cop8scr9lva8 National Semiconductor Corporation, cop8scr9lva8 Datasheet - Page 24

• Manufacturer Part Number: cop8scr9lva8
• Description: 8-bit Cmos Flash Based Microcontroller With 32k Memory, Virtual Eeprom And Brownoutff
• Manufacturer: National Semiconductor Corporation

www.national.com
10.0 Functional Description
10.8 OSCILLATOR CIRCUITS
The device has two crystal oscillators to facilitate low power
operation while maintaining throughput when required. Fur-
ther information on the use of the two oscillators is found in
Section 7.0 Power Saving Features. The low speed oscillator
utilizes the L0 and L1 port pins. References in the following
text to CKI will also apply to L0 and references to G7/CKO
will also apply to L1.
10.8.1 Oscillator
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 is provided to reduce system part
count. 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
on-chip, for the high speed oscillator, and is shown for
reference. Figure 12 shows the crystal oscillator connection
diagram. A ceramic resonator of the required frequency may
be used in place of a crystal if the accuracy requirements are
not quite as strict.
(Continued)
R1 (kΩ)
FIGURE 11. Reset Circuit Using Power-On Reset
5.6
0
0
0
0
TABLE 2. Crystal Oscillator Configuration,
R2 (MΩ)
On Chip
On Chip
On Chip
On Chip
20
T
A
= 25˚C, V
C1 (pF)
18–36
100
18
18
**
CC
= 5V
100–156
C2 (pF)
18–36
18
18
**
10138914
CKI Freq.
32.768
(MHz)
0.455
kHz
10
5
1
*
24
The crystal and other oscillator components should be
placed in close proximity to the CKI and CKO pins to mini-
mize printed circuit trace length.
The values for the external capacitors should be chosen to
obtain the manufacturer’s specified load capacitance for the
crystal when combined with the parasitic capacitance of the
trace, socket, and package (which can vary from 0 to 8 pF).
The guideline in choosing these capacitors is:
Manufacturer’s specified load cap = (C
C
C
should be less than or equal to C
Note: The low power design of the low speed oscillator
makes it extremely sensitive to board layout and load ca-
pacitance. The user should place the crystal and load ca-
pacitors within 1cm. of the device and must ensure that the
above equation for load capacitance is strictly followed. If
these conditions are not met, the application may have
problems with startup of the low speed oscillator.
10.8.2 Clock Doubler
This device contains a frequency doubler that doubles the
frequency of the oscillator selected to operate the main
microcontroller core. The details of how to select either the
high speed oscillator or low speed oscillator are described in,
Power Saving Features. When the high speed oscillator
connected to CKI operates at 10 MHz, the internal clock
frequency is 20 MHz, resulting in an instruction cycle time of
0.5 µs. When the 32 kHz oscillator connected to L0 and L1 is
selected, the internal clock frequency is 64 kHz, resulting in
an instruction cycle of 152.6 µs. The output of the clock
doubler is called MCLK and is referenced in many places
within this document.
*
**
Applies to connection to low speed oscillator on port pins L0 and L1 only.
parasitic
2
32 kHz (low speed oscillator)
See Note below.
can be trimmed to obtain the desired frequency. C
CKI Frequency
3.33 MHz
455 kHz
10 MHz
1 MHz
TABLE 3. Startup Times
1
.
Startup Time
1
10–30 ms
* C
1–10 ms
3–10 ms
3–20 ms
2–5 sec
2
) / (C
1
+ C
2
) +
2