Bit masking on WAITPEQ

Sniper King

In the case of WAITPEQ what would be the bit mask for 10

Is it 1010 or is it 1000000000
considering the example has

waitpeq(%0100, %1100, 0) 'Wait for P3 & P2 to be low & high

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·- Ouch, thats not suppose to be hot!··


Michael King
Application Engineer
R&D
Digital Technology Group

Paul Baker

The easiest way to form bitmasks is using the |< operator. Say you want to mask P7 and P3, the mask would be := |< 7 | |< P3. Also make sure that your state variable does not have a bit set in any position in which the mask for that bit is 0, this will cause the cog to hang.

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

potatohead

The bits are associated with the pins, one for one. So, pin 10 (assuming that's decimal and not binary) would be the 10 bit!

10_0000_0000 = 512 = 2^9.

A series of masks for the pins 0 through 3 would be:

0001 = 1 = 2^0
0010 = 2 = 2^1
0100 = 4 = 2^2
1000 = 8 = 2^3


What Paul is doing is using SPIN shortcuts to shift a single bit over X times, and OR'ing that with other single bits shifted X times.

That's the "<" operator above, followed by the number of shifts.

Set a bit to a 1 and you've impacted the pin associated with that particular bit, one bit per pin, for a total of 32 bits and 32 pins!

If you want pins 1 and 2, then it's this:

0110 = 2 + 4 = 6.

The addition is like an OR, in that adding single bit values (powers of two) together sets those bits in the mask.

0100
or
0010
equals
0110

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Post Edited (potatohead) : 8/28/2008 10:22:25 PM GMT