Basic Syntax for programming MSP430 - beginner questions

Brock,

I think this is a fairly common questions for beginners :)

Checkout the section on "Bitwise Operators" in this Wikipedia link:

http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B

To help you get started; the |= operator does a bitwise OR and the &= ~ does a bit clear.

P5SEL = BIT4 | BIT3 | BIT2; is the same as

P5SEL |= BIT4 + BIT3 + BIT2;

where BITx  represents a bit position 0 through 7 in the PSEL register.

This way the code is easier to understand instead of

P5SEL |= 0x0E; which does the same thing.

Here is another example:

LCDCTL |= LCD4MUX;

By ORing the register with this bit you are setting the bit to 1.

LCDCTL &= ~LCD4MUX;

By ANDing the register with the bit complement you are clearing the bit.

Regards,

Priya

Priya Thanigai said:

Brock,

I think this is a fairly common questions for beginners :)

Checkout the section on "Bitwise Operators" in this Wikipedia link:

http://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B

To help you get started; the |= operator does a bitwise OR and the &= ~ does a bit clear.

P5SEL = BIT4 | BIT3 | BIT2; is the same as

P5SEL |= BIT4 + BIT3 + BIT2;

where BITx  represents a bit position 0 through 7 in the PSEL register.

This way the code is easier to understand instead of

P5SEL |= 0x0E; which does the same thing.

Here is another example:

LCDCTL |= LCD4MUX;

By ORing the register with this bit you are setting the bit to 1.

LCDCTL &= ~LCD4MUX;

By ANDing the register with the bit complement you are clearing the bit.

 

Regards,

Priya

Priya I understand the operators but what I'm trying to ask is for example what you posted :

LCDCTL |= LCD4MUX;

By ORing the register with this bit you are setting the bit to 1.

To get the result of 1 the bit would have to be set to 1 right? How do you know the bit is 1 here? Is that assumed?

OK, I understand your question better now...

YEs, these bit definitions and the values they represent are defined in msp430xxxx.h

See an example below:

[Extract from msp430f6638.h]

The various bits for the LCDBCTL0 are defined as:

// LCDBCTL0
#define LCDON               (0x0001)  /* LCD_B LCD On */
#define LCDSON              (0x0004)  /* LCD_B LCD Segments On */
#define LCDMX0              (0x0008)  /* LCD_B Mux Rate Bit: 0 */
#define LCDMX1              (0x0010)  /* LCD_B Mux Rate Bit: 1 */

The easiest way to decipher code is to look for the functionality of the bits in the User's Guide and  then compare it to the # defines in the header file.

Sometimes these #defines are used to represent a more complex setting/ functionality such as:

#define WDT_ARST_1000       (WDTPW+WDTCNTCL+WDTSSEL0+WDTIS2)                         /* 1000ms  " */

This definition is used to set the timer to run off ACLK and have an interval of 1000ms.

Regards,

Priya

Perhaps some even more basic informations are necessary, to fully answer the original question.

Unlike the 80x86 processors (and some others), the MSP does not have a separate I/O port. All peripherals are occupying an address (or more) in the common address space. There you find not only RAM and FLASH, but also configuration registers for all the hardware the MSP has built-in.

For convenience, the memory locations of these registers are mapped to 'variables' of the proper type (unsigned char/unsigned int) together with the information that they are placed at a fixed address and not in the common data segment.

These mappings are in the variuos include files which are automatically loaded and compiled when you include the proper io.h or msp430xxxx.h header files (depends on the compiler) into your source code.

Furthermore, also the meanings of the bits or bit combinations withing these registers are defined with meaningful names (such as LCDON). The names used usually correspond with the descriptions of the MSP hardware inthe MSP family guides and device manuals.

Writing to such a register can be considered as writing an 8-bit/16 bit  value into a latch with 8/16 digital output lines controlling some built-in hardware circuitry.

in case of the port 6, writing a value to P6OUT results in 8 digital lines set to 0 or 1 respectively, and these lines enable or disable the output buffers of the port pins. Similarly, writing to P5SEL, you 'program' 8 digital control lines which control a switch that will map either the port6.x output buffer (if 0) or some peripherals input or output signal (if 1) to the corresponding processor pin.

So P6OUT is an alias to a memory address which is connected to the P6 output control hardware rather than just a memory cell.  From a programmers view, P6OUT and his brothers and sisters can be handled like just a normal global C variable. But any writing (and sometime also reading) access results in a direct reaction of some hardware module that is lurking on the MSPs address/data bus, waiting for someone accessing a certain memory address.