cop8fg National Semiconductor Corporation, cop8fg Datasheet - Page 16

• Manufacturer Part Number: cop8fg
• Description: 8-bit Cmos Rom Based And Otp Microcontrollers With 8k To 32k Memory, Two Comparators And Usart
• Manufacturer: National Semiconductor Corporation

www.national.com
5.0 Functional Description
5.8 OSCILLATOR CIRCUITS
There are four clock oscillator options available: Crystal Os-
cillator with or without on-chip bias resistor, R/C Oscillator
with on-chip resistor and capacitor, and External Oscillator.
The oscillator feature is selected by programming the ECON
register, which is summarized in Table 1 .
5.8.1 Crystal Oscillator
The crystal Oscillator mode can be selected by programming
ECON 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 pro-
gramming ECON Bit 3 to 1 with the crystal oscillator option
selection. The value of the resistor is in the range of 0.5M to
2M (typically 1.0M). Table 2 shows the component values re-
quired for various standard crystal values. Resistor R2 is
only used when the on-chip bias resistor is disabled. Figure
13 shows the crystal oscillator connection diagram.
ECON4 ECON3
R1 (k )
0
1
0
1
5.6
0
0
0
TABLE 2. Crystal Oscillator Configuration,
0
0
1
1
R2 (M )
With On-Chip Bias Resistor
TABLE 1. Oscillator Option
1
1
1
1
External Oscillator
Crystal Oscillator without Bias Resistor
R/C Oscillator
Crystal Oscillator with Bias Resistor
T
A
= 25˚C, V
C1 (pF)
100
18
20
25
Oscillator Option
DS101116-17
CC
= 5V
100–156
C2 (pF)
18
20
25
(Continued)
FIGURE 13. Crystal Oscillator
CKI Freq.
(MHz)
0.455
15
10
4
16
5.8.2 External Oscillator
The External Oscillator mode can be selected by program-
ming ECON Bit 3 to 0 and ECON 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. G7/
CKO is available as a general purpose input G7 and/or Halt
control. Figure 14 shows the external oscillator connection
diagram.
5.8.3 R/C Oscillator
The R/C Oscillator mode can be selected by programming
ECON Bit 3 to 1 and ECON Bit 4 to 0. In R/C oscillation
mode, CKI is left floating, while G7/CKO is available as a
general purpose input G7 and/or HALT control. The R/C con-
trolled oscillator has on-chip resistor and capacitor for maxi-
mum R/C oscillator frequency operation. The maximum fre-
quency is 5 MHz
temperature range of −40˚C to +85˚C. For max frequency
operation, the CKI pin should be left floating. For lower fre-
quencies, an external capacitor should be connected be-
tween CKI and either V
cillator to external noise can be improved by connecting one
half the external capacitance to V
PC board trace length on the CKI pin should be kept as short
as possible. Table 3 shows the oscillator frequency as a
function of external capacitance on the CKI pin. Figure 15
shows the R/C oscillator configuration.
* Assumes 3-5 pF board capacitance.
Capacitor (pF)*
External
6100
125
52
Without On-Chip Bias Resistor
0
9
TABLE 3. R/C Oscillator Configuration,
−40˚C to +85˚C, V
OSC Freq. Variation of
±
35% for V
R/C OSC Freq
CC
32 kHz
(MHz)
DS101116-18
or GND. Immunity of the R/C os-
5
4
2
1
CC
CC
= 4.5V to 5.5V,
between 4.5V to 5.5V and
CC
and one half to GND.
±
35%
Instr. Cycle
312.5
(µs)
2.0
2.5
5.0
10