Bitmasking Tutorial - Revision history

Proj user14 at 11:21, 18 December 2016

2016-12-18T11:21:58Z

2012-09-18T17:22:15Z

2012-09-18T01:30:38Z

‎Bit-masking using Bit Structures

2012-09-12T20:54:28Z

‎Unions

2012-09-12T20:48:08Z

‎Bit Masking Simplified

2012-09-12T20:47:57Z

‎Bit-masking using Bit Structures

2012-09-11T03:52:52Z

‎Bit-masking using Bit Structures

2012-09-11T03:51:52Z

‎Bit-masking using Bit Structures

2012-09-11T03:49:19Z

2012-09-11T03:44:58Z

‎GPIO Example of LPC17xx

@@ Line 83: / Line 83: @@
 LPC_GPIO1->FIODIR &= ~(1 << 14);
-// Set P1.19 as OUTPUT for the LED:
+// Set P1.15 as OUTPUT for the LED:
 LPC_GPIO1->FIODIR |=  (1 << 15);

@@ Line 113: / Line 113: @@
 <br/>
-== Bit-masking using Bit Structures ==
+== Read More ==
-If you've got comfortable with bit-masking, you can also let the compiler handle bit-masking. Let's now modify our example to use bit structures:
+You can read the advanced bit-masking over at the [[Bit Structures | Bit structure and union page]]

@@ Line 160: / Line 160: @@
 // Woohoo!  We read bit 14 and set bit 15 of FIOPIN with smart #define
 </syntaxhighlight>
 <br/>
 === Unions ===
 In the bit-struct example, '''struct''' was enclosed inside a '''union'''.  A '''union''' is a structure whose members share memory.  In the next example, you should notice that the '''full32bit''' overlaps the memory of the structure and the structure size is also 32-bits.  The '''packed attribute''' is needed such that the compiler will not optimize the structure memory and it will indeed use precisely the bits you requested for each variable.  The memory looks like this:

@@ Line 171: / Line 171: @@
 | byte3 | byte 2| byte 1 | byte 0 |
 </code>
-<br/>
+<br/><br/>
-Modifying byte 0 will also affect full32bit member.  Here is an example:
+Modifying byte 0 will also affect full32bit member.  Here is the code:
 <syntaxhighlight lang="c">

@@ Line 148: / Line 148: @@
 </syntaxhighlight>
 === Bit Masking Simplified ===
 Let's assume that you've got the struct above, so let's show you the power of the bit-structures while taking the complexity out:
@@ Line 162: / Line 163: @@
 <br/>
 === Unions ===
 In the bit-struct example, '''struct''' was enclosed inside a '''union'''.  A '''union''' is a structure whose members share memory.  In the next example, you should notice that the '''full32bit''' overlaps the memory of the structure and the structure size is also 32-bits.  The '''packed attribute''' is needed such that the compiler will not optimize the structure memory and it will indeed use precisely the bits you requested for each variable.  The memory looks like this:

← Older revision		Revision as of 20:47, 12 September 2012
Line 146:		Line 146:
	pFioPin1->b15 = pFioPin1->b14;		pFioPin1->b15 = pFioPin1->b14;

		+	</syntaxhighlight>
		+
		+	=== Bit Masking Simplified ===
		+	Let's assume that you've got the struct above, so let's show you the power of the bit-structures while taking the complexity out:
		+	<syntaxhighlight lang="c">
		+	#define BIT(reg) (((BitStructType)&reg))
		+
		+	void my_foo()
		+	{
		+	BIT(LPC_GPIO1->FIOPIN).b15 = BIT(LPC_GPIO1->FIOPIN).b14;
		+	}
		+	// Woohoo! We read bit 14 and set bit 15 of FIOPIN with smart #define
	</syntaxhighlight>		</syntaxhighlight>

@@ Line 123: / Line 123: @@
      unsigned int full32bit;
      struct {
          unsigned b0  :1; unsigned b1  :1; unsigned b2  :1; unsigned b3  :1;
@@ Line 133: / Line 134: @@
          unsigned b28 :1; unsigned b29 :1; unsigned b30 :1; unsigned b31 :1;
      }__attribute__((packed));
 } BitStructType;

@@ Line 22: / Line 22: @@
 </syntaxhighlight>
 == Bit RESET ==
 To reset a bit, the logic is similar, but instead of '''OR'''ing a bit, we will AND a bit.  '''Remember that AND gate resets a bit if you AND it with 0 so we need to use a tilde (~) to come up with the correct logic:
@@ Line 41: / Line 42: @@
 REG &= ~( (1 << 21) | (1 << 23) );
 </syntaxhighlight>
 <br/>
 == Bit checking ==
 Suppose you want to wait for a register's bit#7 to set:
@@ Line 64: / Line 65: @@
 while(REG & (1 << 9));
 </syntaxhighlight>
 <br/>
@@ Line 94: / Line 96: @@
+}
 </syntaxhighlight>
 <br/>
 LPC also has dedicated registers to set or reset an IOPIN with hardware AND and OR logic:
 == Bit-masking using Bit Structures ==
 If you've got comfortable with bit-masking, you can also let the compiler handle bit-masking. Let's now modify our example to use bit structures:
@@ Line 126: / Line 141: @@
 </syntaxhighlight>
 <br/>

@@ Line 72: / Line 72: @@
 *  GPIO read: IOPIN register
 *  GPIO write: IOPIN register
 Suppose a switch is connected to GPIO Port P1.14 and an LED is connected to Port P1.15.  Note that if a bit is set of FIODIR register, the pin is OUTPUT otherwise the pin is INPUT.  '''So... 1=OUTPUT, 0=INPUT'''
 <syntaxhighlight lang="c">
 // Set P1.14 as INPUT for the switch: