PIC16C717-I/SS View Datasheet(PDF) - Microchip Technology
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Description
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PIC16C717-I/SS Datasheet PDF : 200 Pages
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PIC16C717/770/771
12.2.4 ER MODE
For timing insensitive applications, the ER (External
Resistor) clock mode offers additional cost savings.
Only one external component, a resistor connected to
the OSC1 pin and VSS, is needed to set the operating
frequency of the internal oscillator. The resistor draws
a DC bias current which controls the oscillation fre-
quency. In addition to the resistance value, the oscilla-
tor frequency will vary from unit to unit, and as a
function of supply voltage and temperature. Since the
controlling parameter is a DC current and not a capac-
itance, the particular package type and lead frame will
not have a significant effect on the resultant frequency.
Figure 12-4 shows how the controlling resistor is con-
nected to the PIC16C717/770/771. For Rext values
below 38k ohms, the oscillator operation may become
unstable, or stop completely. For very high Rext values
(e.g. 1M), the oscillator becomes sensitive to noise,
humidity and leakage. Thus, we recommend keeping
Rext between 38k and 1M ohms.
FIGURE 12-4: EXTERNAL RESISTOR
REXT
PIC16C717/770/771
RA6/OSC2/CLKOUT
RA7/OSC1/CLKIN
12.2.6 CLKOUT
In the INTRC and ER modes, the PIC16C717/770/771
can be configured to provide a clock out signal by pro-
gramming the configuration word. The oscillator fre-
quency, divided by 4, can be used for test purposes or
to synchronize other logic.
In the INTRC and ER modes, if the CLKOUT output is
enabled, CLKOUT is held low during reset.
12.2.7 DUAL SPEED OPERATION FOR ER AND
INTRC MODES
A software programmable dual speed oscillator is avail-
able in either ER or INTRC oscillator modes. This fea-
ture allows the applications to dynamically toggle the
oscillator speed between normal and slow frequencies.
The nominal slow frequency is 37KHz. In ER mode, the
slow speed operation is fixed and does not vary with
resistor size. Applications that require low current
power savings, but cannot tolerate putting the part into
sleep, may use this mode.
The OSCF bit in the PCON register is used to control
dual speed mode. See the PCON Register,
Register 2-8, for details.
When changing the INTRC or ER internal oscillator
speed, there is a period of time when the processor is
inactive. When the speed changes from fast to slow, the
processor inactive period is in the range of 100 µS to
300 µS. For speed change from slow to fast, the pro-
cessor is in active for 1.25 µS to 3.25 µS.
The Electrical Specification section shows the relation-
ship between the Rext resistance value and the operat-
ing frequency as well as frequency variations due to
operating temperature for given Rext and VDD values.
The ER oscillator mode has two options that control the
OSC2 pin. The first allows it to be used as a general
purpose I/O port. The other configures the pin as CLK-
OUT. The ER oscillator does not run during reset.
12.2.5 INTRC MODE
The internal RC oscillator provides a fixed 4 MHz (nom-
inal) system clock at VDD = 5V and 25°C, see “Electrical
Specifications” section for information on variation over
voltage and temperature. The INTRC oscillator does
not run during reset.
DS41120A-page 128
Advanced Information
© 1999 Microchip Technology Inc.
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