COP8SAC7SLB8 datasheet(16/62 Pages) TI

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5.0 Pin Descriptions (Continued)

Port D is an 8-bit output port that is preset high when RESET

goes low. The user can tie two or more D port outputs

(except D2) together in order to get a higher drive.

Note: Care must be exercised with the D2 pin operation. At RESET, the

external loads on this pin must ensure that the output voltages stay

above 0.7 VCC to prevent the chip from entering special modes. Also

keep the external loading on D2 to less than 1000 pF.

6.0 Functional Description

The architecture of the device is a modified Harvard archi-

tecture. With the Harvard architecture, the program memory

EPROM is separated from the data store memory (RAM).

Both EPROM and RAM have their own separate addressing

space with separate address buses. The architecture,

though based on the Harvard architecture, permits transfer

of data from EPROM to RAM.

6.1 CPU REGISTERS

The CPU can do an 8-bit addition, subtraction, logical or shift

operation in one instruction (t

C) cycle time.

There are six CPU registers:

A is the 8-bit Accumulator Register

PC is the 15-bit Program Counter Register

PU is the upper 7 bits of the program counter (PC)

PL is the lower 8 bits of the program counter (PC)

B is an 8-bit RAM address pointer, which can be optionally

post auto incremented or decremented.

X is an 8-bit alternate RAM address pointer, which can be

optionally post auto incremented or decremented.

SP is the 8-bit stack pointer, which points to the subroutine/

interrupt stack (in RAM). With reset the SP is initialized to

RAM address 02F Hex (devices with 64 bytes of RAM), or

initialized to RAM address 06F Hex (devices with 128 bytes

of RAM).

All the CPU registers are memory mapped with the excep-

tion of the Accumulator (A) and the Program Counter (PC).

6.2 PROGRAM MEMORY

The program memory consists of 1024, 2048, or 4096 bytes

of EPROM or ROM.

Table 1 shows the program memory

sizes for the different devices. These bytes may hold pro-

gram instructions or constant data (data tables for the LAID

instruction, jump vectors for the JID instruction, and interrupt

vectors for the VIS instruction). The program memory is

addressed by the 15-bit program counter (PC). All interrupts

in the device vector to program memory location 0FF Hex.

The program memory reads 00 Hex in the erased state.

6.3 DATA MEMORY

The data memory address space includes the on-chip RAM

and data registers, the I/O registers (Configuration, Data and

Pin), the control registers, the MICROWIRE/PLUS SIO shift

with the timers (with the exception of the IDLE timer). Data

memory is addressed directly by the instruction or indirectly

by the B, X and SP pointers.

The data memory consists of 64 or 128 bytes of RAM.

Table

1 shows the data memory sizes for the different devices.

Fifteen bytes of RAM are mapped as “registers” at ad-

dresses 0F0 to 0FE Hex. These registers can be loaded

immediately, and also decremented and tested with the

DRSZ (decrement register and skip if zero) instruction. The

memory pointer registers X, SP and B are memory mapped

into this space at address locations 0FC to 0FE Hex respec-

tively, with the other registers (except 0FF) being available

for general usage. Address location 0FF is reserved for

future RAM expansion. If compatibility with future devices

(with more RAM) is not desired, this location can be used as

a general purpose RAM location.

The instruction set permits any bit in memory to be set, reset

or tested. All I/O and registers (except A and PC) are

DS012838-10

FIGURE 5. I/O Port Configurations

DS012838-12

FIGURE 6. I/O Port Configurations — Output Mode

DS012838-11

FIGURE 7. I/O Port Configurations — Input Mode

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