A little help with shifting out Arrays?

[Ryan7777]

I'll maybe try both ways and break out my logic analyzer. I get confused with the MSB/LSB stuff though. I guess the analyzer captures on the screen the first bit it sees on the left hand side of the screen and goes from there. So we'll see. I just have to think about what numbers to use to get a result I can fit my little brain around. high voltage modulators and vacuum tubes at work to microcontrollers and digital circuits at home. Maybe I should take up wood working as a hobby instead!

[SteveB]

Henrik,
What "low order bits" means in this case is that "\X" will always shift out the x lowest bits. So myByte\4 or myWord\4 or myLong\4 would all just output the 4 lowest bits. Now that the correct bits have been determined, it will shift them out based on the mode. So you can think of "\X" as "Shift out the lowest X bits of the variable"

This is why I'm curious about the actual output of the the shift statement used, which could be understood as, "Shift out the lowest 24 bits in the byte called myArray". As you can see, it doesn't make a lot of sense, but it's going to try to do something, and I am wondering what that something is.

[SteveB]

myArray VAR BYTE[3]

myArray[0] = 1
myArray[1] = 127
myArray[2] = 255

SHIFTOUT PORTB.0, PORTB.1, 1, [myArray\24]

Prediction:
The above code will shift out the following:
00000000 00000000 00000001

With myArray[0] = 127:
00000000 00000000 01111111

With myArray[0] = 255:
00000000 00000000 11111111

The values in myArray[1] and myArray[2] have no effect on the outcome.

[Ryan7777]

Sorry for not getting back to this. I've had my logic analyzer tied up for something else and haven't been able to get back to that project let alone this one due to some things beyond my control. I will however report back when I do get to it. I hate starting a question and not getting to the answer.

Best Regards,

Ryan

[SteveB]

I haven't had a chance to test it either. But my prediction is based on looking at the asm code produced. Although I noticed that I compiled it using PBPLong.

Using the non-long PBP, the results will be a little different, but likely still have 0000000000 00000001 as part of the result, since the internal temporary variable PBP uses is a 2-byte word, and it clears the top byte before entering the shift-out routine, then shifts out 24 bytes.

Bottom line, to do what you want, either of the following will work:
- Use 3 seperate BTYE variables
- 1 BYTE and 1 WORD variable
- 1 LONG variable

There may be some other, more creative way to work out the problem, but these are striaght foward and simple. The code may not look as 'tight', but it will work the way you expect/desire.

SB

[HenrikOlsson]

Hi guys,
I just did some tests here. Here's the testcode:

Leader VAR BYTE
myArray VAR BYTE[3]
Trailer VAR byte

Leader = 170
Trailer = 85

Test1:
  myArray[0] = 1
  myArray[1] = 127
  myArray[2] = 255
  PortB.5 = 0
  SHIFTOUT PortB.6, PortB.7, 1, [myArray\24]
  PortB.5 = 1

PauseUs 10

Test2:
  myArray[0] = 255
  myArray[1] = 127
  myArray[2] = 1
  PortB.5 = 0
  SHIFTOUT PortB.6, PortB.7, 1, [myArray\24]
  PortB.5 = 1

Pauseus 10

Test3:
  myArray[0] = 1
  myArray[1] = 127
  myArray[2] = 255

  PortB.5 = 0
  SHIFTOUT PortB.6, PortB.7, 1, [myArray[0], myArray[1], myArray[2]]
  PortB.5 = 1

Pause 100
END

The idea with the Leader and Trailer bytes was to see if the memory is alocated in the order it declared and if the shiftout would "fall into" those - turns out that's not the case in this example. Here are the captured results:

I was a bit surprised that both test 1 and test 2 gave the same results but it did.

/Henrik.

[SteveB]

I was a bit surprised that both test 1 and test 2 gave the same results but it did.

That IS a bit suprising.

But, it difinately shows that shifting out the array in the original post won't work, but doing it as seperate bytes of the array will work. So that is a 4th method that will work.

Thanks Henrik!