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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Surprising Gotcha when writing to OSCTUNE register |
 Posted: Thu Mar 23, 2017 3:32 pm |
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Here is my code (for a PIC 16F1825, PCM compilier):
| Code: | | OSCTUNE = (ramOSCTUNE>>2); //..high order bits don't matter. Only the lower-order 6 bits. |
where ramOSCTUNE is a byte that has been previously adjusted in the signed range of 0x80 (-128) to 0x7f (+127). But the PIC was not behaving as expected whenever the negative portion of this range was being used. A look at the object code showed why:
| Code: | 0036: RRF 23,W (grabbing ramOSCTUNE and shifting one bit right)
0037: MOVLB 01 (switch to bank to access OSCTUNE)
0038: MOVWF 18 (save intermediate result to OSCTUNE) (what the??)
0039: RRF 18,F (perform remaining shift OSCTUNE in place)
003A: MOVLW 3F
003B: ANDWF 18,F (unnecessary masking of bits 7 and 6) |
It is clear what happened. The compiler assumed that the target of the operation was readable and writable in all 8 bits. But OSCTUNE is not. The high two bits are unimplemented and always read back as 0. So using OSCTUNE to hold intermediate results is not valid. Of course the solution was easy:
| Code: | {
unsigned int temp;
temp = ramOSCTUNE>>2;
OSCTUNE = temp;
}
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which compiles to the very nice
| Code: | 0036: RRF 23,W (Grab and shift ramOSCTUNE)
0037: MOVWF 34 (store in temp)
0038: RRF 34,F (rotate temp in place)
0039: MOVLW 3F
003A: ANDWF 34,F (masking, still unnecessary for my use)
003B: MOVF 34,W (grab final temp)
003C: MOVLB 01 (switch bank to access OSCTUNE)
003D: MOVWF 18 (clean store to OSCTUNE)
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_________________
Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19215
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| Posted: Fri Mar 24, 2017 12:15 pm |
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Not actually surprising, but a nice potential problem when writing to any SFR.
If you do maths 'feeding' a register, the compiler may well use the register as 'temporary' for the maths. It doesn't know there is anything 'special' about the register. In this case it is 7bit, but even worse, bits may turn on other things. Lesson is that except for basic things like &, which can be used to mask individual bits, values should be prepared before trying to feed them to a SFR.... |
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