number of PIC pins & 7-seg display

[archendekta]

I'm new to using the 7-seg led display and from what i have seen so far, it looks like it requires a lot of pins from the PIC. is there a way to lessen the number of pins used to drive the display?

[sayzer]

If you use 74HC595, then you need 3 (three) pins to drive a display.


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[rsocor01]

You can use multiplexed LCDs. The number of required pins can go down by 1/3rd or more depending on the bias you are using. If you choose a multiplexed LCD you will have to use a PIC that has LCD driving capabilities like the 16F914.

Beware, that these multiplexed LCDs must of the times are custom made parts.

Robert

[elec_mech]

archendekta,

How many seven segment displays do you want to control? If you only need one or two, use a BCD (binary coded decimal) to seven segment IC. Check out this circuit:

http://www.aaroncake.net/circuits/counter.asp

You would replace the 74LS90 with four pins from your PIC to input BCD to the 74LS47. If you're just counting up and resetting, then you could use the entire circuit from above and just use one pin to send a pulse and count up one digit at a time.

Now, if need to control several displays, I'm using a MAX7221 which can control up to 8 common cathode digits simultaneously.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339

It is controlled similarly to the 74HC595 through SPI communication which requires only three pins from the PIC.

If you need to keep it on the cheap side, you can use a 74HC595, but you may need to use transistors or MOSFETs to feed the pins of your seven segment digits if they require more current or possibly voltage than the '595 can handle. An alternate IC is the STP16DP05 or STP08DP05. They are designed to drive LEDs at a set current (so all the segments will be have the same brightness) and allow you to use up to ~20V which you'll need if you decide to use big seven segments, plus they use SPI communication. Another even cheaper IC is the A6278EA-T. I have not used this one, but it about half the cost of the STP16DP05 and is rated up to 17V.

Hope this helps.

[Byte_Butcher]

I like to drive 7-segment displays with a CAT4016 constant current LED driver.
http://www.catsemi.com/datasheets/4016.pdf

One CAT4016 has 16 output channels, so it can drive two 7-seg displays with decimal point. It takes 3-4 pins from your PIC to drive it (Serial In, Clock, Latch, and Blank if needed) and many units can be daisy chained together.

Here's a board with 2 CAT4016's on the back and a 4 digit LED display on the front...






Steve

[rsocor01]

Ooops, I thought you were asking about LCDs. Anyways, there are multiplexed LEDs, which use a lot less pins. I like driving the segments directly from the pics using BJT transistors. Maybe it's not the best way but it's simple and uses less components.

Robert

[revelator]

I'm looking at maxim 7219 which I believe can drive up to 8 digits, 8 segments using only 3 pins (load,Din,clk) from a pic say RB0-Rb2 on a
16f84

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339

[Archangel]

I'm looking at maxim 7219 which I believe can drive up to 8 digits, 8 segments using only 3 pins (load,Din,clk) from a pic say RB0-Rb2 on a
16f84

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339

Hi archendekta,
You can buy an LED display that uses SPI or Shiftout commands on 3 pins from BRUCE at RENTRON, and others . . . http://www.rentron.com/SLED-C4.htm Or just buy the driver chip from him.Here is another brand from China.http://cgi.ebay.com/1-5-Character-He...3A7%7C294%3A50
and they can be daisy chained.

[archendekta]

wow, a lot of great input here to keep me thinking and tinkering. my display looks like the one in "Byte_Butcher's" post.

my display: ltc-4727jr
my pic: 18f2620 i/sp #although i don't [have] to use this model

a friend dropped by a cd74hct138e 3-to-8 decoder saying it would help my pin usage issue. we'll see...

many thanks to all for your input. i'll study all the suggestions and links and see what i can do. thanks again for your time.