Question about sampling rate

[Reychard]

Hi everyone,

I have a resistive humidity sensor which I'm using an ADC (ADC0804) to get values off. The specification of my project specify that the sensor has to sample the humdity at a frequency of 5Hz. I'm using a Spartan-3 board to interface the ADC. How am I able to do this? Obviously, I don't provide a 5Hz input clock to the ADC, so what is the correct way to go about doing this?

Kind regards

 

[FoxyRick]

TI's datasheet offers a good explanation of the ADC0804's logic: **broken link removed**

Basically, run the ADC0804 at whatever conversion clock speed suites you, say 1MHz. Have another clock running at 5Hz to initiate a conversion and read the result (reading by /INT or another timer) by manipulating the ADC0804 /WR, /CS and /RD lines.

To save power, you could shut the conversion clock down between samples.

 

[Reychard]

Thanks FoxyRick. I'm running my ADC at 50Khz and using a state-machine to go through the procedure. My VHDL code is as follows;

START_FSM	:	PROCESS (CURR_STATE, INTR)
					BEGIN
					
					READ_DATA <= '0';
					WR <= '0';
					
					CASE CURR_STATE IS
							
							WHEN STARTUP =>
								WR <= '0';
								READ_DATA <= '0';
								NEXT_STATE <= CONVERT;
								
							WHEN CONVERT =>
								IF (INTR = '0' AND STREAM = '1') THEN
									NEXT_STATE <= WAIT500;
								ELSIF (INTR = '0' AND STREAM = '0') THEN
									NEXT_STATE <= READ1;
								ELSE
									NEXT_STATE <= CONVERT;
								END IF;
								WR <= '1';
								READ_DATA <= '0';
															
							WHEN WAIT500 =>
								IF (COUNTER_WAIT = 5000) THEN
									NEXT_STATE <= READ1;
								ELSE
									NEXT_STATE <= WAIT500;
								END IF;
								COUNTER_WAIT <= COUNTER_WAIT + 1;
								
							WHEN READ1 =>
								NEXT_STATE <= CONVERT;
								WR <= '1';
								READ_DATA <= '1';
								
							WHEN OTHERS =>
								NEXT_STATE <= STARTUP;
					END CASE;
					END PROCESS;

Now, whenever READ_DATA is 1, I read the ADC output. So are you suggesting that I have a separate process running at 5hz where I check the status of READ_DATA and obtain the 8-bit ADC output? I've tried doing this and I get no values at all. If I run the clock at 50Mhz, I get values just fine.