Countdown Timer
 
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Programming your PIC 16F690 with the timer code will turn it into a convenient dedicated timer chip, ready to be incorporated in your projects. The diagram on the right shows the effective pin layout. You can find detailed description on each pin function below. Also, check the sample schematic diagram at the end of this page and the Sample Applications section.


Pin Description
  • VDD, VSS. Power supply, +5V and GND.
  • OSC1, OSC2. Connect to a 3.579545 MHz (NTSC) crystal.
  • SET, RST, +1, -1. Button inputs. When the buttons are pressed, the level on these pins should be set to LOW.
  • +10, -10. Inputs for the optional +10 and -10 buttons. If those are not used, the +/-10 pins should be tied to +5V through a 10k resistor.
  • A, B, C, D, E, F, G. Outputs for the LED cathodes for the corresponding segments of the 7-segment indicators. Depending on your indicator type, you may want to connect those through 220 Ohm resistors. See sample application below.
  • DIG. Indicator digit selector. The two indicators are multiplexed; this output is in HIGH state when the TENS indicator data is on, and in LOW state when the ONES indicator data is on.
  • U / Inv.DIG. Depending on your code settings, this is either an unused pin (you can modify the code to make use of it), or its output is the inverse of DIG (this way, you can save any extra components and connect DIG and Inv.DIG directly to the LED indicators common anodes).
  • OUT. Timer output. This pin will be in HIGH state while the timer is running. It will switch to LOW state when countdown ends, or when timer is paused/stopped.
    

Sample Application
The following sample schematics uses the PIC16F690 timer to control a single LED (the one on the left, connected to the OUT pin). If you don't want to customize pin 10 (see U / Inv.DIG above), you can configure it as Inv.DIG output and get rid of the two transistors and resistors used for digit selection. In that case, you can connect DIG directly to the digit 1 cathode (+1), and Inv.DIG to digit 2 cathode (+2).




Click on the schematics for higher resolution image.


PCB layout
In the Timer Applications section you can find a link to PCB layout files by Eirik H. Snerthammer. Eirik, thanks for sharing!
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