DS80C390 Datasheet, PDF (15/58 Pages) Dallas Semiconductor – Dual CAN High-Speed Microprocessor

DS80C390
The reset default of one Stretch cycle results in a three cycle MOVX for any external access. Therefore,
the default off-chip RAM access is not at full speed. This is a convenience to existing designs that utilize
slower RAM. When maximum speed is desired, software should select a Stretch value of zero. When
using very slow RAM or peripherals, the application software can select a larger Stretch value.
The specific timing of MOVX instructions as a function of Stretch settings is provided in the Electrical
Specifications section of this data sheet. As an example, Table 8 shows the read and write strobe widths
corresponding to each Stretch value.
DATA MEMORY CYCLE STRETCH VALUES Table 8
MD2 MD1 MD0
Stretch
Cycle
Count
MOVX
Machine
Cycles
RD , WR Pulse Width (in oscillator clocks)
t_MCS
t_MCS
t_MCS
t_MCS
(4X/ 2X = 1 (4X/ 2X = 0 (4X/ 2X = X (4X/ 2X = X
CD1:0 = 00) CD1:0 = 00) CD1:0 = 10) CD1:0 = 11)
000
0*
2
0.5 t_CLCL
1 t_CLCL
2 t_CLCL
2048 t_CLCL
001
1**
010
2
3
t_CLCL
2 t_CLCL
4 t_CLCL
4096 t_CLCL
4
2 t_CLCL
4 t_CLCL
8 t_CLCL
8192 t_CLCL
011
3
100
4
101
5
5
3 t_CLCL
6 t_CLCL
12 t_CLCL 12288 t_CLCL
9
4 t_CLCL
8 t_CLCL
16 t_CLCL 16384 t_CLCL
10
5 t_CLCL
10 t_CLCL
20 t_CLCL 20480 t_CLCL
110
6
111
7
11
6 t_CLCL
12 t_CLCL
24 t_CLCL 24576 t_CLCL
12
7 t_CLCL
14 t_CLCL
28 t_CLCL 28672 t_CLCL
*All internal MOVX operations execute at the 0 Stretch setting.
** Default Stretch setting for external MOVX operations following reset.
EXTENDED STACK POINTER
The DS80C390 supports both the traditional 8-bit and an extended 10-bit stack pointer that improves the
performance of large programs written in high-level languages such as C. The 10-bit stack pointer
feature is enabled by setting the Stack Address Mode bit, SA (ACON.2). The bit is cleared following a
reset, forcing the device to use an 8-bit stack located in the Scratchpad RAM area. When the SA bit is
set, the device will address up to 1kB of stack memory in the first 1kB of the internal MOVX memory.
The 10-bit stack pointer address is generated by concatenating the lower two bits of the Extended Stack
Pointer (ESP;9Bh) and the traditional 8051 Stack Pointer (SP;81h). The 10-bit stack pointer cannot be
enabled when the 4kB of SRAM is mapped as both program and data memory.
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