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參數(shù)資料
| 型號： | UCD9501 |
| 廠商： | Texas Instruments, Inc. |
| 元件分類： | 數(shù)字信號處理 |
| 英文描述： | Digital Signal Processors |
| 中文描述： | 數(shù)字信號處理器 |
| 文件頁數(shù)： | 49/134頁 |
| 文件大小： | 1133K |
| 代理商： | UCD9501 |
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3.6.2
Watchdog Block
TMS320F2809, TMS320F2808, TMS320F2806
TMS320F2802, TMS320F2801, UCD9501
TMS320C2802, TMS320C2801 Digital Signal Processors
SPRS230G–OCTOBER 2003–REVISED FEBRUARY 2006
Table 3-16. Possible PLL Configuration Modes
SYSCLKOUT
(CLKIN)
OSCCLK/2
PLL MODE
REMARKS
PLLSTS[CLKINDIV]
Invoked by the user setting the PLLOFF bit in the PLLSTS register. The PLL block
is disabled in this mode. This can be useful to reduce system noise and for low
power operation. The PLLCR register must first be set to 0x0000 (PLL Bypass)
before entering this mode. The CPU clock (CLKIN) is derived directly from the
input clock on either X1/X2, X1 or XCLKIN.
PLL Bypass is the default PLL configuration upon power-up or after an external
reset (XRS). This mode is selected when the PLLCR register is set to 0x0000 or
while the PLL locks to a new frequency after the PLLCR register has been
modified. In this mode, the PLL itself is bypassed but the PLL is not turned off.
Achieved by writing a non-zero value n into the PLLCR register. Upon writing to the
PLLCR the device will switch to PLL Bypass mode until the PLL locks.
0
PLL Off
1
OSCCLK
0
OSCCLK/2
PLL Bypass
1
OSCCLK
PLL Enable
0
OSCCLK*n/2
3.6.1.3
Loss of Input Clock
In PLL-enabled and PLL-bypass mode, if the input clock OSCCLK is removed or absent, the PLL will still
issue a "limp-mode" clock. The limp-mode clock continues to clock the CPU and peripherals at a typical
frequency of 1-5 MHz. Limp mode is not specified to work from power-up, only after input clocks have
been present initially. In PLL bypass mode, the limp mode clock from the PLL is automatically routed to
the CPU if the input clock is removed or absent.
Normally, when the input clocks are present, the watchdog counter decrements to initiate a watchdog
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.
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 3-13
shows the various functional blocks within the watchdog module.
Functional Overview
49
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