Post by thread:inverting a single bit in C

David Wong wrote: > > If I want to invert a single bit in C how do I do that. For example I want > to invert RA3. > > I tried > > RA3 = !RA3; Try: RA3 ^= 0x04; or RA3 = RA3 ^ 0x04; -- Phil -- Philip Restuccia Senior Principal Software Engineer Periphonics Corporation spam_OUTphilip.restucciaTakeThisOuTperi.com ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify systems administration at .....postmasterKILLspam@spam@peri.com. This footnote also confirms that this email message has been scanned for the presence of commonly detectable virus-types. **********************************************************************

At 10:48 AM 4/14/99 -0400, you wrote: >If I want to invert a single bit in C how do I do that. For example I want >to invert RA3. > >I tried > >RA3 = !RA3; I would do it this way: #define bit3 0b00001000 PORTA = PORTA ^ bit3; The "right" way would be to use a shadow register for the update, just writing the changed value to the port: UINT8 shadow_RA; // Always use this to write to the port shadow_RA ^= bit3; PORTA = shadow_RA; Andy ================================================================== Montana Design - http://www.montanadesign.com - Electronics & Model Boats ==================================================================

> > If I want to invert a single bit in C how do I do that. For example I want > to invert RA3. > > I tried > > RA3 = !RA3; > > But that doesn't seem to work. Bit selection isn't a standard C operation. As shown in other posts you use the XOR operator on the whole register. I'm in the processing of developing yet another HLL for microcontrollers named NPCI. I addressed the issue by adding a bit selection operator. In NPCI this is done by: porta:3 = !porta:3; Which is very similar to what you thought it should originally be. Unfortunately NPCI isn't in a usable state right now because of a design flaw for local variables in parameters. Hope to fix in the next few months. Just a comment. BAJ

I'm using the CCS compiler and it works for me. Below is a fragment of code for inverting the bit driving a piezo buzzer. The piezo is driven from pin RA1 of a 16F84. 0000 00083 .................... #define PIEZO a_port.a1 // Piezo beeper pin 0000 01005 .................... PIEZO = !(PIEZO); // Toggle the output pin. 012E 3002 01006 MOVLW 02 012F 0685 01007 XORWF 05,F Regards, Peter. --- Peter Homann Email: peterhKILLspamadacel.com.au Adacel Technologies Ltd _/_/_/ _/_/_/ _/ Phone: +61 3 9596 2991 250 Bay St, Brighton _/ _/ _/ _/ Fax: +61 3 9596 2960 Victoria 3186 _/_/_/ _/ _/ Home: +61 3 9555 5603 AUSTRALIA _/ _/ _/ _/_/_/Mobile: 0414-494578 ------------------------------------------------------------------------ |{Original Message removed}

On Wed, 14 Apr 1999 13:33:54 -0400 Ralph Stickley <.....rstickleyKILLspam.....DATALUX.COM> shared some of Paul Britton's code for turning a bit number (0-7) into a bit mask (1, 2, 4, 8, 16...128). It was nine instructions long, and isosynchronous. My code is less clever, but faster: ;---------- ; TWO_POWER: two raised to the power of W ;---------- TWO_POWER: #ifdef SAFE ANDLW 0x7 #endif ADDWF PCL, F RETLW B'00000001' RETLW B'00000010' RETLW B'00000100' RETLW B'00001000' RETLW B'00010000' RETLW B'00100000' RETLW B'01000000' RETLW B'10000000' So, to invert a single bit: MOVF bit_to_invert, W CALL TWO_POWER XORWF word_to_have_its_bit_inverted Paul Britton's code is easier to inline, but in the code I've written, I've found that the function is used often enough that I usually want to turn it into a subroutine anyway. Brian

|If I want to invert a single bit in C how do I do that. For example I want |to invert RA3. |I tried |RA3 = !RA3; |But that doesn't seem to work. If you have RA3 declared to be a bit variable at [5.3], then the code you have written should work. Basically, there are four ways to invert a bit [CCS syntax shown, but similar technique should work in all comp- ilers]: /* Method 1 */ #byte PORTA = 0x05 #define RA3mask = 0x08 ... PORTA ^= RA3mask; ... /* Method 2 */ #bit RA3 = 0x05.3 ... RA3 = !RA3; ... /* Method 3 */ #bit RA3 = 0x05.3 ... RA3 ^= 1; ... /* Method 4*/ #bit RA3 = 0x05.3 ... if (RA3) RA3 = 0; else RA3 = 1; ... Note that methods (1) and (4) have clear unambiguous counterparts in machine code, and the machine code version of (1) is smaller than (4). Methods (2) and (3) will probably be evaluated the same as (1) or (4) depending upon the compiler. Note also that if you do something 'creative' like... #bit RA3 = 0x05.3 #bit foo = 0x05.3 ... RA3 = !foo; ... ...you should not necessarily expect the compiler to produce 'correct' code since it may not see the interaction between 'foo' and 'RA3', and could rewrite that expression as ... RA3 = 0; if (!foo) RA3 = 1; ... which would indeed be the fastest possible code for a 'normal' bit ass- ignment. Note, btw, that assigning boolean quantities to I/O ports may cause undesired behavior if brief changes in port state 'matter'; in some cases where the changes don't cause problems, they can even be handy as a means of seeing [on a scope] where the bit-assignments take place. Interesting, no?