PIC18F87K90-E/PT Microchip Technology, PIC18F87K90-E/PT Datasheet - Page 416

• Manufacturer Part Number: PIC18F87K90-E/PT
• Description: 128kB Flash, 4kB RAM, 1kB EE, 16MIPS, NanoWatt XLP, LCD, 5V 80 TQFP 12x12x1mm TR
• Manufacturer: Microchip Technology
• Series: PIC® XLP™ 18Fr

Specifications of PIC18F87K90-E/PT

• Core Processor: PIC
• Core Size: 8-Bit
• Speed: 64MHz
• Connectivity: I²C, LIN, SPI, UART/USART
• Peripherals: Brown-out Detect/Reset, LCD, POR, PWM, WDT
• Number Of I /o: 69
• Program Memory Size: 128KB (64K x 16)
• Program Memory Type: FLASH
• Eeprom Size: 1K x 8
• Ram Size: 4K x 8
• Voltage - Supply (vcc/vdd): 1.8 V ~ 5.5 V
• Data Converters: A/D 24x12b
• Oscillator Type: Internal
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 80-TQFP
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

PIC18F87K90 FAMILY
27.4.2
There is a small amount of capacitance from the
internal A/D Converter sample capacitor, as well as
stray capacitance from the circuit board traces and
pads that affect the precision of capacitance measure-
ments. A measurement of the stray capacitance can be
taken by making sure the desired capacitance to be
measured has been removed.
After removing the capacitance to be measured:
1.
2.
3.
4.
5.
6.
Where:
• I is known from the current source measurement
• t is a fixed delay
• V is measured by performing an A/D conversion
DS39957D-page 416
step
Initialize the A/D Converter and the CTMU.
Set EDG1STAT (= 1 ).
Wait for a fixed delay of time, t .
Clear EDG1STAT.
Perform an A/D conversion.
Calculate the stray and A/D sample capacitances:
C
OFFSET
CAPACITANCE CALIBRATION
= C
STRAY
+ C
AD
= (I • t)/V
This measured value is then stored and used for
calculations of time measurement or subtracted for
capacitance measurement. For calibration, it is
expected that the capacitance of C
approximately known; C
An iterative process may be required to adjust the time,
t , that the circuit is charged to obtain a reasonable volt-
age reading from the A/D Converter. The value of t may
be determined by setting C
and solving for t . For example, if C
calculated to be 11 pF, and V is expected to be 70% of
V
or 63  s.
See
capacitance calibration.
DD
or 2.31V, t would be:
Example 27-3
(4 pF + 11 pF) • 2.31V/0.55 A
 2009-2011 Microchip Technology Inc.
for a typical routine for CTMU
AD
OFFSET
is approximately 4 pF.
to a theoretical value
STRAY
STRAY
is theoretically
+ C
AD
is