I'd like to use 7 segment leds and led arrays. Are there any pros and cons regarding using common cathode or common anode devices - ?
Thank you.
I'd like to use 7 segment leds and led arrays. Are there any pros and cons regarding using common cathode or common anode devices - ?
Thank you.
The specs for the uC will tell you the max. current a pin can sink or source. Also consider the max. total current for the chip.
There is an interesting 8 pin latch/buffer chip (see note 1) used by the 2x16 parallel LCD vendors on ebay so that it becomes a I2C Serial LCD. It seems to me that one of those would be very useful in your project unless you are looking for a total low current circuit. If you understand multiplexing and I hope you do, you can use one of each of the common types (cathode/anode). A single bit (see note 2) will let you toggle between the two 7-segment leds (the hex alpha numeric ones, right?). For instance, loading the latch with 0xxx-xxxx (binary) will light up one display's segments corresponding to the bit pattern xxx-xxxx. Likewise, loading the latch with 1xxx-xxxx will light up the other display's segments corresponding to the bit pattern xxx-xxxx (see note 3). I think it is correct to say that you will need 1 resistor for each of the output pins of each buffer chip. The 7 pins of the latch/expander form a "bus" with the two segment led displays listening but only changing state when directed to by the toggle bit. Required refresh rate is 60 Hz (cycles per second). Easily managed by most uCs.
Note 1: http://www.taydaelectronics.com/datasheets/A-905.pdf
Note2: The toggle bit needs to control the correct N (2N3904) or P (2N3906) type transistors to connect the "common" pin of each display to the appropriate power rail. This approach solves a problem with unequal brightness between different segments and/or characters.
Note 3: One of the displays will require that its bit pattern xxx-xxxx be sent out as 1's complement representation. 
Why is that Prof. Nico?
Special Note: Remember: The I2C bus you define from the uC to the expander(s) CAN control more that one!! The 3 bit address of each expander chip is defined by how its pins 1-3 are connected to Vcc or Vdd individually. ie 000 to 111 Isn't that marvelous!
