參數(shù)資料
型號(hào): SM320F2812HFGM
廠商: TEXAS INSTRUMENTS INC
元件分類: 數(shù)字信號(hào)處理
英文描述: 16-BIT, 150 MHz, OTHER DSP, CQFP172
封裝: NCTB, CERAMIC, QFP-172
文件頁數(shù): 89/147頁
文件大?。?/td> 1721K
代理商: SM320F2812HFGM
Functional Overview
46
December 2004
SGUS053
3.8.1
Loss of Input Clock
In PLL enabled mode, if the input clock XCLKIN or the oscillator clock is removed or absent, the PLL will still
issue a “l(fā)imp-mode” clock. The limp-mode clock will continue to clock the CPU and peripherals at a typical
frequency of 14 MHz. The PLLCR register should have been written to with a non-zero value for this feature
to work.
Normally, when the input clocks are present, the watchdog counter will decrement to initiate a watchdog reset
or WDINT interrupt. However, when the external input clock fails, the watchdog counter will stop decrementing
(i.e., the watchdog counter does not change with the limp-mode clock). This condition could be used by the
application firmware to detect the input clock failure and initiate necessary shut-down procedure for the
system.
3.9
PLL-Based Clock Module
The F2812 has an on-chip, PLL-based clock module. This module provides all the necessary clocking signals
for the device, as well as control for low-power mode entry. The PLL has a 4-bit ratio control to select different
CPU clock rates. The watchdog module should be disabled before writing to the PLLCR register. It can be
re-enabled (if need be) after the PLL module has stabilized, which takes 131 072 XCLKIN cycles.
The PLL-based clock module provides two modes of operation:
Crystal-operation
This mode allows the use of an external crystal/resonator to provide the time base to the device.
External clock source operation
This mode allows the internal oscillator to be bypassed. The device clocks are generated from an external
clock source input on the X1/XCLKIN pin.
External Clock Signal
(Toggling 0 VDD)
Cb1
(see Note A)
X2
X1/XCLKIN
X2
Crystal
Cb2
(see Note A)
(a)
(b)
NC
NOTE A: TI recommends that customers have the resonator/crystal vendor characterize the operation of their device with the DSP chip. The
resonator/crystal vendor has the equipment and expertise to tune the tank circuit. The vendor can also advise the customer regarding
the proper tank component values that will ensure start-up and stability over the entire operating range.
Figure 38. Recommended Crystal / Clock Connection
Table 315. Possible PLL Configuration Modes
PLL MODE
REMARKS
SYSCLKOUT
PLL Disabled
Invoked by tying XPLLDIS pin low upon reset. PLL block is completely
disabled. Clock input to the CPU (CLKIN) is directly derived from the clock
signal present at the X1/XCLKIN pin.
XCLKIN
PLL Bypassed
Default PLL configuration upon power-up, if PLL is not disabled. The PLL
itself is bypassed. However, the /2 module in the PLL block divides the clock
input at the X1/XCLKIN pin by two before feeding it to the CPU.
XCLKIN/2
PLL Enabled
Achieved by writing a non-zero value “n” into PLLCR register. The /2 module
in the PLL block now divides the output of the PLL by two before feeding it to
the CPU.
(XCLKIN * n) / 2
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