An elegant way to reverse 8 bits on the smaller chips?

[SgtB]

Hello everyone,

I'm trying to port Hippy's DS1302 code to an 18M2. Basically the way you pull data out of the DS1302 is to do a REV 8, and then BCD2BIN on each byte. I've figured out BCD to BIN, and BIN to BCD substitutes for the X2 commands.

I can replace REV with my included code, but I'm wondering if anyone knows a better way. My bit math is a little basic. Thanks!

Code I'm replacing: 
secs  = secs  Rev 8 : secs  = BcdToBin secs


My REV replacement: 
bit8 = bit7
bit9 = bit6
bit10 = bit5
bit11 = bit4
bit12 = bit3
bit13 = bit2
bit14 = bit1
bit15 = bit0

 

[Buzby]

Hi sgtB,

Sorry, I don't know a quicker way.

But when we get a 64-bit PICAXE you could try this 3 instruction routine http://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64BitsDiv

Unfortunately, I have run out of brain cells trying to follow how it works !.

Cheers,

Buzby

 

[Buzby]

Seriously now, I don't know hippy's code, but are the bits pulled in by bit-banging ?.

If so, just bit-bang the other way, then you don't need to reverse.

 

[Buzby]

Just did a forum search on 'DS1302', ( I'm sure you did a search too, didn't you ? ), and found this : http://www.picaxeforum.co.uk/showthread.php?20417-Please-help-with-ds1302-RTC&highlight=ds1302

 

[SgtB]

I'm using HSPI so I have to take what I get. The 1302 is temperamental as well. Here is the link: http://www.picaxeforum.co.uk/showthread.php?14320-Interfacing-20x2-to-that-blasted-DS1302-chip&p=124306&viewfull=1#post124306 Hippy's code is the last post.

I wish you could reference bits with a variable: bit. Oh well. I also should see if it actually runs any faster with all of the conversions compared to bb'ing it. Thanks!

 

[hippy]

b0 = b0 REV 8 : b0 = BCD2BIN b0

b1 = bit0 * 2 + bit1 * 2 + bit2 * 2 + bit3 * 10
b0 = bit4 * 2 + bit5 * 2 + bit6 * 2 + bit7 + b1

 

[hippy]

I wish you could reference bits with a variable: bit

You can achieve "value = bit(index)" with ...

LookUp index, (bit0, bit1, bit2 ... bit31 ), value

 

[srnet]

For an 08M2 use one of these.

 

[SgtB]

b0 = b0 REV 8 : b0 = BCD2BIN b0

b1 = bit0 * 2 + bit1 * 2 + bit2 * 2 + bit3 * 10
b0 = bit4 * 2 + bit5 * 2 + bit6 * 2 + bit7 + b1

Is this just a reverse, or is that the entire operation? Lookup is nice, but I can't index "value". I'll have to time the code and see if a couple of loops are slower or faster than 8 lets.

Thank you for looking at this. I really appreciate the original code you wrote too. It's really the only HSPI code available. I got it working on a 28X1 easy, but am trying to prune all the X code out of it for the M chips without resorting to bit banging.

 

[SgtB]

For an 08M2 use one of these.

Very nice Sir! where can I order a +2M08?

 

[vttom]

I think this is equivalent to bit8=bit7: bit9=bit6 etc..

b1 = bit0*128 + bit1*64 + bit2*32 + bit3*16 + bit4*8 + bit5*4 + bit6*2 + bit7

 

[hippy]

b1 = bit0*128 + bit1*64 + ...

Not quite because PICAXE precedence is strictly left to right so you would actually be calculating ...

b1 = ( ( bit0 * 128 ) + bit1 ) * 64 ....

 

[vttom]

Oh, wait.... I forgot about PICAXE's order of operations... Hippy's example from before was a complete BCD2BIN function with backwards bits...

Well done, sir. Elegant, indeed!

 

[SgtB]

Agreed. Thanks you very much Hippy!