
S3F84B8_UM_REV 1.00
4 CONTROL REGISTERS
4-35
4.1.38 SYM — SYSTEM MODE REGISTER: DEH, BANK0
Bit Identifier
.7
.6
.5
.4
.3
.2
.1
.0
Reset Value
0 – – x x x 0 0
Read/Write
R/W – – R/W
R/W R/W R/W R/W
Tri-state External Interface Control Bit
0
Normal operation (disables tri-state operation).
.7
1
Sets the external interface lines to high impedance
(enables tri-state operation).
.6–.5
Not used for S3F84B8.
Fast Interrupt Level Selection Bits
0 0 0 IRQ0
0 0 1 IRQ1
0 1 0 IRQ2
0 1 1 IRQ3
1 0 0 IRQ4
1 0 1 IRQ5
1 1 0 IRQ6
.4–.2
1 1 1 IRQ7
0
Disables fast interrupt processing.
.1
1
Enables fast interrupt processing.
Global Interrupt Enable Bit
0
Disables all interrupt processing.
.0
1
Enables all interrupt processing.
NOTE:
1. Since an external interface is not implemented, SYM.7 must always be ‘0’.
2. You can select only one interrupt level at a time for fast interrupt processing.
3. Setting SYM.1 to “1” enables fast interrupt processing for the interrupt level currently selected by SYM.2-SYM.4.
4. Following a reset, you must enable global interrupt processing by executing an EI instruction (not by writing a “1” to
SYM.0).