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3.6.1
OSC and PLL Block
X1
XCLKIN
(3.3-V clock input)
On chip
oscillator
X2
xor
PLLSTS[OSCOFF]
OSCCLK
PLL
VCOCLK
4-bit PLL Select (PLLCR)
OSCCLK or
VCOCLK
CLKIN
OSCCLK
0
PLLSTS[PLLOFF]
n
n
≠
0
/2
PLLSTS[CLKINDIV]
External Clock Signal
(Toggling 0V
DDIO
)
XCLKIN
X2
NC
X1
External Clock Signal
(Toggling 0V
DD
)
XCLKIN
X2
NC
X1
C
L1
X2
X1
Crystal
C
L2
XCLKIN
TMS320F28044
Digital Signal Processor
SPRS357B–AUGUST 2006–REVISED MAY 2007
Figure 3-6
shows the OSC and PLL block on the F28044.
Figure 3-6. OSC and PLL Block Diagram
The on-chip oscillator circuit enables a crystal/resonator to be attached to the F28044 device using the X1
and X2 pins. If the on-chip oscillator is not used, an external oscillator can be used in either one of the
following configurations:
1. A 3.3-V external oscillator can be directly connected to the XCLKIN pin. The X2 pin should be left
unconnected and the X1 pin tied low. The logic-high level in this case should not exceed V
DDIO
.
2. A 1.8-V external oscillator can be directly connected to the X1 pin. The X2 pin should be left
unconnected and the XCLKIN pin tied low. The logic-high level in this case should not exceed V
DD
.
The three possible input-clock configurations are shown in
Figure 3-7
through
Figure 3-9
Figure 3-7. Using a 3.3-V External Oscillator
Figure 3-8. Using a 1.8-V External Oscillator
Figure 3-9. Using the Internal Oscillator
36
Functional Overview
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