>Brooke asked:
>>
>>I would like to use a few 74HC595 shift registers to get 24 bits
>>of latched data output.  Could someone help me with the correct
>>

Does this device have a Data Out pin for data that overflows the shift
register?  If so, they can be daisy-chained together: the processor to the
data in pin of the first shift register, the data out of the first to the
data in of the second, etc...  tie all the clock and load lines together,
and rotate all three bytes out on the pin.  Something like this:


  mov COUNTER, #24               ;we're going to rotate 24 bits
LOOP
  rl  BYTE3                      ;rotate byte 3.  MSB goes into carry bit.
  rl  BYTE2                      ;carry goes into LSB, MSB goes into carry
  rl  BYTE1                      ;same idea
  movb C,DATAOUTPIN              ;move carry bit to the output pin
  setb CLOCKPIN                  ;strobe the clock pin
  clrb CLOCKPIN
  djnz COUNTER,LOOP              ;repeat for all 24 bits
  setb LOADPIN                   ;strobe the load pin
  clrb LOADPIN


This is written in Parallax's version of the instruction set but shouldn't
be too hard to follow.  the 'djnz' instruction is decrement and jump if not
zero, and like many of the mnemonics compiles into several opcodes.
Obviously, BYTE1, BYTE2, and BYTE3 are the data bytes to shift into the
shift registers, and DATAOUTPIN, CLOCKPIN, and LOADPIN are the output pins
that go to the data, clock, and load inputs to the shift register.  COUNTER
should be self-explanatory.

No guarantees on the code as written, it's off the top of my head, but the
idea is sound-- I used it to drive a Maxim 7219 LED display driver, and
it's blinking away here next to my desk.  (And the temperature in my
bedroom is 70 degrees F, if you are at all curious.)

Hope this is useful to someone.

-Matt


  "DOS Computers manufactured by companies such as IBM, Compaq, Tandy, and
millions of others are by far the most popular, with about 70 million
machines in use wordwide. Macintosh fans, on the other hand, may note that
cockroaches are far more numerous than humans, and that numbers alone do
not denote a higher life form."