em78p257 ELAN Microelectronics Corp, em78p257 Datasheet - Page 47

• Manufacturer Part Number: em78p257
• Description: 8-bit Microcontroller
• Manufacturer: ELAN Microelectronics Corp

This specification is subject to change without prior notice.
2. Crystal Oscillator/Ceramic Resonators(XTAL)
EM78P257A/B can be driven by an external clock signal through the OSCI pin as shown in Fig.18 below.
In most applications, pin OSCI and pin OSCO can be connected with a crystal or ceramic resonator to generate
oscillation. Fig. 19 depicts such circuit. The same thing applies whether it is in the HXT mode or in the LXT mode.
Table 17 provides the recommended values of C1 and C2. Since each resonator has its own attribute, user
should refer to its specification for appropriate values of C1 and C2. RS, a serial resistor, may be necessary for AT
strip cut crystal or low frequency mode.
<Note> The transient point of system frequency between HXT and LXT is around 400 KHz.
Table 16 The summary of maximum operating speeds
ERC(External RC oscillator mode)
HXT(High XTAL oscillator mode)
LXT(Low XTAL oscillator mode)
Two clocks
Conditions
EM78P257A/B
E M 7 8 P 2 5 7 A / B
Fig. 18 Circuit for External Clock Input
Fig. 19 Circuit for Crystal/Resonator
OSCO
OSCI
O S C O
O S C I
XTAL
RS
47
1
0
0
VDD
2.3
3.0
5.0
C2
C1
0
0
0
Fxt max.(MHz)
20
4
8
1
1
0
EM78P257
07.27.2004 (V1.4)
OTP ROM