Freescale Semiconductor MC68HC908MR16, MC68HC908MR32 manual VRS74 VCO Range Select Bits, Interrupts

Interrupts

NOTE

The multiplier select bits have built-in protection that prevents them from being written when the PLL is on (PLLON = 1).

VRS[7:4] — VCO Range Select Bits

These read/write bits control the hardware center-of-range linear multiplier L, which controls the hardware center-of-range frequency fVRS. See 4.3.2.1 PLL Circuits, 4.3.2.4 Programming the PLL and

4.5.1PLL Control Register. VRS[7:4] cannot be written when the PLLON bit in the PLL control register (PCTL) is set. See 4.3.2.5 Special Programming Exceptions. A value of $0 in the VCO range select bits disables the PLL and clears the BCS bit in the PCTL. See 4.3.3 Base Clock Selector Circuit and 4.3.2.5 Special Programming Exceptions for more information.

Reset initializes the bits to $6 to give a default range multiply value of 6.

NOTE

The VCO range select bits have built-in protection that prevents them from being written when the PLL is on (PLLON = 1) and prevents selection of the VCO clock as the source of the base clock (BCS = 1) if the VCO range select bits are all clear.

The VCO range select bits must be programmed correctly. Incorrect programming may result in failure of the PLL to achieve lock.

4.6 Interrupts

When the AUTO bit is set in the PLL bandwidth control register (PBWC), the PLL can generate a CPU interrupt request every time the LOCK bit changes state. The PLLIE bit in the PLL control register (PCTL) enables CPU interrupts from the PLL. PLLF, the interrupt flag in the PCTL, becomes set whether interrupts are enabled or not. When the AUTO bit is clear, CPU interrupts from the PLL are disabled and PLLF reads as logic 0.

Software should read the LOCK bit after a PLL interrupt request to see if the request was due to an entry into lock or an exit from lock. When the PLL enters lock, the VCO clock, CGMVCLK, divided by two can be selected as the CGMOUT source by setting BCS in the PCTL. When the PLL exits lock, the VCO clock frequency is corrupt, and appropriate precautions should be taken. If the application is not frequency-sensitive, interrupts should be disabled to prevent PLL interrupt service routines from impeding software performance or from exceeding stack limitations.

NOTE

Software can select the CGMVCLK divided by two as the CGMOUT source even if the PLL is not locked (LOCK = 0). Therefore, software should make sure the PLL is locked before setting the BCS bit.

4.7 Wait Mode

The WAIT instruction puts the MCU in low power-consumption standby mode.

The WAIT instruction does not affect the CGM. Before entering wait mode, software can disengage and turn off the PLL by clearing the BCS and PLLON bits in the PLL control register (PCTL). Less power-sensitive applications can disengage the PLL without turning it off. Applications that require the PLL to wake the MCU from wait mode also can deselect the PLL output without turning off the PLL.

MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1

Freescale Semiconductor 69