Interfacing EPROM -> LED - Page 1
Forum section: electronics / projects

author: SgtSiff
author_rank: Contributor
timestamp: April 12, 2014, 07:45:39 pm
content: HiHow 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

author: dannyf
author_rank: Super Contributor
timestamp: April 12, 2014, 08:01:50 pm
content: Yes, you can connect them. I usually use copper wires but pcb traces will work as well.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2014, 08:02:31 pm
content: What are you trying to do?

author: abyrvalg
author_rank: Frequent Contributor
timestamp: April 12, 2014, 10:35:55 pm
content: 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.

author: David_AVD
author_rank: Super Contributor
timestamp: April 12, 2014, 11:31:41 pm
content: Quote from: SgtSiff on April 12, 2014, 08:08:33 pmSo 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?

author: David Hess
author_rank: Super Contributor
timestamp: April 13, 2014, 07:33:50 am
content: Quote from: abyrvalg on April 12, 2014, 10:35:55 pmUsing the high efficiency displays that are available now rated from 1 to 4 milliamps per segment would make this a non-issue.

author: SeanB
author_rank: Super Contributor
timestamp: April 13, 2014, 09:18:50 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: April 13, 2014, 07:38:29 pm
content: 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.pdfhttp://www.ti.com/lit/ds/scas171b/scas171b.pdf

author: abyrvalg
author_rank: Frequent Contributor
timestamp: April 13, 2014, 09:47:13 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 14, 2014, 02:46:18 am
content: 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.