Texas Instruments TM4C1294NCPDT, Datasheet

Page: 238 / 1890

If the main oscillator provides the clock reference to the PLL, the translation provided by hardware
and used to program the PLL is available for software in the
PLL Frequency n (PLLFREQn) registers
(see page 292). The internal translation provides a translation within ± 1% of the targeted PLL VCO
frequency. Table 5-7 on page 238 shows the actual PLL frequency and error for a given crystal
choice.
Table 5-7 on page 238 provides examples of the programming expected for the PLLFREQ0 and
PLLFREQ1 registers. The first column specifies the input crystal frequency and the last column
displays the PLL frequency given the values of
MINT
and
N
, when
Q
=0.
Table 5-7. Actual PLL Frequency
a
PLL Frequency
(MHz)
Reference
Frequency
(MHz)
b
N MINT (Hexadecimal
Value)
MINT (Decimal
Value)
Crystal
Frequency
(MHz)
320 5 0x0 0x40 64 5
320 2 0x2 0x35 160 6
320 8 0x0 0x28 40 8
320 10 0x0 0x20 32 10
320 4 0x2 0x50 80 12
320 16 0x0 0x14 20 16
320 2 0x8 0xA0 160 18
320 20 0x0 0x10 16 20
320 8 0x2 0x28 40 24
320 5 0x4 0x40 64 25
480 5 0x0 0x60 96 5
480 6 0x0 0x50 80 6
480 8 0x0 0x3C 60 8
480 10 0x0 0x30 48 10
480 12 0x0 0x28 40 12
480 16 0x0 0x1E 30 16
480 6 0x2 0x50 80 18
480 20 0x0 0x18 24 20
480 24 0x0 0x14 20 24
480 5 0x4 0x60 96 25
a. For all examples listed, Q=0
b. For a given crystal frequency, N should be chosen such that the reference frequency is within 4 to 30 MHz.
PLL Operation
If a PLL configuration is changed, the PLL output frequency is unstable until it reconverges (relocks)
to the new setting. The time between the configuration change and relock is T
READY
(see Table
27-16 on page 1835). During the relock time, the affected PLL is not usable as a clock reference.
Software can poll the
LOCK
bit in the PLL Status (PLLSTAT) register to determine when the PLL
has locked.
Modification of the PLL VCO frequency may not be performed while the PLL serves as a clock
source to the system. All changes to the PLL must be performed using a different clock source until
the PLL has locked frequency. Thus, changing the PLL VCO frequency must be done as a sequence
from PLL to PIOSC/MOSC and then PIOSC/MOSC to new PLL.
June 18, 2014 238
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