Hi Art,
Yes, you'd need 8 PNP or P-FET row driver transistors but then you could get rid of the 18 column driver transistors --> fewer transistors, more current limiting resistors on the columns now instead of the rows, and more than double the "average" LED current (and brightness).
If you've already committed to a PCB design then I would agree it's a bit late for this hardware change, but then again, if you're designing a new PCB then why would you limit yourself to the design that just fits on the prototyping board that you happened to have?
Here's my (unsolicited) idea for a modular system that will support a single 8x8 module or up to eight daisy-chained 8x8 modules. Start by designing a single 2.1"x2.1" PCB for those large 2.2-inch 8x8 matrix displays which can be used either as a "master" 8x8 display module (with cpu) or as a daisy-chained 8x8 display module. Install a PIC, column driver IC, and row driver transistors on the first 2.1"x2.1" PCB for a stand-alone "master" 8x8 display module. Install just the column driver IC on an additional 2.1"x2.1" board and daisy chain this new 8x8 module to the first module for an 8x16 display. Likewise, add additional modules as desired. The modules connect via a simple 10 pin powered buss (plus +5V and GND lines) and the 1/8th (12.5%) duty cycle remains unchanged wether you're using a single 8x8 module or all eight 8x8 modules.
In summary, a single relatively inexpensive 2.1"x2.1" PCB can be used as the basic building block for a high performance full brightness mono-color display of 8x8 up to 8x64 in size.
Kind regards, Mike
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