Functional Overview
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 “limp-mode” clock. The limp-mode clock will continue to clock the CPU and peripherals at a typical
frequency of 1−4 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 F281x and C281x have 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 131072 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.
X1/XCLKIN
X2
X1/XCLKIN
X2
External Clock Signal
C
C
L1
(see Note A)
L2
(see Note A)
(Toggling 0−V
DD
)
Crystal
(a)
NC
(b)
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 3−10. Recommended Crystal/Clock Connection
Table 3−16. Possible PLL Configuration Modes
PLL MODE
REMARKS
SYSCLKOUT
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.
PLL Disabled
XCLKIN
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.
PLL Bypassed
PLL Enabled
XCLKIN/2
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
53
April 2001 − Revised December 2004
SPRS174L
TI [ TEXAS INSTRUMENTS ]
