C8051T620/1/6/7 & C8051T320/1/2/3
124
Rev. 1.2
SFR Address = 0xFF
20.3. External Reset
The external RST pin provides a means for external circuitry to force the device into a reset state. Assert-
ing an active-low signal on the RST pin generates a reset; an external pullup and/or decoupling of the RST
pin may be necessary to avoid erroneous noise-induced resets. See Table 7.4 for complete RST pin spec-
ifications. The PINRSF flag (RSTSRC.0) is set on exit from an external reset.
20.4. Missing Clock Detector Reset
The Missing Clock Detector (MCD) is a one-shot circuit that is triggered by the system clock. If the system
clock remains high or low for more than the MCD time-out, a reset will be generated. After a MCD reset,
the MCDRSF flag (RSTSRC.2) will read 1, signifying the MCD as the reset source; otherwise, this bit reads
0. Writing a 1 to the MCDRSF bit enables the Missing Clock Detector; writing a 0 disables it. The state of
the RST pin is unaffected by this reset.
20.5. Comparator0 Reset
Comparator0 can be configured as a reset source by writing a 1 to the C0RSEF flag (RSTSRC.5).
Comparator0 should be enabled and allowed to settle prior to writing to C0RSEF to prevent any turn-on
chatter on the output from generating an unwanted reset. The Comparator0 reset is active-low: if the non-
inverting input voltage (on CP0+) is less than the inverting input voltage (on CP0-), the device is put into
the reset state. After a Comparator0 reset, the C0RSEF flag (RSTSRC.5) will read 1 signifying
Comparator0 as the reset source; otherwise, this bit reads 0. The state of the RST pin is unaffected by this
reset.
SFR Definition 20.1. VDM0CN: V
DD
Monitor Control
Bit
7
6
5
4
3
2
1
0
Name
VDMEN
VDDSTAT
Type
R/W
R
R
R
R
R
R
R
Reset
Varies
Varies
Varies
Varies
Varies
Varies
Varies
Varies
Bit
Name
Function
7
VDMEN
V
DD
Monitor Enable.
This bit turns the V
DD
monitor circuit on/off. The V
DD
Monitor cannot generate sys-
tem resets until it is also selected as a reset source in register RSTSRC (SFR Def-
inition 20.2). Selecting the V
DD
monitor as a reset source before it has stabilized
may generate a system reset. In systems where this reset would be undesirable, a
delay should be introduced between enabling the V
DD
Monitor and selecting it as a
reset source. See Table 7.4 for the minimum V
DD
Monitor turn-on time.
0: V
DD
Monitor Disabled.
1: V
DD
Monitor Enabled.
6
VDDSTAT
V
DD
Status.
This bit indicates the current power supply status (V
DD
Monitor output).
0: V
DD
is at or below the V
DD
monitor threshold.
1: V
DD
is above the V
DD
monitor threshold.
5:0
Unused
Read = Varies; Write = Don’t care.