
C1
C2
MXI1/CLKIN
MXO1
VSS_MX1
0.1 F
L1
VDDA_PLL1
VSSA_PLL1
Crystal
19.2MHz
24MHzor
36MHz
0.1 F
C
L
C
1C2
(C
1
C
2)
SPRS457E
– MARCH 2009 – REVISED JUNE 2011
6.6
Oscillators and Clocks
The device has one oscillator input/output pair (MXI1/MXO1) usable with external crystals or ceramic
resonators to provide clock inputs. The optimal frequencies for the crystals are 19.2 MHz, 24 MHz, 27
MHz, and 36 MHz. Optionally, the oscillator inputs are configurable for use with external clock oscillators.
If external clock oscillators are used, to minimize the clock jitter, a single clean power supply should power
both the device and the external oscillator circuit and the minimum CLKIN rise and fall times must be
observed. The electrical requirements and characteristics are described in this section.
The timing parameters for CLKOUT[3:1] are also described in this section. The device has three output
clock pins (CLKOUT[3:1]). See
Section 3.3 for more information on CLKOUT[3:1].
Note: Please ensure that the appropriate oscillator input pin (GIO81/OSCCFG) frequency range setting is
set correctly. For more details on this pin setting, see
Section 3.7.6.
6.6.1
MXI1 Oscillator
The MXI1 (typically 24 MHz, can also be 19.2 MHz, 27 MHz, or 36 MHz) oscillator provides the primary
reference clock for the device. The on-chip oscillator requires an external crystal connected across the
MXI1 and MXO1 pins, along with two load capacitors, as shown in
Figure 6-5. The external crystal load
capacitors must be connected only to the oscillator ground pin (VSS_MX1). Do not connect to board ground
(VSS). Also, the PLL power pin (VDDA_PLL1) should be connected to the power supply through a ferrite
bead, L1 in the example circuit shown in
Figure 6-5.
Note: If an external oscillator is to be used, the external oscillator clock signal should be connected to the
MXI1 pin with a 1.8V amplitude. The MXO1 should be left unconnected and the VSS_MX1 signal should
be connected to board ground (Vss).
Figure 6-5. MXI1 Oscillator
The load capacitors, C1 and C2, should be chosen such that the equation is satisfied (typical values are
C1 = C2 = 10 pF). CL in the equation is the load specified by the crystal manufacturer. All discrete
components used to implement the oscillator circuit should be placed as close as possible to the
associated oscillator pins (MXI1 and MXO1) and to the VSS_MX1 pin.
Copyright
2009–2011, Texas Instruments Incorporated
Peripheral Information and Electrical Specifications
81