PIC16F877A-I/ML Microchip Technology, PIC16F877A-I/ML Datasheet - Page 39

• Manufacturer Part Number: PIC16F877A-I/ML
• Description: IC MCU FLASH 8KX14 A/D 44QFN
• Manufacturer: Microchip Technology
• Series: PIC® 16Fr

Specifications of PIC16F877A-I/ML

• Core Size: 8-Bit
• Program Memory Size: 14KB (8K x 14)
• Core Processor: PIC
• Speed: 20MHz
• Connectivity: I²C, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, POR, PWM, WDT
• Number Of I /o: 33
• Program Memory Type: FLASH
• Eeprom Size: 256 x 8
• Ram Size: 368 x 8
• Voltage - Supply (vcc/vdd): 4 V ~ 5.5 V
• Data Converters: A/D 8x10b
• Oscillator Type: External
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 44-QFN
• Controller Family/series: PIC16F
• No. Of I/o's: 33
• Eeprom Memory Size: 256Byte
• Ram Memory Size: 368Byte
• Cpu Speed: 20MHz
• No. Of Timers: 3
• Processor Series: PIC16F
• Core: PIC
• Data Bus Width: 8 bit
• Data Ram Size: 368 B
• Interface Type: I2C, SPI, USART
• Maximum Clock Frequency: 20 MHz
• Number Of Programmable I/os: 33
• Number Of Timers: 1 x 16 bit
• Operating Supply Voltage: 2 V to 5.5 V
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• 3rd Party Development Tools: 52715-96, 52716-328, 52717-734
• Development Tools By Supplier: PG164130, DV164035, DV244005, DV164005, PG164120, ICE2000, DM163022, DV164120
• Minimum Operating Temperature: - 40 C
• On-chip Adc: 8
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: XLT44QFN2 - SOCKET TRAN ICE 44QFN/40DIPAC164322 - MODULE SOCKET MPLAB PM3 28/44QFN444-1001 - DEMO BOARD FOR PICMICRO MCU
• Lead Free Status / Rohs Status: Details

Section 2. Oscillator
2.3.3.1
Determining Best Values for Crystals, Clock Mode, C1, C2, and Rs
The best method for selecting components is to apply a little knowledge and a lot of trial, mea-
surement, and testing.
Crystals are usually selected by their parallel resonant frequency only, however other parame-
ters may be important to your design, such as temperature or frequency tolerance. Application
Note AN588 is an excellent reference if you would like to know more about crystal operation and
their ordering information.
The PICmicros™ internal oscillator circuit is a parallel oscillator circuit, which requires that a par-
allel resonant crystal be selected. The load capacitance is usually specified in the 20 pF to 32 pF
range. The crystal will oscillate closest to the desired frequency with capacitance in this range. It
may be necessary to sometimes juggle these values a bit, as described later, in order to achieve
2
other benefits.
Clock mode is primarily chosen by using the F
parameter specification
(parameter
1A) in the
OSC
device’s data sheet, based on frequency. Clock modes (except RC) are simply gain selections,
lower gain for lower frequencies, higher gain for higher frequencies. It is possible to select a
higher or lower gain, if desired, based on the specific needs of the oscillator circuit.
C1 and C2 should also be initially selected based on the load capacitance as suggested by the
crystal manufacturer and the tables supplied in the device data sheet. The values given in the
Microchip data sheet can only be used as a starting point, since the crystal manufacturer, supply
voltage, and other factors already mentioned may cause your circuit to differ from the one used
in the factory characterization process.
Ideally, the capacitance is chosen (within the range of the recommended crystal load preferably)
so that it will oscillate at the highest temperature and lowest V
that the circuit will be expected
DD
to perform under. High temperature and low V
both have a limiting affect on the loop gain, such
DD
that if the circuit functions at these extremes, the designer can be more assured of proper oper-
ation at other temperatures and supply voltage combinations. The output sine wave should not
be clipped in the highest gain environment (highest V
and lowest temperature) and the sine
DD
output amplitude should be great enough in the lowest gain environment (lowest V
and highest
DD
temperature) to cover the logic input requirements of the clock as listed in the device data sheet.
A method for improving start-up is to use a value of C2 greater than C1. This causes a greater
phase shift across the crystal at power-up, which speeds oscillator start-up.
Besides loading the crystal for proper frequency response, these capacitors can have the effect
of lowering loop gain if their value is increased. C2 can be selected to affect the overall gain of
the circuit. A higher C2 can lower the gain if the crystal is being over driven (see also discussion
on Rs). Capacitance values that are too high can store and dump too much current through the
crystal, so C1 and C2 should not become excessively large. Unfortunately, measuring the watt-
age through a crystal is tricky business, but if you do not stray too far from the suggested values
you should not have to be concerned with this.
A series resistor, Rs, is added to the circuit if, after all other external components are selected to
satisfaction, the crystal is still being over driven. This can be determined by looking at the OSC2
pin, which is the driven pin, with an oscilloscope. Connecting the probe to the OSC1 pin will load
the pin too much and negatively affect performance. Remember that a scope probe adds its own
capacitance to the circuit, so this may have to be accounted for in your design, i.e. if the circuit
worked best with a C2 of 20 pF and scope probe was 10 pF, a 30 pF capacitor may actually be
called for. The output signal should not be clipping or squashed. Overdriving the crystal can also
lead to the circuit jumping to a higher harmonic level or even crystal damage.
1997 Microchip Technology Inc.
DS31002A-page 2-9