COP8TAB9 Datasheet, PDF (19/59 Pages) National Semiconductor (TI) – 8-Bit CMOS Flash Microcontroller with 2k Byte or 4k Byte Memory

10.0 Functional Description
(Continued)
10.7.1 R/C Oscillator
The device features an R/C oscillator with two modes of
operation:
1. R/C with internal R operating at a fixed frequency (R/C
mode)
2. R/C with external frequency control resistor (R/C+R
mode).
If the Oscillator Selection bits of the Option Byte remain
unprogrammed (equal to zero), the internal R/C Oscillator
mode will be selected. In internal R/C oscillation mode, CKI
may be left unconnected, while G7/CKO is available as a
general purpose input G7 and/or HALT control. The internal
R/C oscillator has on-chip resistor and capacitor for prede-
termined R/C oscillator frequency operation. The predeter-
mined frequency is 10 MHz ± 20% for temperature range of
−40˚C to +85˚C.
The R/C Oscillator with external frequency control resistor
mode (R/C+R) can be selected by programming Option Bit 3
to 1 and Option Bits 6 and 4 to 0. In R/C+R mode, the
frequency of oscillation is controlled by the current through a
resistor connected from the CKI pin to GND. G7/CKO is
available as a general purpose input G7 and/or HALT con-
trol. The maximum frequency is 10 MHz ± 20% for tempera-
ture range of −40˚C to +85˚C. For lower frequencies, an
external resistor should be connected between CKI and
GND. 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
approximate external resistance on the CKI pin. Figure 16
shows the R/C oscillator configuration.
TABLE 3. R/C+R Oscillator Configuration,
−40˚C to +85˚C,
OSC Freq. Variation of ± 20%
External Resistor
(kΩ)
60
100
200
500
OSC Freq
(MHz)
15
10
5
2
Instr. Cycle
(µs)
0.65
1.0
2.0
5.0
20047525
FIGURE 14. Crystal Oscillator With External Bias
Resistor
20047526
FIGURE 15. External Oscillator
20047524
FIGURE 13. Crystal Oscillator With On-Chip Bias
Resistor
19
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