Texas Instruments MSC1210 manual SFR Definitions

SFR Definitions

3.5 SFR Definitions

This section will endeavor to quickly overview each of the SFRs found in the SFR chart map of Table 3−1. It is not the intention of this section to fully explain the functionality of each SFR—this information will be covered in separate chapters. This section is to just give a general idea of what each SFR does.

P0 (Port 0, Address 80H, Bit-Addressable): This is input/output port 0. Each bit of this SFR corresponds to one of the pins on the microcontroller. For exam- ple, bit 0 of port 0 is pin P0.0, bit 7 is pin P0.7. Writing a value of 1 to a bit of this SFR sets a high level on the corresponding I/O pin, whereas a value of 0 brings it to a low level.

Note:

Even though the MSC1210 has four I/O ports (P0, P1, P2, and P3), if the hardware uses external RAM or external code memory (i.e., if the program is stored in an external ROM or EPROM chip, or if external RAM chips are being used), P0 or P2 may not be used. This is because the MSC1210 uses ports P0 and P2 to address the external memory (refer to Section 15.1 for the detailed control of the port usages). Thus, if external RAM or code memory is being used, only ports P1 and P3 (except P3.6 and P3.7) may be used by the application.

SP (Stack Pointer, Address 81H): This is the stack pointer of the microcontroller. This SFR indicates where the next value to be taken from the stack will be read from Internal RAM. If a value is pushed onto the stack, the value will be written to the address of SP + 1. That is to say, if SP holds the value 07H, a PUSH instruction will push the value onto the stack at address 08H. This SFR is modified by all instructions that modify the stack, such as PUSH, POP, LCALL, RET, RETI, and whenever interrupts are triggered by the microcontroller.

Note:

The SP SFR, on startup, is initialized to 07H. This means the stack will start at 08H and will grow to larger addresses of internal RAM. It is necessary to initialize SP in the program to some other value if alternate register banks and/or bit me- morywill be used because alternate register banks 1, 2, and 3, as well as the user bit variables, occupy internal RAM from addresses 08H through 2FH. It is not a bad idea to initialize SP to 2FH as the first instruction of every one of the programs, unless there is complete confidence that the program will not be us- ing register banks and bit variables.

Special Function Registers (SFRs) 3-5