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RichardF
Registered Member

Date Joined Mar 2007
Total Posts : 154

Posted 3/28/2008 4:47 AM (GMT -7)

I need to be able to read and write individual bits within a byte variable in order to keep track of eight switch positions (on/off). This is easy to do in pBasic using the .BITn form, but I don't see an easy way (or frankly any way) to do this in Spin. Would appreciate some suggestions.

Thanks,

Richard

Mike Green
Registered Member

Date Joined Oct 2004
Total Posts : 15579

Posted 3/28/2008 6:25 AM (GMT -7)

byteVar &= !Mask ' To set a bit off
byteVar |= Mask ' To set a bit on
byteVar ^= Mask ' To toggle a bit
if byteVar & Mask ' To test if a bit is on
where
CON Mask = %10000000 ' or something similar

Rayman
Registered Member

Date Joined Jul 2007
Total Posts : 3127

Posted 3/28/2008 6:27 AM (GMT -7)

Usually, I'd do something like

Mask:=1<<bit

to make a mask

tekochip
Registered Member

Date Joined Feb 2007
Total Posts : 56

Posted 3/28/2008 6:33 AM (GMT -7)

Here's some examples with functions to do the same:

CON
#0, Serve, Dispense, Flash 'Declare some bits

PUB Demo |coils

OnBit(@coils, Serve)
ToggleBit(@coils, Flash)

if TestBit(@coils, Flash)
OffBit(@coils, Serve)

PUB OnBit(x, bit)
{{Turn a bit on}}
long[x]|=1<<bit

PUB OffBit(x, bit)
{{Turn a bit off}}
long[x]&= !(1<<bit)

PUB ToggleBit(x, bit)
{{Toggle a bit}}
long[x]^=1<<bit

PUB TestBit(x, bit)
{{Return true if a bit is on}}
return long[x] & (1<<bit)

RichardF
Registered Member

Date Joined Mar 2007
Total Posts : 154

Posted 3/28/2008 7:41 AM (GMT -7)

Thanks everyone. Makes me appreciate pBasic (smile)
Richard

hippy
Registered Member

Date Joined Feb 2007
Total Posts : 1951

Posted 3/28/2008 8:02 AM (GMT -7)

For reading bits you could create a method which handles that, "var.bit4" becomes "bit4(var)", and option which is consistent and works for read and write is "GetBit4(@var)" and "SetBit4(@var,bitVal)", something like ...

PUB GetBit4( varPtr )
result := GetBitX(4, varPtr )
PUB GetBitX( bitNum, varPtr )
result := ( byte[ varPtr ] & |< bitNum ) <> 0

etc

Ariba
Registered Member

Date Joined Apr 2007
Total Posts : 712

Posted 3/28/2008 8:57 AM (GMT -7)

Another idea is to use the outb and dirb registers as bitaddressable variables (Prop 1 has this registers but no PortB hardware):

1)
  outb[ 4] := 1
  if outb[ 5] == 1
    outb[ 4] := 0
2)
  outb[ 4]~~
  if outb[ 5]
    outb[ 4]~

1) is better readable and 2) is more optimized.
You get up to 64 bits with this methode. And if you don't use the timers, also frqa, frqb, phsa, phsb are good for bitaddressing.

Andy

Edit: the space after [ is only for the forum software

Post Edited (Ariba) : 3/28/2008 4:02:15 PM GMT

RichardF
Registered Member

Date Joined Mar 2007
Total Posts : 154

Posted 3/28/2008 9:40 AM (GMT -7)

Two more great ideas, thank you.
Ariba, your use of the b registers is really cool. I have always assumed they were not addressable, since there are no associated pins.

Beau Schwabe (Parallax)
IC Layout Engineer

Date Joined Aug 2004
Total Posts : 3967

Posted 3/28/2008 10:07 AM (GMT -7)

Ariba,

outb is ok to use on the Current Propeller Chip, but it will not be backwards compatible on the next Propeller Chip

Instead, if you are not using any or one of the counters, you could use the frqa, frqb, phsa, and phsb registers to do basically the same thing that you are suggesting.

Stay away from the ctra, and ctrb registers as you could startup one of the counters without intending to if you write to them.

Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Rayman
Registered Member

Date Joined Jul 2007
Total Posts : 3127

Posted 3/28/2008 10:43 AM (GMT -7)

Ariba, Beau,

I like this idea! I rarely use frqa, frqb, phsa, or phsb in the main cog. So, this is a very nice way to access bits!

Javalin
Got a Propeller, need some SPIN?

Date Joined Jul 2004
Total Posts : 768

Posted 3/28/2008 12:09 PM (GMT -7)

Chaps,

Surely:

if byteVar & Mask ' To test if a bit is on
where
'CON Mask = %10000000 ' or something similar

should be

if byteVar & Mask == Mask ' to test if a bit is on

so - if byteVar is %00001111 & %00001000 would leave %00001000 so surely wouldn't equate to TRUE/FALSE or 1 or 0?

??

James

Post Edited (Javalin) : 3/28/2008 7:37:55 PM GMT

Rayman
Registered Member

Date Joined Jul 2007
Total Posts : 3127

Posted 3/28/2008 1:10 PM (GMT -7)

I think zero == false and non-zero == true...

Paul Baker
Registered Member

Date Joined Jul 2004
Total Posts : 6323

Posted 3/28/2008 7:45 PM (GMT -7)

correct, just like C, false is 0 and anything else is true, so it is equivalent to "if (byteVar & Mask)<>0"

Paul Baker

Propeller Applications Engineer

Parallax, Inc.

Post Edited (Paul Baker (Parallax)) : 3/29/2008 2:52:31 AM GMT

Javalin
Got a Propeller, need some SPIN?

Date Joined Jul 2004
Total Posts : 768

Posted 3/29/2008 2:12 AM (GMT -7)

ah - ok!

J

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