display: ‘8’.
LE: latch enable. Pull low to read the inputs and pass through to the seven segment outputs. Pull high to latch the outputs.
Note that BCD values above ‘9’ are considered void and the display will be blank. This renders the 4511 unsuitable for hexadecimal display.
The 4511 can source or sink current for the displays. See the datasheet for ratings.
Flicker-free 7-segment display
Figure 6. By using a 4511 latch per display a flicker-free display is possible.
Figure 5 shows how to use a 4511 latch per digit. In this case the data for each digit is set up in turn on D0 to D3 and the digit’s /LE (latch enable) pin is pulled low. The seven-segment display will immediately update and will be latched when the /LE pin is pulled high again. This, for example, would allow the micro to handle other tasks without having to update the display until required. The D0 to D3 IO pins could even be used as inputs with suitable input resistors while the diplay update is not required.
Note that the display decimal points are not controlled by the 4511. This leaves the designer with several options:
No decimal point.
A fixed decimal point by wiring one of the DP pins to Vss.
Program controlled decimal point. This will require one extra controller output pin per decimal point.
