 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Ah yes. That's the fun thing about doing digital electronics. You can't
just *buy* a 7400. No, you have to decide whether you want a 74LS00 or a
74HC00 or a 74HCT00 or...
Reading the datasheets is like walking into another world. (Especially
since the datasheets are usually poorly-scanned grainy PDFs that don't
match the part number you actually asked about!)
However, I just found the following pages:
http://www.kpsec.freeuk.com/components/ic.htm#logic
http://www.kpsec.freeuk.com/components/74series.htm
http://www.kpsec.freeuk.com/components/cmos.htm
Suddenly everything seems so much clearer. For example...
"74LS series: Inputs 'float' high to logic 1 if unconnected."
Oh crap. Well that would explain a thing or two! >_<
(I foolishly assumed that being unconnected would register as 0V - i.e.,
logic 0. Damn, this is going to make interactive testing *so* much harder!)
Also, it's nice to have some indication of the power requirements,
switching frequencies, and what the hell all the letter codes mean!
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
> "74LS series: Inputs 'float' high to logic 1 if unconnected."
>
> Oh crap. Well that would explain a thing or two! >_<
You should never be leaving any inputs as floating anyway, that's just
asking for trouble (for example if you use a different IC series later).
Best to always tie your inputs to whatever you want them to be.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
scott wrote:
>> "74LS series: Inputs 'float' high to logic 1 if unconnected."
>>
>> Oh crap. Well that would explain a thing or two! >_<
>
> You should never be leaving any inputs as floating anyway, that's just
> asking for trouble (for example if you use a different IC series later).
> Best to always tie your inputs to whatever you want them to be.
Sure. But I'm sitting with the IC between my fingers, poking the pins
with the end of a wire. Not so easy to tie it all down just for a quick
test.
So it seems that every input pin is supposed to be connected to one rail
or the other. (Or an output, which I guess is going to be the most
common case...)
The page also claims that you can configure an output pin to drive an
LED so that it lights when the pin goes *low*... Man, electronics is weird!
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
> The page also claims that you can configure an output pin to drive an LED
> so that it lights when the pin goes *low*... Man, electronics is weird!
Why is that weird? The LED doesn't care which leg is connected to the logic
output pin, it's just a voltage. You could even connect the LED between two
output pins*, then it would only turn on when one was high and the other was
low.
* Using a series resistor if required...
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
>> The page also claims that you can configure an output pin to drive an
>> LED so that it lights when the pin goes *low*... Man, electronics is
>> weird!
>
> Why is that weird? The LED doesn't care which leg is connected to the
> logic output pin, it's just a voltage. You could even connect the LED
> between two output pins*, then it would only turn on when one was high
> and the other was low.
I always thought of it as logic high meaning "connected to the +V rail"
and logic low meaning "not connected to anything". From the looks of
things, it seems that actually logic low means "connected to the -V
rail". So it's like a bipolar setup...
> * Using a series resistor if required...
Yes, that's the other fun thing. Depending on which type of gates you're
using, they supply about 20 mA. But some of the LEDs I'm looking it draw
40 mA...
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Mmm, 74HC00, 79p. That's quite cheap...
sound quite so cheap any more. o_O
Remind me, *why* am I embarking on this crazy project? ;-)
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Am 19.05.2010 13:07, schrieb Invisible:
> Ah yes. That's the fun thing about doing digital electronics. You can't
> just *buy* a 7400. No, you have to decide whether you want a 74LS00 or a
> 74HC00 or a 74HCT00 or...
>
> Reading the datasheets is like walking into another world. (Especially
> since the datasheets are usually poorly-scanned grainy PDFs that don't
> match the part number you actually asked about!)
Texas Instruments has a nice collection of 74xx data sheets in a single
proper PDF file at <http://focus.ti.com/lit/ug/scyd013b/scyd013b.pdf>
("Digital Logic Pocket Data Book"), apparently covering all their
current products (as of 2007) in the 74xx line, from single-gate 5-pin
thingies like the 74x1G00 (single NAND gate) to 100-pin monsters like
the 74x32543 (36-bit registered bus transceivers with 3-state outputs).
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Am 19.05.2010 14:51, schrieb Invisible:
> Yes, that's the other fun thing. Depending on which type of gates you're
> using, they supply about 20 mA. But some of the LEDs I'm looking it draw
> 40 mA...
That's why normally you'd use an additional transistor to drive the LED
(if you have multiple LEDs to drive you may want to use a transistor
array in an IC package).
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
> I always thought of it as logic high meaning "connected to the +V rail"
> and logic low meaning "not connected to anything".
Logic "high" and "low" are just names given to a certain agreed range of
voltages. The difference between "not connected to anything" (floating) and
"logic low" is very important when working with data busses. If you have
several devices all with their outputs connected to a data bus, then only
one device at a time must be driving the data lines to logic low/high, the
rest must set their outputs to floating.
> From the looks of things, it seems that actually logic low means
> "connected to the -V rail". So it's like a bipolar setup...
But don't assume a "logic low" output will be exactly -V, it is only
guaranteed to be within the range of voltages given for "logic low" in the
data sheet.
> Yes, that's the other fun thing. Depending on which type of gates you're
> using, they supply about 20 mA. But some of the LEDs I'm looking it draw
> 40 mA...
What you mean is, they draw *up to* 40 mA? How much they actually draw
depends on what value series resistor you put in there. Using Ohm's law and
the V-I curve of the LED from the datasheet you can work out exactly what
resistor size you need for a given current.
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Am 19.05.2010 15:29, schrieb scott:
>> I always thought of it as logic high meaning "connected to the +V
>> rail" and logic low meaning "not connected to anything".
>
> Logic "high" and "low" are just names given to a certain agreed range of
> voltages. The difference between "not connected to anything" (floating)
> and "logic low" is very important when working with data busses. If you
> have several devices all with their outputs connected to a data bus,
> then only one device at a time must be driving the data lines to logic
> low/high, the rest must set their outputs to floating.
... except of course in data buses that operate with one dominant and
one recessive symbol (e.g. CAN), in which sending the recessive symbol
is perfectly equivalent to not sending anything - which allows for some
neat tricks to deal with collisions (i.e. the situation where two
devices start sending simultaneously).
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
