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Use of enable on intelligent LED displays. Answered

I've found some early intelligent LED displays (datasheet here, I have confirmed I have the decimal-only variant) and have been trying to load digits onto them using the enable input. Currently, they will display whatever value is applied to the input pins when power is applied, but changing the value requires interrupting power to the display so it restarts with the new value.

When enable is pulled to 0V, the display turns off and will not turn back on until enable has been released and the display has been restarted. The rise time of the power supply seems to be important in whether the display will start properly, as the display will usually not start without at least a10uF capacitor on the power supply.  Also, the decimal point is non functional.

How do I get the decimal point and the enable input to work so that I don't have to switch the whole display on and off?

Thanks in advance for any ideas.

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user
steveastroukBest Answer (author)2012-08-26

The RISING EDGE of the Enable signal is the thing that does the enabling, not the LEVEL.

Steve

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user

The display doesn't seem to care what the enable pin is doing, and nothing appears to happen when the pin transitions from low to high or from high to low other than the display going off when the pin is low (and staying off until the display is restarted. Perhaps it might respond better to the signal from a microcontroller than manually connecting wires?

I'm fairly sure that the problem is not on the display, as all 14 of them behave the same.

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user

I've never driven them from manually connected wires....

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user

I've been connecting each data pin either to +5V or to 0V using jump leads to input values. However, using this on the enable pin causes the display to turn off when tied to 0V. I don't think there should be any problems with this, as the signals will be the same as from a microcontroller.

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user

Have you put pull downs on all the inputs ?

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user

10K pull down on all data and enable pins, although a couple of the displays seem to need a bit more to prevent one of the pins being read as high, so I'll try changing them all to 1K tomorrow. Other than that, everything seems to be behaving now with the debouncing capacitor, so I just need to confirm that the problem has been resolved by driving the displays with a microcontroller.

Thanks for all the help so far.

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user

220uF a BIT big for a debounce BTW. I'd try 100pF !

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user

Just tried it with 1K pull down on all pins and a clock signal from a microcontroller and the displays are now fully functional.

Thanks for all the help.

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user

I thought it might be a bit excessive, but 10uF is unreliable, and 0.1uF fails to operate at all. I'll see what it does with a clean signal from a microcontroller.

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user

I've just put a 220uF debouncing capacitor an the enable pin to test the microcontroller theory, and the display seems to be working. It doesn't appear to be due to the capacitor causing the supply voltage to drop out when connected, as switching it in and out of the power supply does not have an effect.

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