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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Better way to return 16 bits? |
 Posted: Fri Jun 17, 2011 8:45 am |
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Is there any better way to do this in a 18F part (PCH compiler):
| Code: | unsigned int16 ReadEEdata16(int addr)
{
EEADR = addr;
EECON1.EEPGD = 0;
EECON1.CFGS = 0;
EECON1.RD = 1;
_RETURN_ = EEDATA; //..low byte..
EEADR++;
EECON1.RD = 1; //..read high byte..
#byte ReturnHighByte = _RETURN_+1
ReturnHighByte = EEDATA; //..high byte
} |
This compiles very efficiently, but I don't like having to mess with the compiler's internal _RETURN_ register, in case future implementations of the PCH compiler handle 16-bit returns with different symbols. I tried various ways of forming expressions like:
| Code: | | return (((unsigned int16)EEDATA)<<8)+k; |
where k was saved in an earlier step, but they all compiled with many more machine instructions.
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Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Re: Better way to return 16 bits? |
| Posted: Mon Jun 27, 2011 5:31 am |
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Well, since on one has any comments on this question, perhaps I could revise the question a little. What are the precautions, if any, of using the _RETURN_ and _RETURN_+1 compiler pseudo registers to form a 16-bit return value, as opposed to doing a normal C-language "return"? I know that the compiler gives a warning that my function is not void and does not return a value (because it does not see that I have done the equivalent thing by setting _RETURN_ and _RETURN_+1 explicitly). But beyond that, is there any risk in writing code this way? For example, is the compiler likely to change in a way that invalidates my use of this symbol?
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Robert Scott
Real-Time Specialties
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jeremiah
Joined: 20 Jul 2010
Posts: 1319
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| Posted: Mon Jun 27, 2011 6:59 am |
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| Did you try the make16() function/macro? It is usually pretty efficient in most of my programs. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jun 27, 2011 11:26 am |
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I didn't respond because you didn't provide the PIC or the #byte and #bit
statements. If you had provided a small compilable program it would
have been easy to drop it in MPLAB and look at it. |
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RLScott
Joined: 10 Jul 2007
Posts: 465
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| Posted: Mon Jun 27, 2011 7:30 pm |
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| jeremiah wrote: | | Did you try the make16() function/macro? It is usually pretty efficient in most of my programs. |
Ah, yes, that was a lot better than the various expressions I had tried. And even though it is just a tiny bit less efficient than my _RETURN_ method, I think I am going to go with your suggestion just because it avoids that "no return value" warning and is not open to compiler changes regarding the _RETURN_ symbol. For the record, here are both of them. (The part is a 18F2553, and the SFRs are the normal ones dealing with access to the EE data.)
| Code: | unsigned int16 ReadEEdata16_myWay(unsigned int addr)
{
0296 C0A5 MOVFF 0xa5, 0xfa9 EEADR = addr;
029A 9EA6 BCF 0xfa6, 0x7, ACCESS EECON1.EEPGD = 0;
029C 9CA6 BCF 0xfa6, 0x6, ACCESS EECON1.CFGS = 0;
029E 80A6 BSF 0xfa6, 0, ACCESS EECON1.RD = 1;
02A0 CFA8 MOVFF 0xfa8, 0x1 _RETURN_ = EEDATA;
02A4 2AA9 INCF 0xfa9, F, ACCESS EEADR++;
02A6 80A6 BSF 0xfa6, 0, ACCESS EECON1.RD = 1;
#byte ReturnHighByte = _RETURN_+1
02A8 CFA8 MOVFF 0xfa8, 0x2 ReturnHighByte = EEDATA;
}
02AC 0012 RETURN 0 |
| Code: | unsigned int16 ReadEEdata16_make16Way(unsigned int addr)
{
int k;
02AE C0A5 MOVFF 0xa5, 0xfa9 EEADR = addr;
02B2 9EA6 BCF 0xfa6, 0x7, ACCESS EECON1.EEPGD = 0;
02B4 9CA6 BCF 0xfa6, 0x6, ACCESS EECON1.CFGS = 0;
02B6 80A6 BSF 0xfa6, 0, ACCESS EECON1.RD = 1;
02B8 CFA8 MOVFF 0xfa8, 0xa6 k = EEDATA;
02BC 2AA9 INCF 0xfa9, F, ACCESS EEADR++
02BE 80A6 BSF 0xfa6, 0, ACCESS EECON1.RD = 1;
02C0 CFA8 MOVFF 0xfa8, 0x3 return make16(EEDATA, k);
02C4 C0A6 MOVFF 0xa6, 0x1
02C8 CFA8 MOVFF 0xfa8, 0x2
}
02CC 0012 RETURN 0 |
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Robert Scott
Real-Time Specialties
Embedded Systems Consulting |
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