DS87C550 Datasheet, PDF (13/50 Pages) Dallas Semiconductor – EPROM High-Speed Micro with A/D and PWM

DS87C550
Stretch setting. When maximum speed is desired, software should select a Stretch value of 0. When using
very slow RAM or peripherals, the application software can select a larger Stretch value. Note that this
affects data memory accesses only and that there is no way to slow the accesses to program memory other
than to use a slower crystal (or external clock).
The specific timing of the variable speed Stretch MOVX is provided in the Electrical Specifications
section of this data sheet. Table 4 shows the resulting MOVX instruction timing and the read or write
strobe widths for each Stretch value.
DATA MEMORY CYCLE STRETCH VALUES Table 4
CKCON.2-0 MOVX MACHINE _{RD OR WR STROBE WIDTH}
M2 M1 M0
CYCLES
IN MACHINE CYCLES
0 0 0 2 (forced internal)
0.5
0 0 1 3 (default external)
1
010
4
2
011
5
3
100
9
4
101
10
5
110
11
6
111
12
7
Dual Data Pointer With Inc/Dec
The DS87C550 contains several new, unique features that are associated with the Data Pointer register. In
the original 8051 architecture, the DPTR was a 16-bit value that was used to address off-chip data RAM
or peripherals. To improve the efficiency of data moves, the DS87C550 contains two Data Pointer
registers (DPTR0 and DPTR1). By loading one DPTR with the source address and the other with the
destination address, block data moves can be made much more efficient. Since DPTR0 is located at the
same address as the single DPTR in the original 8051 architecture, code written for the original
architecture will operate normally on the DS87C550 with no modification necessary.
The second data pointer, DPTR1 is located at the next two register locations (up from DPTR0) and is
selected using the data pointer select bit SEL (DPS.0). If SEL = 0, then DPTR0 is the active data pointer.
Conversely, if SEL = 1, then DPTR1 is the active data pointer. Any instruction that reference the DPTR
(ex. MOVX A, @ DPTR) refers to the active data pointer as determined by the SEL bit. Since the bit
adjacent to SEL in the DPS register is not used, the fastest means of changing the SEL (and thereby
changing the active data pointer) is with an INC instruction. Each INC DPS Instruction will toggle the
active data pointer.
Unlike the standard 8051, the DS87C550 has the ability to decrement as well as increment the data
pointers without additional instructions. When the INC DPTR instruction is executed, the active DPTR is
incremented or decremented according to the ID1, ID0 (DPS.7-6), and SEL (DPS.0) bits as shown. The
inactive DPTR is not affected.
ID1 ID0 SEL RESULT OF INC DPTR
X0
0 INCREMENT DPTR0
X1
0 DECREMENT DPTR0
0 X 1 INCREMENT DPTR1
1 X 1 DECREMENT DPTR1
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