What is the significance of the overline on SRCLR and OE (pins 10 and 13 )?
In my RexQualis Lesson 8 Shift Register with 1 Digit 7 Segment Display I followed the setup that left shift register pin 9 QH' unpinned. The Texas Instrument datasheet for the shift register states:
Those pins are low-active. I.E. drive the OE line low to enable the outputs.
Another way to say the same thing is to say the line means NOT, so NOT SRCLR, meaning that when SRCLR is high it is not active.
Newbie here with newbie questions. Thanks for replies. LOW =low impedance=low active. When pulled HIGH=high impedance. Low impedance=low resistance so current can flow? High impedance=high resistance so current can't flow? Or what does that do to the performance of the Qa-Qh output pins. Output pins hold memory or don't hold memory?
No. The reference is to binary logic levels.
LOW = logic zero = false,
HIGH = logic 1 = true.
Impedance comes into play in the physical electronics realm. Plus, resistors have only negligible impedance. Impedance is a characteristic of capacitors and inductors.
On my project, Pin 13 goes to ground. QH inverted. I'll have to look that up.
When it is said that it is " pulled HIGH" what does that mean? Does it refer to that it has been sent 1 and if it is pulled LOW it is sent a 0?
Just reread @
PerryBebbington)
So HIGH sent 0 LOW receives a 1?
When wiring a digital circuit, sometime an input should be made LOW or HIGH permanently if that pin's function is to be unchanged.
So you want to permanently enable something, for example. You'd need to know if LOW enabled it or HIGH.
This is also called "tying high" or "tying low". Just how to do that varies with habit and the details of the logic family the chip comes from.
Tying LOW could just mean grounding the input pin. Tying HIGH might be done with a resistor to Vcc of a value guaranteed to be seen as HIGH logically.
Signal names should carry with them an indication of whether LOW or HIGH means it is "active".
/OE short for output enable would mean LOW enables output. Also seen as OE with a bar over it.
OE would mean HIGH disables output.
/SRCLR means taking that pin low is what clears the shift register. If you don't need that function of the chip, it should therefore be tied HIGH.
Just one more opportunity to get things exactly backwards every first time. 
a7
Whoeee! Okay, let me wade in this and hope not to drown: My shift register is 74HC595N. The Texas Instrument datasheet shows pin 13 as /OE, and pin 10 as /SRCLR (bar over).
In my display project, /OE AND /SRCLR are pinned to ground.
So both connected to ground, both permanently set at, or tied low (LOW state). The reason is they are unused pins for purposes of this project? [quote="alto777, post:9, topic:1211523"]
When wiring a digital circuit, sometime an input should be made LOW or HIGH permanently if that pin's function is to be unchanged.
[/quote]
Ok... When one says HIGH = input is that the same as it has received an input or that is ready to receive and input? Based on the following quote, I interpret it means that the state has been readied for input by setting it HIGH.
Wrong. If that is working I can't explain it.
Unused pins must be tied appropriately, not just low because they are not used by you, but carefully put in the condition that makes the chip work like you want it to.
You do want the output enabled: /OE is active low, so it should be tied low. Active. So the output is enabled.
You do not want the shift register to be cleared: /SRCLR is active low, so it should be tied high. Inactive. So the shift register is not cleared.
Here's the wokwi reference page for that part. They use /MR master reset but it is the same pin as your document's /SRCLR.
at the bottom are some fully functional demo links.
HTH
a7
Yes, my mistake, Only OE is pinned to ground. (Page 3: Connect GND and OE pins on 74HC595 to GND pin on Arduino hardware.) SRCLR pin 10, is pinned to + (but isn't mentioned in instructions on Page 3 is shown on setup picture.
Lesson 8 74HC595 And Segment Display.pdf (685.5 KB)
.I can get this setup to work but it is very flaky. Wiggle test brings power to the board intermittently. Swapped UNO R3 to another board it does same. This appears to be my problem all along. To me this is significant: some jumper wires are magnetic.
Add a decoupling capacitor close to the Vcc pin, 100nF ceramic.
SRCLR is active low. In this project, we don't want the register cleared. Pinning SRCLR to Vcc/5V creates a high impedance state or electrical block making it inactive.
OE is active low. In this project, we want output enabled. To make OE active, it is pinned to ground achieving a low impedance state that enables its output.
The reason the register is not to be cleared, is because the same data is being sent in a repeating loop?
The reason output is enabled is so it can repeatedly send outputs to the display segments?
Without the Wokwi 74595 documentation, how would I know what state made a pin active or inactive? [quote="alto777, post:9, topic:1211523"]
You'd need to know if LOW enabled it or HIGH.
[/quote]
Learn to read the 'language' of the datasheet?
'595 datasheet
In particular, the timing diagram (section 6.1) and the functional description(8.2) are your go-to.
Just leave impedance out of it. 5 volts on an input is above the switching threshold voltage, so is seen as logical one.
O volts is below the threshold for interpretation as logical zero.
What makes it active or inactive is how the chip's designers decided things woukd work. Kinda like the choice you have to see a pressed button as HIGH or LOW depending on how you wire it up.
The data sheet for any device, as @camsysca notes, is the hole truth. Sometimes you can get only what you need, it takes practice. Usually a good working example and some informal prose found through searching or asking works well enough.
Inputs on the UNO are very high impedance. All this means practically is that there will be very little current drawn form the source applying the voltage.
i hope you are not struggling with an inferior breadboard. Are you using wire of the correct gauge, with the ends exposed to a good length and placing wires carefully so they aren't having any trouble staying put?
a7
@crabcrawler That's good advice.
If you've just begun this journey, it's wise to buy a 'kit' of pre-stripped fixed length breadboarding wires. More hair has been pulled due to poor wiring on breadboards... Believe me, it's worth it.
That breadboard image is a telltale, too. Does your breadboard look similar? Does it have that extra-special wide gap in the center of the power wiring on both sides? If so, put a jumper across that gap, right now, on both sides, both power strips. Because the visible larger gap is indicative that the power strip to the left, and to the right ARE NOT CONNECTED. Your wiring, as shown, depends on those strips being continuous. So, check your board.
If you became competent with the IDE and the program/verify/download/cry rinse and repeat routine, you have all it takes to use the wokwi simulator.
I would say it is intuitive. That may be a step too far, but it is straightforward enough.
The beauty of it is that it completely eliminates all kindsa problems in the real world and leaves only the software to blame for misbehaviour.
I think it is good to play with real components, absolutely, and anything you end up making will have real issues.
But it is nice to arrive at a point where you know it ain't the software. I can usually get the logic perfect, even when the simulator doesn't have exactly the parts I'll need IRL. Many times a bit of thinking will suggest proxies like LEDs or pushbuttons or repurposing a part there is available.
a7
Yeah, that's great advice. It's taking awhile. When I don't understand the information presented on the datasheet I ask questions. Pointing to the datasheet, particularly to Table 6, is a good answer.
You'll get used to looking for datasheets if you spend long with integrated circuits, transistors, and other active devices. Otherwise, you'll Google and find 27 different answers to the same question, half of which are wrong, another quarter of which are out-of-context - and the last quarter you won't be able to tell, because they're just plain confused. So who to trust? The manufacturer.
Did you resolve the difference between HIGH, LOW, and high impedance? If not, we can try to confuse you some more.
