What oscillator/clock should I use

[wot_gorilla]

Hello.

I am designing a simple controller for a home-scale automated irrigation system. I am using some flip-flops and counters, so I will need a clock. I am familiar with digital circuits and I have done some VHDL, Verilog and FPGA programming, but I am a starter with the design of digital circuits using discrete elements. I have bought some parts, but I don't know what kind of clock I should use and where to find it.

I looked on the Internet, and I found references to the following:
IC8038
MAX038
ICL8038
XR2206

Are these what I'm looking for? How do you know what the frequency of these oscillators is? When you buy an oscillator, can you adjust its frequency? I am asking the question because I my counter will be counting minutes and perhaps even hours. My clock needs to be very slow, or otherwise I will be needing 10 or more 4-bit counters, which is ridiculous. Of course, if I can save some money, I'd like to. I'm not going to buy a 25 MHz clock if even a 1 Hz clock would do.

Thanks in advance for your help.

[rehanessa]

555 timer is a very cheap and easily available timer/clock ic. i designed a digital clock with it and it worked very well.

[audioguru]

The frequency of a 555 oscillator isn't nearly stable enough for a clock. You need a crystal oscillator with divider, or sync with the mains frequency.

[Kevin Weddle]

There are good oscillators and better ones. The best oscillators are the ones that have the inventers names attached to them. In the case of clock oscillators, there is always a capacitor that is at the heart of the oscillator. Then you are left with a logic circuit which works equally well from device to device. I would choose the one that requires a minimal number of devices. However, you will note that there are a number of factors in how you link the circuits together. You just have to be mindful of the current and voltage of the free running oscillator. I would suspect slight variations in the way the clock is produced with respect to threshold levels and the unknown zone of the gates. A perfect pulse is hard to come by. In fact you will surely be stuck with something that cannot be corrected.

I might suggest an alternative. What I see is the best way to produce a clock is to start with a regular oscillator and high gain it until it is a pulse. This is for you if you are concerned with the integrity of the ramps. I think the ramps are the crucial part because any lack of integrity can cause small oscillation or delay in triggering.

[audioguru]

In the good old days I made an extremely accurate clock with an 8-pin, $2.00 MM5369AA Cmos IC, and a very accurate $1.50 colour TV 3.58MHz crystal. The output was exactly 60Hz but 50Hz and 100Hz versions were also available. Since nobody makes their own clocks anymore, it was discontinued in 1997.

Have you seen this website's clock? It must be using a 555 as a clock oscillator because it is 23 minutes fast.

[MP]

wot_gorilla,
check the data sheet for the MAX038.
It is a stable device.

MP

[audioguru]

Have you seen a time-of-day clock now-a-days that uses an analog timing reference?

Yeah, the 200 parts per million frequency drift of a MAX038 looks impressive, but is more than an ordinary 555 (150) and isn't its max.
Its 600ppm max drift works out to a timing error of almost 1 minute per day, with a temp change of only 1 degree. Plus the tempco of its timing cap which could cancel the IC's error or quadruple it.

Let's use the MAX038 in a temp controlled room or housing to keep it stable. Then try to set its frequency. An ordinary frequency counter isn't accurate enough with its short gate time, so you will need a PPL and counter circuit to multiply its frequency so you can set the timing error to within minutes per day. Accurate enough?

That counter circuit which you were trying to eliminate, should be used in an accurate and stable quartz crystal clock circuit like real clocks and watches, even cheap ones, use. ::)

[MP]

I really do not think that the loss of a minute or 2 or 3 or more is going to interfere much with an irrigation control. There is no reason to make something like this crystal controlled and expensive for home use. I personally would make it with a microcontroller and 2 crystals, add a display, a keypad entry system. and datalog the wind direction and wind speed so as not to turn it on when the wind will interfere, but that is because this is what I do and I have the parts. The guy is not making a precision clock; he is just turning a valve on and off at intervals. The MAX038 is a good solution from his 'list of parts' in the original post because it is simple to get a low frequency output with minimal parts.

Audioguru: It is not very helpful to keep going onto posts and instead of offering solutions, to tell people all of the ideas are not good. If you want to offer a better way, you should give the guy some instructions on how to build it.
Wot_gorilla: do not be discouraged. Go make this thing work and tell us about it.

MP

[audioguru]

I found another idea in this post that is not very good, but also found a good solution.

Have you seen the price of a MAX 038? More than $23US! And it won't be very accurate but is probably OK to time an irrigation system.

I would use inexpensive but very accurate parts such as a through hole but very small 32.768KHz watch crystal ($.35US), A CD4060 oscillator/counter ($.55US) for a clock pulse every 1/2 second, add a CD4013 ($.48) for exactly 1 second, or a CD4541 oscillator/counter ($.50US) for a clock pulse every 2 seconds. I got the prices today from Digikey.

[MP]

Maxim will give you the parts free as samples. As many as 4 each. If you are looking at cost only, go through their catalog and use as many Maxim parts as possible.

http://www.maxim-ic.com/samplescart.cfm
You will have to register on their site with a business name.

MP