SM320F2808-EP Datasheet, PDF (41/118 Pages) Texas Instruments – Digital Signal Processors

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SM320F2808-EP, SM320F2806-EP
SM320F2801-EP
Digital Signal Processors
SGLS316A – MARCH 2006 – REVISED FEBRUARY 2007
reset or WDINT interrupt. However, when the external input clock fails, the watchdog counter stops
decrementing (i.e., the watchdog counter does not change with the limp-mode clock). In addition to this,
the device will be reset and the “Missing Clock Status” (MCLKSTS) bit will be set. These conditions could
be used by the application firmware to detect the input clock failure and initiate necessary shut-down
procedure for the system.
NOTE
Applications in which the correct CPU operating frequency is absolutely critical should
implement a mechanism by which the DSP will be held in reset, should the input clocks
ever fail. For example, an R-C circuit may be used to trigger the XRS pin of the DSP,
should the capacitor ever get fully charged. An I/O pin may be used to discharge the
capacitor on a periodic basis to prevent it from getting fully charged. Such a circuit would
also help in detecting failure of the flash memory and the V_DD3VFL rail.
3.6.2 Watchdog Block
The watchdog block on the 280x is similar to the one used on the 240x and 281x 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 3-12 shows the various functional blocks within the watchdog module.
OSCCLK
WDCR (WDPS(2:0))
WDCR (WDDIS)
Watchdog ^WDCLK
/512
Prescaler
WDCNTR(7:0)
8-Bit
Watchdog
Counter
CLR
Clear Counter
Internal
Pullup
XRS
WDKEY(7:0)
Watchdog
55 + AA
Key Detector
Good Key
Core-reset
WDCR (WDCHK(2:0))
Bad
WDCHK
Key
Generate ^WDRST
Output Pulse _WDINT
(512 OSCCLKs)
SCSR (WDENINT)
WDRST^(A)
101
A. The WDRST signal is driven low for 512 OSCCLK cycles.
Figure 3-12. Watchdog Module
The WDINT signal enables the watchdog to be used as a wakeup from IDLE/STANDBY mode.
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Functional Overview
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