[sdiy] 7-segment LED displays with 4511 driver chip - max current?

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sat Aug 8 23:27:15 CEST 2015

On Aug 8, 2015, at 10:43 AM, Tim Ressel <timr at circuitabbey.com> wrote:
> My go-to for this would be a 74HC595 shift register. I would multiplex the displays, either with 2 '595s or with a single and use 3 proc lines to drive the display common pins. Multiplexing might be off-putting if you haven't done it before, but it is really quite easy.  '595s are groovy because you can clock them fast, and you can use SPI or bit-banging to drive them. With an Atmel AVR at 20 MHz and SPI you should be able to get in and out of the display routine in a few micro seconds. Also with multiplexing only one display is on at a time, thus reducing total current draw.
The '595 is certainly popular, but not without drawbacks.

Example: When scanning a multiplexed matrix of LEDs, each step requires that either all Rows be disabled or the whole Column must be disabled before the next Row and Column are enabled. This is a synchronous operation, because you have to wait for the disable to take effect before you enable the next set, otherwise you see ghosting of incorrect LED coordinates. The problem with those shift registers is that it takes some number of cycles before the value written to the SPI peripheral actually appears on the chip's outputs. This usually means that you have to burn CPU cycles before you can take the next step (enable must wait for disable to finish).

However, if you have an interrupt-driven queue for an SPI shift register, then you could feasibly queue up a sequence of commands that would work, but you end up with more interrupts per matrix scan step.

Bit-banging is certainly a waste of CPU cycles, so I hope that a new hardware design would not rely on that technique.

Often, there are spare CPU cycles available to be wasted, so in some sense it doesn't matter. But whenever you're talking about a battery-powered device, or even a USB-powered device running from a laptop, the wasteful circuit designs just end up draining your battery faster.


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