Functional Overview
52
March 2004 Revised October 2004
SGUS051A
3.10
External Reference Oscillator Clock Option
The typical specifications for the external quartz crystal for a frequency of 30 MHz are listed below:
Fundamental mode, parallel resonant
C
L
(load capacitance) = 12 pF
C
L1
= C
L2
= 24 pF
C
shunt
= 6 pF
ESR range = 25 to 40
Watchdog Block
3.11
The watchdog block on the F281x and C281x is identical to the one used on the 240x devices. The watchdog
module generates an output pulse, 512 oscillator clocks wide (OSCCLK), whenever the 8-bit watchdog up
counter has reached its maximum value. To prevent this, the user disables the counter or the software must
periodically write a 0x55 + 0xAA sequence into the watchdog key register which will reset the watchdog
counter. Figure 311 shows the various functional blocks within the watchdog module.
/512
OSCCLK
WDCR (WDPS(2:0))
WDCLK
WDCNTR(7:0)
WDKEY(7:0)
Bad Key
Good Key
1
0
1
WDCR (WDCHK(2:0))
Bad
WDCHK
Key
WDCR (WDDIS)
Clear Counter
SCSR (WDENINT)
Watchdog
Prescaler
Generate
Output Pulse
(512 OSCCLKs)
8-Bit
Watchdog
Counter
CLR
WDRST
WDINT
Watchdog
55 + AA
Key Detector
XRS
Core-reset
NOTE A: The WDRST signal is driven low for 512 OSCCLK cycles.
WDRST
(See Note A)
Internal
Pullup
Figure 311. Watchdog Module
The WDINT signal enables the watchdog to be used as a wakeup from IDLE/STANDBY mode timer.
In STANDBY mode, all peripherals are turned off on the device. The only peripheral that remains functional
is the watchdog. The WATCHDOG module will run off the PLL clock or the oscillator clock. The WDINT signal
is fed to the LPM block so that it can wake the device from STANDBY (if enabled). See Section 3.12,
Low-Power Modes Block, for more details.