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Count-down timer

On this page, I will introduce the Count-down timer with PIC16F84A.
I am putting a count-down timer by the hardware on Electronic Circuits Application Garage.
The function of the timer is the same as it approximately. (The stop switch is added)
Because this unit was made compactly, the wiring of the back becomes quite complicated.

I tyied to use following technique for the software of this equipment.

Mode changing method of I/O port
7 segment LED (Output), BCD switch (Input) and Start switch (Input) are connected with PORTB in parallel. Each device is switched using the decoder. The mode of the input/output of PORTB is changed according to it.
Control method of the 7 segment LED

The control code of 7 segments LED is controlled by the binary-coded decimal code(BCD).
Display control mothod in case of borrow

The next of 00 seconds, it is displayed with 59 seconds. When each digit becomes 0, the time out processing is done.
Precision of the hardware timer

The timer is count-downed every second by hardware timer.
I examined this accuracy.

Processor Frequency10MHz
Time range1 minute - 99 minutes
Timer setting methodBCD switches
Display Minute : 00 - 99
Second : 00 - 59
Output During count-down, it controls a voltageless contact.
It lights up an LED as the display during control.

Circuit drawing

Pattern drawing

Circuit explanation

Parts explanation

Flow chart


Processing explanation