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Where is V comming from using 74hCT259 LATCH — Parallax Forums

Where is V comming from using 74hCT259 LATCH

metron9metron9 Posts: 1,100
edited 2005-10-03 21:03 in BASIC Stamp
I was playing with a 74HCT259 tonight support.radioshack.com/support_supplies/doc14/14824.htm

I hooked up +5 at the VCC pin 16 using a 470 ohm resistor
Fed the address with stamp pins and toggled the data input with a fourth pin
Blinking 8 leds to test out the chip.
when I unplugged the VCC from the positive voltage the thing still works

So question is, without the VCC on pin 16 not hooked up is the voltage comming from one of the stamps pins?


www.semiconductors.philips.com/acrobat_download/various/HCT_FAMILY_SPECIFICATIONS.pdf
www.semiconductors.philips.com/acrobat_download/datasheets/74HC_HCT259_CNV_2.pdf

Question 2, I am not sure how much current the pins can source/sink as the radioshack data sheet says 20mA and it looks like only 4mA on some of the other datasheets.
Please help me read the datasheets as far as mA current for the full 8 pins source and each pin correctly as it is the only way I can advance in using components without specific circuits designed by someone else.

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2005-10-03 07:19
    metron9 -

    Answering question two first, according to my reading of the Philips data sheet (the Radio Shack one is ANCIENT!) the DC output current capability is 25 mA, source or sink. Whenever you see a data sheet with the term "fanout" on it, as the Radio Shack data sheet does, go looking for a more recent data sheet. That's a dead giveaway as to how old it really is.

    Insofar as question one is concerned, there isn't enough detail in the data sheet to clearly indicate how that might occur. My guess is that the address pins ("Stamp pins") are indeed permitting current flow to the output section. I suspect that's merely leakage current however, which should be quite limited. Often leakage current is a measured parameter, and is indicated as one of the IC's parameters on the data sheet, but such seems not to be the case with this chip.

    Beau may be better able to answer question one, since he has dealt with the internals of IC's in a good deal of detail.

    Regards,

    Bruce Bates
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-10-03 12:33
    In an IC there are many transistors connected to the source and ground, it is possible under the right circumstances to power a chip through another pin besides the Vcc (most typically though an output pin, but special care must be taken that the chip never tries to drive the pin low, otherwise a short will occur inside the chip and it will destruct). Some people have designed battery backup for some microcontrollers by using this property, but if you don't understand the circuit inside the chip and its particular method of operation, you should not attempt to do this because you are seriously risking destruction of the chip. As an aside you should not place a resistor in series with the power supply, this increases the source resistance and cause problems with the operation of the chip, especially when switching at high speeds. Also you should never intentionally disassemble a circuit when it is still powered, alway cut the power to the circuit before making any changes.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    ·1+1=10
  • metron9metron9 Posts: 1,100
    edited 2005-10-03 13:37
    After getting up this morning your point about the resistor came to mind, I was thinking that only one of the 8 pins would be high at any one time so I could use one resistor on the vcc input instead of 8 resistors. Yes switching speeds has a direct relationship with voltage as I have learned with playing with fuses and clock speeds on other microcontrollers and I just did not think as this chip is not a microcontroller but it is still a device. As far as leakage I am going to put a scope on it tonight I ran out of time last night as it was the power I pulled from the vcc on the chip when I noticed one led was still on so the V came thru the BASE of the transistors because the DATA PIN 13 was HIGH.

    So that's it then, as you said the internal components of the chip are transistors, I realize I have no resistor going to the data input, Obviously if I did this with one transistor I would eventually blow it up, and lucky I just blinked the leds in the experiment I was doing or I would have fried this one (0.98 cents ) but I would also imagine the stamp could be damaged as well if the chip fried in this way.

    So back to resistors. I have 3- 10k resistors pulling the three address inputs to ground, again the stamp pins are tied directly to the address input pins, Should I use resistors hear as well or diodes to protect the stamp pins?

    I will hook up just 2 leds using 2 resistors on the output side to continue experimenting with this chip but I would like to know what I need to look at on the datasheet to make a decision on what size resistor to use on the data input pin 13, and what on the datasheet would tell me if I need resistors on the three address pins.

    Looking in depth on a simple device should help ring some bells for me I would hope.



    Ground
    >10k/\---->[noparse][[/noparse]stamp pin]----> address Input of chip
    or

    Ground
    >10k/\---->[noparse][[/noparse]stamp pin]----? xk/\ or diode?
    > address Input of chip
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-10-03 18:44
    You really should not put any resistance on the chip's VCC or GND leads. This is an HCT chip. It expects to connect to +5 and GND with NO resistance in between. The chip is designed to supply current to its output leads via switching transistors inside, that expect NO resistance to VCC or GND.

    It looks like you're trying to second-guess the designer of the chip, by putting current limitation on the VCC lead. There's LOTS of stuff inside that chip -- registers, latches, etc -- and you may current starve them. In any event, your LED's each need their own current limiting resistor -- that is NOT a function of the chip.

    Bottom line -- the chip requires certain things, for it to do its function. Namely, a low-resistance connection to VCC and GND (also known as a wire). Only then do the truth tables for the chip work as advertised, and the specifications for the chip apply.
  • metron9metron9 Posts: 1,100
    edited 2005-10-03 21:03
    Somebody said...
    It looks like you're trying to second-guess the designer of the chip,

    Not at all, I simply just dont know what I am doing most of the time.

    But this actual experience doing it wrong (no smoke yet) even though it actually works (I can turn on any of 8 LED's by setting up the 3 input pins) I understand it is not correct and I will make the changes outlined above.

    Thank's for the help on this one smile.gif

    I just got in the mail a ULN2803 Darlington Driver (I was waiting for this when I happened to spot the afor mentioned 74HCT259 on the wall at my local Radio Shack store while I was picking up a rechargable battry+charger for my Stamp (it's getting lots of use).
    Also in the box MAX7219 and a 4N28 Phototrans Optocoupl and a few crystals and caps to play with on my other microcontrollers as well.

    Should keep be busy for a few days.
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