COP888GD Datasheet, PDF (12/42 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 16k

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 (ex-
cept D2) together in order to get a higher drive.
Note: Care must be exercised with the D2 pin operation. At RESET, the ex-
ternal loads on this pin must ensure that the output voltages stay
above 0.8 V_CC to prevent the chip from entering special modes. Also
keep the external loading on D2 to <1000 pF.
Functional Description
The architecture of the device is modified Harvard architec-
ture. With the Harvard architecture, the control store pro-
gram memory (ROM) is separated from the data store
memory (RAM). Both ROM and RAM have their own sepa-
rate addressing space with separate address buses. The ar-
chitecture, though based on Harvard architecture, permits
transfer of data from ROM to RAM.
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). The SP is initialized to RAM ad-
dress 06F with reset.
S is the 8-bit Data Segment Address Register used to extend
the lower half of the address range (00 to 7F) into 256 data
segments of 128 bytes each.
All the CPU registers are memory mapped with the excep-
tion of the Accumulator (A) and the Program Counter (PC).
PROGRAM MEMORY
The program memory consists of 16 kbytes of ROM. These
bytes may hold program instructions or constant data (data
tables for the LAID instruction, jump vectors for the JID in-
struction, and interrupt vectors for the VIS instruction). The
program memory is addressed by the 15-bit program
counter (PC). All interrupts in the devices vector to program
memory location 0FF Hex.
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
register, and the various registers, and counters associated
with the timers (with the exception of the IDLE timer). Data
memory is addressed directly by the instruction or indirectly
by the B, X, SP pointers and S register.
The data memory consists of 256 bytes of RAM. Sixteen
bytes of RAM are mapped as “registers” at addresses 0F0 to
0FF 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, B and S are memory mapped into this space
at address locations 0FC to 0FF Hex respectively, with the
other registers being available for general usage.
The instruction set permits any bit in memory to be set, reset
or tested. All I/O and registers (except A and PC) are
memory mapped; therefore, I/O bits and register bits can be
directly and individually set, reset and tested. The accumula-
tor (A) bits can also be directly and individually tested.
Note: RAM contents are undefined upon power-up.
Data Memory Segment RAM
Extension
Data memory address 0FF is used as a memory mapped lo-
cation for the Data Segment Address Register (S).
The data store memory is either addressed directly by a
single byte address within the instruction, or indirectly rela-
tive to the reference of the B, X, or SP pointers (each con-
tains a single-byte address). This single-byte address allows
an addressing range of 256 locations from 00 to FF hex. The
upper bit of this single-byte address divides the data store
memory into two separate sections as outlined previously.
With the exception of the RAM register memory from ad-
dress locations 00F0 to 00FF, all RAM memory is memory
mapped with the upper bit of the single-byte address being
equal to zero. This allows the upper bit of the single-byte ad-
dress to determine whether or not the base address range
(from 0000 to 00FF) is extended. If this upper bit equals one
(representing address range 0080 to 00FF), then address
extension does not take place. Alternatively, if this upper bit
equals zero, then the data segment extension register S is
used to extend the base address range (from 0000 to 007F)
from XX00 to XX7F, where XX represents the 8 bits from the
S register. Thus the 128-byte data segment extensions are
located from addresses 0100 to 017F for data segment 1,
0200 to 027F for data segment 2, etc., up to FF00 to FF7F
for data segment 255. The base address range from 0000 to
007F represents data segment 0.
Figure 5 illustrates how the S register data memory exten-
sion is used in extending the lower half of the base address
range (00 to 7F hex) into 256 data segments of 128 bytes
each, with a total addressing range of 32 kbytes from XX00
to XX7F. This organization allows a total of 256 data seg-
ments of 128 bytes each with an additional upper base seg-
ment of 128 bytes. Furthermore, all addressing modes are
available for all data segments. The S register must be
changed under program control to move from one data seg-
ment (128 bytes) to another. However, the upper base seg-
ment (containing the 16 memory registers, I/O registers,
control registers, etc.) is always available regardless of the
contents of the S register, since the upper base segment
(address range 0080 to 00FF) is independent of data seg-
ment extension.
The instructions that utilize the stack pointer (SP) always ref-
erence the stack as part of the base segment (Segment 0),
regardless of the contents of the S register. The S register is
not changed by these instructions. Consequently, the stack
(used with subroutine linkage and interrupts) is always lo-
cated in the base segment. The stack pointer will be initial-
ized to point at data memory location 006F as a result of
reset.
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