Interfacing EPROM -> LED

[SgtSiff]

Hi

How would I go about connecting a seven segment display to an EEPROM (AT28C), can I connect it directly, or do I need to use a transistor?

Thanks

[dannyf]

Yes, you can connect them. I usually use copper wires but pcb traces will work as well.

[Zero999]

What are you trying to do?

[abyrvalg]

Looks like AT28Cxx's max output current is around 4mA - not enough to drive typical LED at full power. Either use transistors or just try limiting the current to 4mA and check if the brightness is still suitable for you.

These unusual EPROM usage tricks were cool in the pre-MCU past: sound players with counter for addresses and resistor ladder DAC at the outputs, state machines with several data lines fed back to addresses.

[David_AVD]

It's just for college work that's all. We are supposed to demonstrate consolidating a logic circuit to eeprom and I'm doing a BCD 7 segment decoder. I've never used an EEPROM before so I wasn't sure if they could sink/source any current.

So you didn't bother to check the data sheet before asking here?

Sorry to sound harsh, but why do some people not check things themselves before asking here?   

[David Hess]

Looks like AT28Cxx's max output current is around 4mA - not enough to drive typical LED at full power. Either use transistors or just try limiting the current to 4mA and check if the brightness is still suitable for you.

Using the high efficiency displays that are available now rated from 1 to 4 milliamps per segment would make this a non-issue.

[SeanB]

As you are going to drive a display simply use a pair of 4050 buffers to provide the required current. If you do not want to, and are willing to drive the LED's with 2mA per segment then arrange using a common anode display and use the device to sink current. As it is TTL compatible it can sink a lot better than source, so program the outputs in your logic table to be active low and use it that way. It is not quite proper TTL, which can do 24mA per output, but is capable of driving a single LSTTL load of 2.1mA. probably will do 5mA but not with the output voltage being a valid logic level any more but for a LED it is no worry.

[Zero999]

Use a line driver. Watch the current through Vcc or GND doesn't exceed the maximum rating.

Here are some examples:
http://www.fairchildsemi.com/ds/74/74VHC541.pdf
http://www.ti.com/lit/ds/scas171b/scas171b.pdf

[abyrvalg]

The problem with datasheet is that there is no max output current specified. But there is a total Icc of just 40mA and some output voltage specified for 2mA and 4mA, so it looks safe to assume those 4mA but definitely not something like usual LED's 20mA (note the total Icc). The fact that it is a CMOS device with a primary function to output data to another logic device votes for that too. I would either use a conservative estimation (adding an unnesesary driver wouldn't fry anything, while not adding a nessesary one will fry) or ask Atmel support.

[David Hess]

The maximum Icc listed is the maximum operating current without any output loading so it is not helpful and the 2 milliamp Isb standby current is even less helpful.

The output low voltage specification of 0.4 volts at 2.1 milliamps just represents the maximum low state voltage at that current.  The typical value will probably be half that.

I would just use a high efficiency LED display and drive it directly with a current limiting resistor for each segment.  It will be plenty bright and the displays are inexpensive.