limiting to "no less than" zero

[HankMcSpank]

I'd figured as much (on account everyone was doing it!)....can the default be changed to 0? (I'd imagine that switches get used with weak pullups or external pullups ....so the switch pressed condition would be a 0).

Re Darrel's entry..

if volume.15 then volume = 0 ; zero if it went negative

how does bit 15 of 'volume' being a '1' indicate negative? Can someone shine a little more light on this one please?

[Ioannis]

Related to the "default" just use the ! before your variable:

If !buttonB then.... meaning If buttonB=0 then...

The 15th bit is the sign bit in two's complement. When the most significant bit is 1, it indicates a negative sign. Just Binary maths...

But is relative. If you use 8-bits then the 8th (most significant bit) is considered the sign bit. As long as you are thinking of positive and negative numbers. So at the end you have from -128 to +127 (1000 0000 to 0111 1111).

http://en.wikipedia.org/wiki/Two's_complement

Ioannis

[Byte_Butcher]

Thanks guys!

Well... I guess the answer to my question is that there isn't really a way that's as easy as using "min" to limit the upper end.

The method that Charles posted is what I've been using. But I was hoping for something that was... simpler.
I guess I'd hoped there was some way as clean and easy as "MAX" but that worked all the way down to zero.

Darrels method using volume.15 is great, but it "wastes" half of a variable.
Most of the variables that I need to limit are declared as bytes, and I need the full 255 step range for some of them.
I *can* use it for my temperature setpoint because that needs a word anyway and I can afford to "lose" part of it.

I'm starting to run low on code space on this project so hoping to find more efficient ways to do a few things...

Thanks for the input!

Steve

[Darrel Taylor]

Darrels method using volume.15 is great, but it "wastes" half of a variable.
Most of the variables that I need to limit are declared as bytes, and I need the full 255 step range for some of them.

With 8-bit values, the CARRY flag is valid after addition or subtraction.

So it can be ...

if buttonB then
    Volume = Volume - Vstep
    IF !STATUS.0 THEN Volume = 0   ; Zero if BORROWED
endif

[Byte_Butcher]

STATUS ?
STATUS.0 ?

Ahhh, Darrel, you confuseth me. From whence comes... STATUS ?

steve

[Darrel Taylor]

STATUS is a register in the PIC that contains the results (status) of arithmetic operations.
Depening on the chip family, it may also have Watchdog Timer status and FSR/page bits.

STATUS.0 is the CARRY flag.
After an addition, if the result is larger than 255 the carry bit will be set (1).
After a subtraction, if the result is less than 0 (BORROW), the carry bit will be cleared (0).

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

[Byte_Butcher]

The carry bit is only valid after addition/subtraction with 8-bit values.
After math with WORD values, the carry bit cannot be used because not all of the math is done by the hardware.

OK, curiosity got me so I tried it with WORD values and it still seems to work...


x = 32768
Y = 32769
x = x + y
Result: X = 1, STATUS.0 = 1
'--------------------------------

x = 32768
Y = 32765
x = x + y
Result: X = 65533, STATUS.0 = 0
'-------------------------------------

x = 32768
Y = 32765
x = x - y
Result: X = 3, STATUS.0 = 1
'--------------------------------------

x = 32765
Y = 32771
x = x - y
Result: X = 65530, STATUS.0 = 0
'------------------------------------

I thought it was only 'sposed to work with 8 bit numbers?


Steve

[Ioannis]

If the previous subtraction produced negative results, bit 0 of STATUS is set (to 1). Then you check this condition and if true, set Volume to 0.

By the way which chip are you controlling?

Ioannis

[Byte_Butcher]

Wow... thanks Darrel. THAT is an interesting item to know about.
I'm sure I can make good use of that!

Ioannis, I'm using a 16F887. And thanks for helping explain the STATUS thing!


steve