The figure on the right is displaying the condition to be displaying 10:32:54 p.m. switching every 500 milliseconds. In the switching time of the actual circuit, it is 1 millisecond. So, all digit sees to be displayed at the same time. The light of the LED is 1/6 compared with the continuation lighting-up.
The specification of the display position is controlled by the binary signal which is output from RA0, RA1 and RA2 port of PIC. This signal is decoded in CPLD and eight kinds of signals are made. This time, six kinds are used. Only the transistor which corresponds to the L level decoder output becomes ON condition. The LED which is connected with the transistor becomes a lighting-up possible condition.
In the tens-hour display, only “1″ is displayed. So, I put the display of AM/PM. 74HC138 can be used for the 3-8 decoder, too.
Segments of the lighting-up of each digit are controlled using 7 ports of RC6 from RC0 of PIC. The output of these ports is common to all the LEDs. However, only the LED of the digit which was specified by RA0-2 is controlled in the lighting-up. The LED lights up when the RC port is an L level.
The brightness of the LED depends on the kind of the LED. When the brightness is different extremely, the resistors which are connected with the RC ports should be separated. This time, because it is approximately the same brightness, resistors are made common.
Because the maximum voltage which is applied to the pin of PIC is 5V, I make the power of the LED 5V.
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