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On-Chip Clock Synthesis

The PLLD bit should not be set when the PLLE bit is set. Both the PLLD bit and the PLLE bit are cleared when the chip is reset.

NOTE:

The STOP instruction does not power down the PLL if the PLL is not powered down (PLLD = 0) when entering stop mode.

Table 10-1. PLL Operations

PLLE

PLLD

Phi Clock

PLL Mode

 

 

 

 

 

 

 

 

0

0

Oscillator clock

Active

 

 

 

 

0

1

Oscillator clock

Power down

 

 

 

 

1

0

Oscillator clock × [YD + 1]

Active

 

 

 

 

1

1

(Reserved)

(Reserved)

 

 

 

 

10.2.1.4 Low Power Stop (LPST)—Bit 12

The low power stop (LPST) control bit is used to place the chip in the lowest power configuration when entering stop mode. If the LPST bit is set when entering stop mode, the clock is disabled at the crystal oscillator. If the LPST bit is cleared when entering stop mode, the oscillator continues running in stop mode. The LPST bit is cleared on DSP reset.

10.2.1.5 Test Enable (TSTEN)—Bit 11

The test enable (TSTEN) control bit allows the prescaler output to drive the CLKO pin when set in conjunction with the CS[1:0] bits, as shown in Table 10-3 on page 10-7. The TSTEN bit is cleared on DSP reset.

10.2.1.6 Prescaler Divider (PS[2:0])—Bits 10–8

The prescaler divider (PS[2:0]) control bits are used to pass, disable, or divide the oscillator clock by several different divide ratios: 16, 64, and 256. The output of the divider can be used as the operating clock for the timer module or for the COP and real-time timers. On reset, the PS[2:0] bits are set to 010, providing a divide-by-16 prescaler rate. This ensures that implementations using a high-speed clock will function properly, because the general-purpose and COP timers are not designed to work at frequencies higher than one-sixteenth of the maximum frequency of the DSP56824.

The prescaler divider is used for systems with higher frequency crystals to provide a slower clocking frequency near 32 kHz for the RTI and COP timers discussed in detail in Chapter 11, “COP and RTI Module.” Typically a user would set the divider to divide by one for systems with a 32.0 kHz or 38.4 kHz crystal, but would use the divider when a higher frequency crystal is used. Likewise, it is possible to disable this divider and its output clock for low-power applications that do not require a real-time or COP timer and do not require a low-frequency clock for the timer module.

The prescaler should always be set up with the correct division ratio before any peripheral using the prescaler clock is enabled. Table 10-2 on page 10-7 shows how to program the PS[2:0] bits.

10-6

DSP56824 User’s Manual

 

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