27.9
Clocks
The following sections provide specifications on the various clock sources and mode.
27.9.1
PLL Specifications
The following tables provide specifications for using the PLL.
Note:
If the integrated Ethernet PHY is used in this device, then F
REF_XTAL
and F
REF_EXT
are
required to be 25 MHz.
Table 27-16. Phase Locked Loop (PLL) Characteristics
Unit
Max
Nom
Min
Parameter Name
Parameter
MHz
25
-
5
Crystal reference
F
REF_XTAL
MHz
25
-
5
External clock reference
F
REF_EXT
MHz
480
-
-
PLL VCO frequency at 1.2V
a
F
PLLR
MHz
480
-
-
PLL VCO frequency at 0.9V
b
F
PLLS
µs
512 * (reference clock
period)
-
-
PLL lock time (enabling the PLL) when PLL is
transitioning from power down to power up
T
READY
µs
128 * (reference clock
period)
-
-
PLL lock time when the PLL VCO frequency is
changed (PLL is already enabled)
µs
128 * (reference clock
period)
-
-
PLL lock time, changing the
OSCSRC
between
MOSC and PIOSC
a. PLL frequency is manually calculated using the values in the PLLFREQ0 and PLLFREQ1 registers.
b. If the LDO is dropped to 0.9V, the system must be run 1/4 of the maximum frequency at most. The
Q
value in the
PLLFREQ1
register must be set to 0x3 rather than using the
PSYSDIV
field in the
RSCLKCFG
register for the divisor.
27.9.1.1
PLL Configuration
The PLL is disabled by default during power-on reset and is enabled later by software if required.
Software specifies the output divisor to set the system clock frequency and enables the PLL to drive
the output. The PLL is controlled using the
PLLFREQ0
,
PLLFREQ1
and
PLLSTAT
registers.
Changes made to these registers do not become active until after the
NEWFREQ
bit in the
RSCLKCFG
register is enabled.
The clock source for the main PLL is selected by configuring the
PLLSRC
field in the
Run and Sleep
Clock Configuration (RSCLKCFG)
register.
The PLL allows for the generation of system clock frequencies in excess of the reference clock
provided. The reference clocks for the PLL are the PIOSC and the MOSC. The PLL is controlled
by two registers,
PLLFREQ0
and
PLLFREQ1
. The PLL VCO frequency (f
VCO
) is determined through
the following calculation:
f
VCO
= f
IN
* MDIV
where
f
IN
= f
XTAL
/(Q+1)(N+1) or f
PIOSC
/(Q+1)(N+1)
MDIV = MINT + (MFRAC / 1024)
The
Q
and
N
values are programmed in the
PLLFREQ1
register. Note that to reduce jitter,
MFRAC
should be programmed to 0x0.
1835
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller