multiple 7 segment displays

[Peter Truman]

Hi All - I have a new project which requires I use small 7 segment led displays. The project has 2 forms, form 1 requires 4 sets of 4 digit displays and for 2 requires 8 sets.

So - each display board has 4 x 7 segment displays (8 if you count the dp) plus 3 discrete indicator led's

Each set will be built on a 'plug in' PCB on a main PCB that has a number of M12 PBC mount sockets - there will be a main PIC on the base board that will handle a range of sensing tasks, plus a rotary encoder / menu system.

I'm thinking about how best to drive my 4 digit displays - Each digit will be multiplexed and strobed to display the necessary data.

I've not really been able to identify a low cost IC to handle this for me (Max7219 is A$26 bucks!)

My thinking now is I could use a PCA9535 I/O expander to handle the digits. The main board PIC will fire off an I2C packet to each display board in turn (max 8 display boards) - that would mean the main PIC is pretty busy servicing all 8 I2C signals fast enough to strobe each display. I don't think that approach is going to cut the mustard!

I'm going around in circles a bit with this - now thinking I should add a low cost PIC on each display board - I would just have my main PIC fire off a serial packet (probably SPI) - the PIC on the display board will pick up the packet and then handle the strobing independent of the main board.

Next - it occurs to me - if I'm going to have a PIC on the display board - and all it has to do is handle the 7 segment displays, why not use a higher pin count PIC and do away with the port expander?

I decided I would post the question here and see what more experienced people would suggest!

Many thanks in anticipation

[kcsl]

Well, hardware costs money, takes board space, increases power requirements and typically pushes up development time, so I always try and reduce the hardware as much as possible. There's also fewer points of failure so less to go wrong.

Most of the time, I can afford the CPU cycles to drive the display via an interrupt routine. But on the odd occasion where I can't, I create a chain of 74HC595's (which have sufficient outputs to drive the 8 segments). You can drive the entire chain with 4 or 5 output pins and you don't get dimming due the multiplexing. The IC's are cheap and easily available. I also like this approach because you can drive any number of digits.
The additional 3 indicator LEDs could be driven by another 595 or just 3 PIC output pins.

Regards,
Joe

[shantanu@india]

Hi,
The best way to drive 7-segment common-anode display modules are through 4094 8-way shift registers which can sink current. Reduce the common anode voltage by 1/2 diodes so that you do not need any current limiting resistance between the 4094 outputs & the 7-segment LED. 'Strobe" & "Clock" signals are common & the 4094's are cascaded by data pin. Just 3 pins of the microcontroller are required.