WATCH DOG TIMER
The FDC37B72x contains a Watch Dog Timer
(WDT). The Watch Dog Time-out status bit may
be mapped to an interrupt through the WDT_CFG
Configuration Register. It can also be brought out
on the GP12 or GP63 pins by programming the
corresponding GPIO configuration register.
The host may force a Watch Dog time-out to occur
by writing a "1" to bit 2 of the WDT_CTRL (Force
WD Time-out) Configuration Register. Writing a
"1" to this bit forces the WDT count value to zero
and sets bit 0 of the WDT_CTRL (Watch Dog
Status). Bit 2 of the WDT_CTRL is self-clearing.
The FDC37B72x's WDT has a programmable
time-out ranging from 1 to 255 minutes with one
minute resolution, or 1 to 255 seconds with 1
second resolution. The units of the WDT timeout
value are selected via bit[7] of the
WDT_TIMEOUT register (LD8:CRF1.7). The WDT
time-out value is set through the WDT_VAL
Configuration register. Setting the WDT_VAL
register to 0x00 disables the WDT function (this is
its power on default). Setting the WDT_VAL to
any other non-zero value will cause the WDT to
reload and begin counting down from the value
loaded. When the WDT count value reaches zero
the counter stops and sets the Watchdog time-out
status bit in the WDT_CTRL Configuration
Register. Note: Regardless of the current state of
the WDT, the WDT time-out status bit can be
directly set or cleared by the Host CPU.
LED
The FDC37B72x can directly drive an LED using
the alternate function of GP13 or GP61 (only one
may be used at at time). These pins are active
under VTR power so the LED may be used in any
system power state. The GPIO used for the LED
will initially default to an input; the corresponding
GPIO configuration register must be programmed
to configure the pin for the LED function and as a
push pull or an open drain output. However, under
VTR power the LED must be configured as open
drain, since the pin cannot drive current under
VTR power. The polarity bit may be chosen as
either non-inverted or inverted (active high or
active low).
The LED can be turned on and off or toggled at a
1 Hertz rate with a 50 percent duty cycle. When
the GP13 or GP61 pin is configured as a non-
inverted, open drain output and the LED function is
chosen, the LED may be turned on by writing ‘1’
the GP1 register bit 3 or the GP6 register bit 1.
Clearing these bits will then turn the LED off. The
LED may be toggled as described below. Note
that the GPIO can control the LED in its default
GPIO function, but it may only toggle if the LED
function is chosen.
There are three system events which can reset the
WDT. These are
a
Keyboard Interrupt, a
Mouse Interrupt, or I/O reads/writes to address
0x201 (the internal or an external Joystick Port).
The effect on the WDT for each of these system
events may be individually enabled or disabled
through bits in the WDT_CFG configuration
register. When a system event is enabled through
the WDT_CFG register, the occurrence of that
event will cause the WDT to reload the value
stored in WDT_VAL and reset the WDT time-out
status bit if set. If all three system events are
disabled the WDT will inevitably time out.
Setting bit 1 of the WDT_CTRL configuration
register will cause the Power LED output driver to
toggle at 1 Hertz with a 50 percent duty cycle.
When this bit is cleared the Power LED output will
drive continuously unless it has been configured to
toggle on Watch Dog time-out conditions. Setting
bit 3 of the WDT_CFG configuration register will
cause the Power LED output driver to toggle at 1
Hertz with a 50 percent duty cycle whenever the
WDT time-out status bit is set. The truth table
The Watch Dog Timer may be configured to
generate an interrupt on the rising edge of the
Time-out status bit. The WDT interrupt is mapped
to an interrupt channel through the WDT_CFG
Configuration Register. When mapped to an
interrupt the interrupt request pin reflects the value
of the WDT time-out status bit.
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SMSC [ SMSC CORPORATION ]
