A Simple Tour of the MSP430

Light LEDs in C LEDs can be connected in two standard ways. Active high circuit, the LED illuminates if the pin is driven high (logic 1) and extinguishes if the pin is driven low (logic 0). Input and output is memory mapped, which means that nothing unusual is needed to control output pins: We simply write in the normal way to the register that controls them. The complete program needs to carry out the following tasks: 1. Configure the microcontroller. 2. Set the relevant pins to be outputs by setting the appropriate bits of PxDIR. 3. Illuminate the LEDs by writing to PxOUT. 4. Keep the microcontroller busy in an infinite, empty loop.

Light LEDs in C

Assignment The basic instruction in assembly language is mov.w source, destination to move a word or mov.b for a byte. A single character denotes the mode in the operand. immediate, #: The value itself (word or byte) is given and stored in the word following the instruction. This is also known as a literal value. absolute, &: The address of a register in memory space is given and stored in the word following the instruction. register, R: This specifies one of the 16 registers in the CPU. The immediate mode is clearly useful only for the source but the other two can be used for either the source or destination, provided that it is possible to write to the destination—it is not in ROM, for instance.

Infinite Loop The flow of control is based on jump instructions, which can be either conditional or unconditional (always taken). You have no choice in assembly language. Normally the CPU executes instructions from memory one after the other. The program counter (PC) is automatically incremented so that it is ready to fetch the next instruction as soon as it is needed. This sequence is broken by a jump, which causes a new address to be loaded into in the PC.

Program Is Stored in Memory Generally the code should go into the flash ROM and variables should be allocated in RAM. We put the code in the most obvious location, which is at the start of the flash memory (low addresses). The Memory Organization section of the data sheet shows that the main flash memory has addresses 0xF000–FFFF. We therefore instruct the assembler to start storing the code at 0xF000.

Program Start In the MSP430, the address of the first instruction to be executed is stored at a specific location in flash memory, rather than the instruction itself. This address, called the reset vector, occupies the highest 2 bytes of the vector table at 0xFFFE:FFFF. It is set up as follows: 1. Put a label in front of the first instruction to be executed. 2. Use an directive to tell the assembler where the reset vector should be stored. It is at the same address in all current MSP430 devices. 3. This is followed by a DW Reset directive for “define word,” which tells the assembler to store the following word (2 bytes) in memory. This is the address of the first instruction to be executed and the assembler replaces the label with its numerical value.

Read Input from a Switch The pull-up resistor R pull holds the input at logic 1 (voltage VCC) while the button is up; closing the switch shorts the input to ground, logic 0 or VSS. The input is therefore active low, meaning that it goes low when the button is pressed. Most MCUs offer internal pull-ups to reduce the number of external components needed.

Read Input from a Switch A program can respond to inputs in two ways. ◦ Polling ◦ Interrupt Polling is simple but carries the overhead of regular checking, which is high if checks must be made frequently. An interrupt needs special hardware and requires tidying up so that normal operation can be resumed after the interrupt has been serviced.

Two Loops, One for Each State of the Button The LED is continually being switched on or off in the first version. The action is repeated continually. The LED is turned on or off only when necessary in the second version, just at the points when the button is pressed or released.