
DSP56374 Data Sheet, Rev. 4.2
External Clock Operation
Freescale Semiconductor
30
If the DSP56374 system clock is an externally supplied square wave voltage source, it is connected to
EXTAL
(Figure 2.). When the external square wave source connects to EXTAL, the XTAL pin is not used.
Figure 2. External Clock Timing
Table 19. Clock Operation
No.
Characteristics
Symbol
Min
Max
Units
6
EXTAL input high 1
(40% to 60% duty cycle)
Eth
3.33
50
ns
7
EXTAL input low2
(40% to 60% duty cycle)
Etl
3.33
50
ns
8
EXTAL cycle time
With PLL disabled
With PLL enabled
Etc
6.67
50
inf
200
ns
9
Instruction cycle time= ICYC = TC
3
With PLL disabled
With PLL enabled
Icyc
6.67
inf
13.33
ns
Suggested component values:
fosc = 24.576 MHz
R = 1 M ±10%
C (EXTAL)= 18 pF
Calculations are for a 12 - 49 MHz crystal with the following parameters:
shunt capacitance (C0) of 10 pF - 12 pF
series resistance 40 Ohm
C (XTAL) = 47 pF
drive level of 10
W
EXTAL
VIL
VIH
Midpoint
Note:
The midpoint is 0.5 (VIH + VIL).
ETH
ETL
ETC
7
8
6