Shift Register 74LS166

How about the clear input? Is it tied high?

Luhan

The clear input is tied to a pushbutton as well. As the clear pin works on a logic low, wat i did was to connect the pushbutton thru an inverter so that whenever the pushbutton is pushed, it sends out a logic low thru the inverter and clears the register.

it seems from your last answer that you are using push buttons to drive the logic high. TTL pulls high by default so you would need pull down resistors on each input. Supply voltage is also critical for lsttl, within 250mV of 5V, a decoupling capacitor (0.1uF) at the chips supply pins is needed as well. Check your logic levels, you need less that

I inserted 100 ohm resistors between each pushbutton and the power supply to prevent overloading of current into the chip. Can this be considered as a pull down resistor as well? I have not inserted a decoupling capacitor though. Does this mean i just to insert a capacitor between the power supply and the supply pin of the shift register ie. Pin 16? Does this capacitor apply to all other supply pins of other chips as well eg. Schmitt Trigger and Inverter? Thanks.

Sorry, wat i meant was I inserted 100 ohm resistors between each pushbutton and ground, the other pin of the pushbutton goes to the supply line. Is this considered a pull down resistor? Cheers.

I inserted 100 ohm resistors between each pushbutton and ground, the other leg of the pushbutton goes to the supply line to prevent overloading of current into the chip. Can this be considered as a pull down resistor as well? I have not inserted a decoupling capacitor though. Does this mean i just to insert a capacitor between the power supply and the supply pin of the shift register ie. Pin 16? Does this capacitor apply to all other supply pins of other chips as well eg. Schmitt Trigger and Inverter? Thanks.

High by default , hmm I am certain of that for vanilla and 'L', I am starting to wonder about LS as it doesn't have the original open emitter input. I am having problems my self with the simplest setup of LS164 after my PIC-ls164 LCD driver wouldn't work. With only clock pulled up and leds on BCD I have 011 after powerup. Man I got to call the ghost busters ..... or move to the coffee table by the window (after I clean it up from the 7 years of crad), away from my

Cheers.

If your data input is static (i.e.. pulled up/down with resistors ) you are probably OK without the caps on the buttons. but usually you'd use RC between power rails shorting the cap with a pushbutton. Better yet use inverter or driver to sharpen the the edge when the cap is charging, before finally feeding the intended input, that way you don't have to worry if you matched active low input with C/button on bottom or active high with C/button on top.

What I am wondering about your circuit, how is the clock controlled. Is it a free running oscillator? ticking while you are loading new values? Hmmm Ok time too look at the SPEC.

???? OH, so you are having trouble loading the data?

Hmmm , clock inhibit has precedence over sh/|ld This could become very complex circuit very fast. Oooooh, I can see an RS flip/flop and some gates in your future, after the loading is solved ;).

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Lincoln,

From your description I think your main problem is lack of understanding logic circuits in general and TTL logic in particular.

So, as for the speed, this circuits are made to handle frequencies up to

To set up the circuit: Every input should be connected either directly to a TTL output or to ground or indirectly to Vcc using a pullup resistor. Everything between - let's say - 1k and 10k will do. If you want to use switches they should be connected between an input and ground.

So clock inhibit connected to ground is ok, shift/load high (using a pullup resistor) too and let's assume your 40Hz clock runs well. As all inputs are pulled high, the serial output should be high too. Every simple multimeter is good enough to verify. If not either the wiring or the chip is bad. Now one simple test can easily be done: Connect the serial input pin to ground. The output should follow almost immediately, that is to say after some (200) milliseconds. If not etc...

Before going on, you have to know that mechanical switches always bounce. The contacts bouncing against each other cause pulses in the milli- en microseconds range, often for milliseconds. But these are valid signals for circuits designed to work up to in the tenth of microseconds range. So for debouncing, a simple Schmitt trigger often will not be enough.

Another thing you do not seem to realize, is that pushing the shift/load button will load the register tens to hundreds of times, but the loaded value will be shifted out in one fifth of a second when the button is released. Nothing meaningfull to see on the oscilloscope especially if the output also is disturbed by bounce. Only if you setup the trigger and the time base correctly you may see some pulses flashing by.

To see the shiftregister in action, the only pushbutton you need is one carefully debounced clock. One push should give one - and only one - pulse. (Forget the 40Hz clock.) One other pushbutton may be used for the reset (clear) but needs no debouncing. All other switches can better be one pole one throw rocker switches. Keep shift/load high and push reset (clear). The output should go low if it wasn't already. Then make (or keep) serial in high and push clock eight times. The output should go high. Compare with the pulse diagram on the datasheet. If the output goes high earlier your debounce has failed. Now set the parallel input switches to some desired pattern, pull shift/load low and push clock one time. The output should become the same as the H-input. More pushing will change nothing as long as you do not touch the other switches. Set shift/load to high. Pushing clock eight times will shift out the loaded pattern, more pushing will continue to shift out the serial input.

petrus bitbyter

Don Lancaster wrote some good books especially for beginners. The TTL Cookbook, CMOS cookbook and Active Filter cookbooks are in my 'DO NOT GET RID OF' pile along with some older National and Motorola databooks. You will not regret getting Don's books. He explains, with minimum pain, how to do what you're attempting. I learned a lot from Don.

BTW, Thanks Don !

Glenn Gundlach

driver

input

free

A big thanks to all with your guidance. Am working on the circuit now and will post results ASAP. Once again, thank you all.

Lincoln