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TMS320C242
DSP CONTROLLER
SPRS063B – DECEMBER 1997 – REVISED DECEMBER 1999
47
POST OFFICE BOX 1443
HOUSTON, TEXAS 77251–1443
CLOCK CHARACTERISTICS AND TIMINGS
clock options
PARAMETER
PLL multiply-by-4
The ’C242 device includes an on-chip PLL which is hardwired for multiply-by-4 operation. This requires the use
of a 5-MHz clock input frequency for 20-MHz device operation. This input clock can be provided from either
an external reference crystal or oscillator.
external reference crystal clock option
The internal oscillator is enabled by connecting a crystal across XTAL1/CLKIN and XTAL2 pins as shown in
Figure 17a. The crystal should be in fundamental operation and parallel resonant, with an effective series
resistance of 30
(typical)
and a power dissipation of 1 mW; it should be specified at a load capacitance of
20 pF.
external reference oscillator clock option
The internal oscillator is disabled by connecting a TTL-level clock signal to XTAL1/CLKIN and leaving the XTAL2
input pin unconnected as shown in Figure 17b.
External
Clock Signal
(toggling 0–5 V)
C2
(see Note A)
C1
(see Note A)
Crystal
XTAL2
XTAL1/CLKIN
XTAL1/CLKIN
XTAL2
NC
(a)
(b)
NOTES: A. For the values of C1 and C2, see the crystal manufacturer’s specification.
B. TI recommends that customers have the resonator/crystal vendor characterize the operation of their device with the DSP chip. The
resonator/crystal vendor has the equipment and expertise to tune the tank circuit. The vendor can also advise customers regarding
the proper component values which ensure startup and stability over the entire operating range.
Figure 17. Recommended Crystal/Clock Connection
If the input frequency is 5 MHz, the series resistances of the crystal can be between 30 to 150
.
A