Chapter 5 Generic Pin Configuration
Applications that do not use the key can leave the key pins unconnected. You can also use the Key Reset (KEYRST*) bit, Key Data (KEYDQ) bit, and Key Clock (KEYCLK*) bit pins as general-purpose, TTL, digital I/O pins.
For more information, see the Key Control Register (KEYREG) section in Chapter 3,
TNT4882 Interface Registers, and the Using the KEY Pins section in Chapter 4, TNT4882
Programming Considerations.
Oscillator Pins
The TNT4882 requires a 40-MHz clock signal. You can generate the clock signal by using one of two methods.
Crystal Oscillator
A CMOS 40-MHz crystal oscillator can drive the clock signal. Connect the crystal oscillator output to the XTALI pin of the TNT4882; leave the XTALO pin unconnected.
Discrete Oscillator Circuit
A circuit based on a 40-MHz quartz crystal can drive the clock signal. Figure 5-2 shows the recommended circuit for a third overtone mode crystal. As Figure 5-2 shows, the oscillator circuit consists of a tank circuit. The capacitors C1PAR and C2PAR are the parasitic capacitances of the oscillator macro cell. The capacitor C3 is a DC block, so the inductor does not short the inverter output to ground.
C
1PAR
XTLI
C
1
R f
40 MHz
C
2
XTLO INTERNAL TO IC
C
2PAR
C
3
L
1
Figure 5-2. Recommended Circuit for a Third Overtone Mode Crystal
© National Instruments Corp.
5-9 TNT4882 Programmer Reference
Generic Pin Configuration
Table 5-2 lists the specifications of the quartz crystal.
Table 5-2. Quartz Crystal Specifications
Frequency
Load Capacitance
Type
Effective Series
Resistance
Drive Level
40 MHz
20 pF
3rd Overtone, Parallel
< 50 ohms
> 500
µ
W
The values of the components of the oscillator circuit are as follows:
C1
C2
C3
L1
Rf
20 pF
68 pF
0.01
µ
F
0.47
µ
H
1 M
Ω
Chapter 5
TNT4882 Programmer Reference 5-10 © National Instruments Corp.
