cop8tac9 National Semiconductor Corporation, cop8tac9 Datasheet - Page 20

• Manufacturer Part Number: cop8tac9
• Description: 8-bit Cmos Flash Microcontroller With 2k Byte Or 4k Byte Memory
• Manufacturer: National Semiconductor Corporation

www.national.com
10.0 Functional Description
10.7.2 Crystal Oscillator
The Crystal Oscillator mode can be selected by program-
ming Option Bit 4 to 1. CKI is the clock input while G7/CKO
is the clock generator output to the crystal. An on-chip bias
resistor connected between CKI and CKO can be enabled
by programming Option Bit 3 to 0. The value of the resistor is
in the range of 0.3M to 2.5M (typically 1.0M).
Table 4 shows the component values required for various
standard crystal values. Resistor R2 is only used when the
on-chip bias resistor is disabled. Figure 14 and Figure 15
show the crystal oscillator connection diagrams.
10.7.3 External Oscillator
The External Oscillator mode can be selected by program-
ming Option Bit 3 to 0 and Option Bit 4 to 0. CKI can be
driven by an external clock signal provided it meets the
specified duty cycle, rise and fall times, and input levels.
(Continued)
R1 (kΩ) R2 (MΩ) C1 (pF) C2 (pF)
FIGURE 15. Crystal Oscillator With External Bias
With External Frequency Control Resistor (R/C+R)
5.6
0
0
0
TABLE 4. Crystal Oscillator Configuration,
1
1
1
1
T
A
= 25˚C, V
100
18
18
45
Resistor
100–156
30–36
CC
18
18
20047525
20047527
= 2.5V
CKI Freq. (MHz)
0.455
15
10
4
FIGURE 17. R/C Oscillator
20
G7/CKO is available as a general purpose input G7 and/or
Halt control. Figure 13 shows the external oscillator connec-
tion diagram.
10.7.4 Clock Prescaler
The device is equipped with a programmable clock prescaler
which allows the user to dynamically adjust the clock speed,
and thus the power dissipation, to the processing needs of
the application. By merely writing an eight-bit value to the
CLKPS register, the user can divide the input oscillator clock
by an integer multiple (1 — 256) and reduce the CPU clock
frequency. The format of the CLKPS Register is shown in
Table 5. The value written to the CLKPS register is one less
than the desired divider. A value of 0 (zero) written to the
CLKPS register yields a CPU clock equal to the input clock
frequency. A value of 255 written to the CLKPS register
yields a CPU clock with a period equal to 256 input clock
periods.
Bit 7
TABLE 5. Clock Prescale Register (CLKPS)
With Fully On-Chip R/C Oscillator.
FIGURE 16. External Oscillator
CLKPS
20047528
20047526
Bit 0