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For a heart rate calculator project by using Basys2 and VHDL, I am trying to connect ADC0808 in order to get 8bit digital output from my pulse sensor's output. However, although I designed the circuit according to datasheet, in the Basys2 board, it is seen that all the eight leds are turned on. Moreover, the output of ADC does not change by cancelling connections and make open circuit. I am not sure where and how to connect "start", "EOC", CLK and ALE. Thanks for your help.

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    \$\begingroup\$ Which part of the datasheet are you having trouble with? \$\endgroup\$ Nov 24, 2014 at 17:44
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    \$\begingroup\$ Wow. I remember the ADC0808. First ADC I ever played with. So yes, as the question stands it's not one we can answer. I for one don't know what a Basys2 is, and I certainly have no clue how you're wiring it all up. There are 8 LEDs lit up. We don't know what those LEDs are supposed to do. Please provide us with schematics, relevant bits of code, etc. \$\endgroup\$
    – Majenko
    Nov 24, 2014 at 18:19
  • \$\begingroup\$ I have problems about the connections. Where will the start, interrupt and ale pins should be connected? \$\endgroup\$
    – Merto
    Nov 24, 2014 at 19:03
  • \$\begingroup\$ To IO pins. Which one depends on your software and the rest of your schematic. \$\endgroup\$
    – Majenko
    Nov 24, 2014 at 19:13
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    \$\begingroup\$ And you want to do that without software...? Ohkay... so you will have to design a circuit to load the right address for the analog pin you are using into the address latch circuit, then pulse the start input and wait until the complete output is set. Then you set the output enable, and you will get some output. Simpler to write some software for your Basys2, whatever that may be. \$\endgroup\$
    – Majenko
    Nov 24, 2014 at 19:22

1 Answer 1

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If you haven't connected Start or CLK to anything then your ADC0808 is just going to sit there doing nothing.

CLK must be connected to a continuously running clock signal at a frequency somewhere between 10kHz and about 1MHz, and Start needs to get a pulse to tell the ADC to start a conversion.
8 clock cycles after your Start pulse, EOC (End-of-Conversion) will tell you that its done and the result is available on the 8 output bits.

ALE (Address-Latch-Enable) is used to lock the input number selected by the 3 address bits, so you can choose which of the 8 analog inputs to use.

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