Moving Message Display

It is possible to process periodically with the hardware timer. While the hardware timer is counting time, the software can do the other processing. In the software, the scroll of the message is periodically done by the interruption processing of hardware timer. And the lighting-up control processing of LEDs is executed in the waiting time of scroll. The Circuit diagram of this project is given below.

 

•  LED control circuit

• As for the left figure, all RA ports are in the L level condition. In this case, only outout-0 of 74HC154 is L level. Then, only TR1 is in the ON condition, the other transistors are in the OFF condition. So, only the LEDs which are connected with TR1 are in the lighting-up possible condition. The lighting-up of the LED is decided in the condition of the RB7-RB0 ports. In case of the left figure, because RB0 and RB4 are L level, D1 and D5(It isn't displayed in figure) light up. The other LEDs are connected with RB0 and RB4 can't be lit up, because the transistors for the power supply to those LEDs are in the OFF condition.
  
  
  The figure on the left shows the situation of the LED blink by the RA ports and the RB ports of PIC16F84A. You can find that the lighting-up time of one LED is 1/16 with this animation. If using latch registers for each LED row, the lighting-up time of the LEDs can be made long. I don't use latch registers because the printed board space needs more.
  Download Source code
  Download Hex File