Greetings Andy/Andrew,
arader wrote: what do you mean by not needing the dual capacitor arrangement, but that I should add more decoupling capacitors? Do you just mean I can get rid of C2, but that I should add more caps elsewhere? Where would I need them most?
Your diagram shows two caps in parallel and close together.
One larger in capacitance. This arrangement is popular in
wide-band analog circuits where the small ceramic cap
operates on high frequency, because the large electrolytic
is assumed to have poor HF performance. This is old school
but valid. Improvements have been made in electrolytics
and with much lower voltage supplies (hence physically
smaller) they operate well up to HF.
Going further, the caps should be spread around the PCB,
and associated with the ICs that draw most current. The
caps serve as a local supply of current, and counter the
effects of long PCB runs or wiring (which you will certainly
have with daisy chained modules).
Place one small footprint electrolytic cap at the power
and/or ground pin of each IC, well at least the AVR and
the high-side and low-side drivers. That's three caps.
You can always remove them after the proto works -
adding extra caps (if needed) is a pain.
arader wrote: When does pulse current come in to play? Is this just the max amount of current I can supply in short bursts?
Yes. Which will be much of the time in a MUX'd LED driver.
The high-side and low-side drivers will take gulps of current,
with little or no restriction at each transistion. Even if the
MUX is cycling at a few hundred hertz (to stop flicker) the
edge speed will be micro-seconds (or faster) causing EMI
in the megahertz range, which could couple into the AVR
or corrupt data in the daisy-chain.
arader wrote: yep, that's a typo :oops: should be 74HC595.
Then it's pushing the limits. The HC family is not intended for
sourcing much current. The spec is 6 to 8mA per output.
Here's an important article on the
twenty or so different sub-familiesof logic.
The AC and ACT family is superior as it can source 24mA, but
you need equality with the low side driver and ability to hit
the 100mA peak of the LED matrix.
This is my position, others will disagree. I believe if you drive
the LED to the max allowed you can always back down (with
either PWM or larger ballast resistors). BUT design it too weak
and you are stuck with a dim display (which might be good
for a bedroom clock but not for a daylight readable sign).
arader wrote: first, when you say high/low side driver, your talking about the 74HC595 attached to either anode or cathode respectively, yes? I share your concern for the high-side driver, and this is what I have been thinking so far: Since I'll be switching through the rows 1 at a time, the high side driver (column driver) will only have to power 1 led per output.
In your scheme how many LEDs could be lit at any moment?
I think it's eight (a whole row or column - the labels are just
semantics - the matrix is square). If you think it's only one
LED, then pulse current has to be eight time higher,
and the pulse limit on the LED is 100mA, so the average
per LED is now just 100/64 or 1.6mA (not enough).
arader wrote:I'll be using TI's 74HC595 Looking at page 5 it states "Continuous output current, IO (VO = 0 to VCC)" ±35 mA". So this should work, yes? also, looking at sparkfun's serial backpack they used the similar 74LS595s: http://www.sparkfun.com/datasheets/Comp ... ematic.pdf
You are reading the Absolute MAX ratings and applying
them to typical operation. It may work, it may not. It may
work today but not tomorrow... The typ ratings for that part
are only six milliamps per output!
For the low side driver consider the power logic version
TPIC6C596N, which has 100mA per output.
I happen to be constructing a display at the moment
that uses six, five by seven LED, matrices. I have the
prototype working with 10mA average LED current,
and 50mA peak current (it scans in blocks of five, not
eight as you have). Let me know if you'd like to see
the schematic and/or PIX of the hardware.
Comments Welcome!