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6.5 External Clock Input From MXI/CLKIN Pin
6.5.1
Clock Input Option 1 – Crystal
MXI/CLKIN
MXO
C1
C2
Crystal
27 MHz
MXV
SS
1.8 V
MXV
DD
RBIAS
(optional)
C
L
C
1
C
2
(
C
1
C
2
)
TMS320DM6443
Digital Media System-on-Chip
SPRS282E–DECEMBER 2005–REVISED MARCH 2007
The DM6443 device has two input pins for an external clock source, MXI/CLKIN and M24XI. The
MXI/CLKIN pin provides the clock source for PLL1 and PLL2 whose optimal frequency is 27 MHz. The
M24XI pin provides the clock source for the USB PLL whose optimal frequency is 24 MHz.
The DM6443 device includes two options to provide an external clock input:
1. Use an on-chip oscillator with external crystal or ceramic resonator circuit (only supporting
parallel-resonant mode; it does not provide overtone support). For more details, see
Section 6.5.1
.
2. Use an external 1.8-V LVCMOS-compatible clock input. For more details, see
Section 6.5.2
.
6.5.1.1
27-MHz Crystal for System Oscillator
In this option, a crystal is used as the external clock input to the DM6443 PLL1 and PLL2.
The 27-MHz oscillator provides the reference clock for all DM6443 subsystems and peripherals. The
on-chip oscillator requires an external 27-MHz crystal connected across the MXI and MXO pins, along
with two load capacitors, as shown in
Figure 6-10
. The external crystal load capacitors
must
be
connected only to the 27-MHz oscillator ground pin (MXV
SS
).
Do not
connect to board ground (V
SS
). The
MXV
DD
pin can be connected to the same 1.8 V power supply as DV
DD18
.
Figure 6-10. 27-MHz System Oscillator
The RBIAS resistor is optional. If the RBIAS resistor is used, it should equal 1 M
±
5%. The load
capacitors, C1 and C2, should be chosen such that the equation is satisfied (typical values are C1 = C2 =
10 pF). C
L
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
(MXI and MXO) and to the MXV
SS
pin.
Table 6-10. Crystal Requirements for a 27-MHz System Oscillator
PARAMETER
MIN
TYP
MAX
UNIT
ms
MHz
ppm
Start-up time (from power up until oscillating at stable frequency of 27 MHz)
Oscillation frequency
ESR
Frequency stability
4
27
60
±
50
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Peripheral and Electrical Specifications
101