
The central processing unit (CPU)
UM0404
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DocID13284 Rev 2
Note:
1
Note that any explicit write request (via software) to an SFR supersedes a simultaneous
modification of the same register, by hardware.
2
Any write operation to a single byte of an SFR clears the non-addressed complementary
byte within the specified SFR. Non-implemented (reserved) SFR bits cannot be modified,
and will always supply a read value of '0'.
3.4.1
The system configuration register SYSCON
This bit-addressable register provides general system configuration and control functions.
The reset value for register SYSCON depends on the state of the PORT0 pins during reset
(see hardware affectable bits).
SYSCON (FF12h / 89h)
SFR
Reset Value: 0xx0h
Reset Value: 0000 0xx0 x000 0000b
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
STKSZ
ROM
S1
SGT
DIS
ROM
EN
BYT
DIS
CLK
EN
WR
CFG
CS
CFG
PWD
CFG
OWD
DIS
BDR
STEN
XPEN
VISI
BLE
XPER-
SHARE
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
RW
Bit
Function
XPER-SHARE
XBUS Peripheral Share Mode Control
‘0’: External accesses to XBUS peripherals are disabled.
‘1’: XRAM1 and XRAM2 are accessible via the external bus during hold
mode. External accesses to the other XBUS peripherals are not guaranteed
in terms of AC timings. See
Section 2.4.1: XRAM access via external
for additional details.
VISIBLE
Visible Mode Control
‘0’: Accesses to XBUS peripherals and XRAM are done internally.
‘1’: XBUS peripheral accesses are made visible on the external pins.
XPEN
XBUS Peripheral Enable bit
‘0’: Accesses to the on-chip X-peripherals and XRAM are disabled.
‘1’: The on-chip X-peripherals are enabled.
BDRSTEN
Bidirectional Reset Enable
‘0’: RSTIN pin is an input pin only. SW Reset or WDT Reset have no effect on
this pin.
‘1’: RSTIN pin is a bidirectional pin. This pin is pulled low during internal reset
sequence.
OWDDIS
Oscillator Watchdog Disable Control
‘0’: Oscillator Watchdog (OWD) is enabled. If PLL is bypassed, the OWD
monitors XTAL1 activity. If there is no activity on XTAL1 for at least 1
μ
s, the
CPU clock is switched automatically to PLL’s base frequency (from 750 kHz
to 3 MHz).
‘1’: OWD is disabled. If the PLL is bypassed, the CPU clock is always driven
by XTAL1 signal. The PLL is turned off to reduce power supply current.