SPRS230M – OCTOBER 2003 – REVISED MARCH 2011
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Table 3-17. Possible PLL Configuration Modes
PLL MODE
REMARKS
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.
PLLSTS[CLKINDIV]
0
1
0
1
0
SYSCLKOUT
(CLKIN)
OSCCLK/2
OSCCLK
OSCCLK/2
OSCCLK
OSCCLK*n/2
PLL Off
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
PLL Bypass
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.
PLL Enable
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.
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.
50
Functional Overview
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