System Integration Module (SIM60)
MOTOROLA
MC68360 USER’S MANUAL
6-41
PLLEN—PLL Enable Bit
The QUICC does not support disabled PLL. The bit is always set to one on reset unless the
MODCK1-MODCK0 pins are driven with 00 during reset. This mode of MODCK (00) is
reserved as indicated in 2.1.10.4 Clock Mode Select (MODCK1–MODCK0).
PLLWP—PLLCR Write Protect
This bit protects accidental writing of the PLLCR. After reset, this bit defaults to zero to
enable writing. Setting this bit prevents further writing (excluding the first write that sets
this bit).
PREEN—Prescaler Enable
This bit controls the divide-by-128 prescaler on the EXTAL signal. This bit is set during
hardware reset only if the MODCK1–MODCK0 pins specify that the divide-by-128 pres-
caler is used. It may be read thereafter as a status. If it is ever modified by software, it
should be changed at the same time that the corresponding change in the MF bits is per-
formed.
0 = The divide-by-128 prescaler is disabled. CLKIN = EXTAL—the PLL input clock fre-
quency is the EXTAL frequency.
1 = The divide-by-128 prescaler is enabled. CLKIN = EXTAL/128—the PLL input clock
frequency is the EXTAL frequency divided by 128.
STSIM—Stop Mode SIMCLK
0 = When the LPSTOP instruction is executed, the SIMCLK is driven from the crystal.
The frequency is either EXTAL/2 or EXTAL/256, depending on the divide-by-128
option. The PLL is disabled to conserve power.
1 = When the LPSTOP instruction is executed, the SIM60 clock is driven from the
VCO.
MF11–MF0—Multiplication Factor
These bits define the multiplication factor that will be applied to the PLL input frequency. The
multiplication factor can be any integer from 1 to 4096. The system frequency is ((MF bits +
1)
×
EXTALDIV), where EXTALDIV is either EXTAL or EXTAL/128, depending on the
MODCK bit configuration. The multiplication factor must be chosen to ensure that the result-
ing VCO output frequency will be in the range from 10 MHz to the maximum allowed clock
input frequency (e.g., 25 MHz for a 25-MHz device). In addition, the VCO outputs a 2
×
fre-
quency signal, which is 2
×
the multiplied value configured in the MF bits. This frequency is
not used in any of the MF calculations.
The value 000 results in a multiplier value of 1; the value $FFF results in a multiplier value
of 4096.
Anytime a new value is written into the MF11–MF0 bits, the PLL will lose the lock condition
and, after a delay, will relock. When the PLL loses its lock condition, all clocks generated by
the PLL are disabled. After a hardware reset, the MF11–MF0 bits default to either 0 or 400
($190 hex), depending on the MODCK1–MODCK0 pins (giving a multiplication factor of 1 or
401). If the multiplication factor is 401, then a standard 32.768-kHz crystal generates an ini-