John De Villiers wrote:

> Let me see if i understand this right.
>
> When cascading 2 MAX7219's and you want to send data to the second one in
> the chain you need to do the following
>
> 1) take load/cs low
> 2) send your 16 bits of data
> 3) send 16 bits of no-op - all zero's
> 4) take load/cs high
>
> Is this right?  I cant make out if i should leave load/cs high or
> low when i
> start sending the no-ops. Datasheet isnt all that clear on that one.

What you are saying is basically correct.

Data is serially clocked into (and also out of) the 7219 all the time the
clock is present.  It doesn't matter if the Load line is high or low (the
later 7221 has a chip select (CS) instead of the simple Load which must be
low to make the clock active).  The data is only transferred to the display
at the low to high transition of the Load line (or CS) - see the functional
diagram in the data sheet.

If you serially interconnect the 7219's and use common Load & Clock lines,
then to write to the second driver do exactly what you said.  You could, of
course, just re-write the data in the first driver at the same time rather
than use NOP's, it's your choice.

Using a PIC to drive the 7219 is simple.  The Clock & Data lines can
obviously be multiplexed with other functions.  Don't forget that you need
to set up the housekeeping functions e.g. number of digits, mode, brightness
etc before you can use the display and I would suggest that you update these
values regularly, particularly in a noisy environment.

Hope that provides some clarification.


Dave