Clocks and Power Control
MPC561/MPC563 Reference Manual, Rev. 1.2
8-4
Freescale Semiconductor
8.2.1
Frequency Multiplication
The PLL can multiply the input frequency by any integer between one and 4096. The multiplication factor
depends on the value of the MF[0:11] bits in the PLPRCR register. While any integer value from one to
4096 can be programmed, the resulting VCO output frequency must be at least 15 MHz. The multiplication
factor is set to a predetermined value during power-on reset as defined in
8.2.2
Skew Elimination
The PLL is capable of eliminating the skew between the external clock entering the chip (EXTCLK) and
both the internal clock phases and the CLKOUT pin, making it useful for tight synchronous timings. Skew
elimination is active only when the PLL is enabled and programmed with a multiplication factor of one or
two (MF = 0 or 1). The timing reference to the system PLL is the external clock input (EXTCLK pin).
8.2.3
Pre-Divider
A pre-divider before the phase comparator enables additional system clock resolution when the crystal
oscillator frequency is 20 MHz. The division factor is determined by the DIVF[0:4] bits in the PLPRCR.
8.2.4
PLL Block Diagram
As shown in
, the reference signal, OSCCLK, goes to the phase comparator. The phase
comparator controls the direction (up or down) that the charge pump drives the voltage across the external
filter capacitor (XFC). The direction depends on whether the feedback signal phase lags or leads the
reference signal. The output of the charge pump drives the VCO. The output frequency of the VCO is
divided down and fed back to the phase comparator for comparison with the reference signal, OSCCLK.
The MF values, zero to 4095, are mapped to multiplication factors of one to 4096. Note that when the PLL
is operating in 1:1 mode (refer to
), the multiplication factor is one (MF = 0). The PLL output
frequency is twice the maximum system frequency. This double frequency is needed to generate GCLK1
and GCLK2 clocks. On power-up, with a 4-MHz or 20-MHz crystal and the default MF settings,
VCOOUT will be 40 MHz and the system clock will be 20 MHz.
The equation for VCOOUT is:
NOTE
When operating with the backup clock, the system clock (and CLKOUT) is
one-half of the ring oscillator frequency, (i.e., the system clock is
approximately 11 MHz). The time base and PIT clocks will be twice the
system clock frequency.
In the case of initial system power up, or if KAPWR is lost, an external circuit must assert power on reset
(PORESET). Once KAPWR is valid, PORESET must be asserted long enough to allow the external
oscillator to start up and stabilize for the device to come out of reset in normal (non limp) mode.
VCOOUT =
OSCCLK
DIVF + 1
x (MF + 1) x 2
Summary of Contents for MPC561
Page 84: ...MPC561 MPC563 Reference Manual Rev 1 2 lxxxiv Freescale Semiconductor...
Page 144: ...Signal Descriptions MPC561 MPC563 Reference Manual Rev 1 2 2 46 Freescale Semiconductor...
Page 206: ...Central Processing Unit MPC561 MPC563 Reference Manual Rev 1 2 3 62 Freescale Semiconductor...
Page 302: ...Reset MPC561 MPC563 Reference Manual Rev 1 2 7 14 Freescale Semiconductor...
Page 854: ...Time Processor Unit 3 MPC561 MPC563 Reference Manual Rev 1 2 19 24 Freescale Semiconductor...
Page 968: ...Development Support MPC561 MPC563 Reference Manual Rev 1 2 23 54 Freescale Semiconductor...
Page 1144: ...Internal Memory Map MPC561 MPC563 Reference Manual Rev 1 2 B 34 Freescale Semiconductor...
Page 1212: ...TPU3 ROM Functions MPC561 MPC563 Reference Manual Rev 1 2 D 60 Freescale Semiconductor...
Page 1216: ...Memory Access Timing MPC561 MPC563 Reference Manual Rev 1 2 E 4 Freescale Semiconductor...