87C748 Philips Semiconductors, 87C748 Datasheet - Page 8

• Manufacturer Part Number: 87C748
• Description: 80C51 8-bit microcontroller family 2K/64 OTP/ROM/ low pin count
• Manufacturer: Philips Semiconductors

Philips Semiconductors
DIFFERENCES BETWEEN THE 8XC748 AND THE
80C51
Memory Organization
The central processing unit (CPU) manipulates operands in two
address spaces as shown in Figure 3. The part’s internal memory
space consists of 2k bytes of program memory, and 64 bytes of data
RAM overlapped with the 128-byte special function register area.
The differences from the 80C51 are in RAM size (64 bytes vs. 128
bytes), in external RAM access (not available on the 83C748), in
internal ROM size (2k bytes vs. 4k bytes), and in external program
memory expansion (not available on the 83C748). The 128-byte
special function register (SFR) space is accessed as on the 80C51
with some of the registers having been changed to reflect changes
in the 83C748 peripheral functions. The stack may be located
anywhere in internal RAM by loading the 8-bit stack pointer (SP). It
should be noted that stack depth is limited to 64 bytes, the amount
of available RAM. A reset loads the stack pointer with 07 (which is
pre-incremented on a PUSH instruction).
Program Memory
On the 8XC748, program memory is 2048 bytes long and is not
externally expandable, so the 80C51 instructions MOVX, LJMP, and
LCALL are not implemented. The only fixed locations in program
memory are the addresses at which execution is taken up in
response to reset and interrupts, which are as follows:
Event
Reset
External INT0
Counter/timer 0
External INT1
Timer I
Counter/Timer Subsystem
The 8XC748 has one counter/timer called timer/counter 0. Its
operation is similar to mode 2 operation on the 80C51, but is
extended to 16 bits with 16 bits of autoload. The controls for this
counter are centralized in a single register called TCON.
Timer I is available for use as a fixed 10-bit time-base, or as a
watchdog.
Counter Timer – Special Function Register
The counter/timer has only one mode of operation, so the TMOD
SFR is not used. There is also only one counter/timer, so there is no
need for the TL1 and TH1 SFRs found on the 80C51. These have
been replaced on the 8XC748 by RTL and RTH, the counter/timer
reload registers. Table 2 shows the special function registers, their
locations, and reset values.
1999 Apr 15
80C51 8-bit microcontroller family
2K/64 OTP/ROM, low pin count
Program Memory
Address
00B
01B
000
003
013
8
Interrupt Subsystem – Fixed Priority
The IP register and the 2-level interrupt system of the 80C51 are
eliminated. Simultaneous interrupt conditions are resolved by a
single-level, fixed priority as follows:
Highest priority:
Lowest priority:
Special Function Register – Interrupt Subsystem
Because the interrupt structure is single level on the 83C748, there
is no need for the IP SFR, so it is not used.
Special Function Register –
Serial Communications
The 8XC748 contains many of the special function registers (SFR)
that are found on the 80C51. Due to the different peripheral features
on the 8XC748, there are several additional SFRs. Since the UART
found on 80C51 has been removed, the UART SFRs SCON and
SBUF have also been removed.
I/O Port Latches (P0, P1, P3)
The port latches function the same as those on the 80C51. Since
there is no port 2 on the 83C748, the P2 latch is not used. Port 0 on
the 83C748 has only 3 bits, so only 3 bits of the P0 SFR have a
useful function.
Data Pointer (DPTR)
The data pointer (DPTR) consists of a high byte (DPH) and a low
byte (DPL). In the 80C51 this register allows the access of external
data memory using the MOVX instruction. Since the 83C748 does
not support MOVX or external memory accesses, this register is
generally used as a 16-bit offset pointer of the accumulator in a
MOVC instruction. DPTR may also be manipulated as two
independent 8-bit registers.
(FFH) 255
(80H) 128
(3FH) 63
(00H) 0
Pin INT0
Counter/timer flag 0
Pin INT1
Timer I
Figure 3. Memory Map
Internal Data
Registers
Function
Special
83C748/87C748
RAM
Preliminary specification
SU00299