Datasheet

 2010 Microchip Technology Inc. Preliminary DS39979A-page 177
PIC18F87J72 FAMILY
17.3.4 DESIGN CONSIDERATIONS FOR
THE LCD CHARGE PUMP
When designing applications that use the LCD regula-
tor with the charge pump enabled, users must always
consider both the dynamic current and RMS (static)
current requirements of the display, and what the
charge pump can deliver. Both dynamic and static
current can be determined by Equation 17-1:
EQUATION 17-1:
For dynamic current, C is the value of the capacitors
attached to LCDBIAS3 and LCDBIAS2. The variable,
dV, is the voltage drop allowed on C2 and C3 during a
voltage switch on the LCD display, and dT is the dura-
tion of the transient current after a clock pulse occurs.
For practical design purposes, these will be assumed
to be 0.047 F for C, 0.1V for dV and 1 s for dT. This
yields a dynamic current of 4.7 mA for 1 s.
RMS current is determined by the value of C
FLY for C,
the voltage across V
LCAP1 and VLCAP2 for dV and the
regulator clock period (T
PER) for dT. Assuming CFLY of
0.047 F, a value of 1.02V across C
FLY and TPER of
30 s, the maximum theoretical static current will be
1.8 mA. Since the charge pump must charge five
capacitors, the maximum current becomes 360 A. For
a real-world assumption of 50% efficiency, this yields a
practical current of 180 A.
Users should compare the calculated current capacity
against the requirements of the LCD. While dV and dT
are relatively fixed by device design, the values of C
FLY
and the capacitors on the LCDBIAS pins can be
changed to increase or decrease current. As always,
any changes should be evaluated in the actual circuit
for its impact on the application.
17.4 LCD Multiplex Types
The LCD driver module can be configured into four
multiplex types:
• Static (only COM0 used)
• 1/2 Multiplex (COM0 and COM1 are used)
• 1/3 Multiplex (COM0, COM1 and COM2 are used)
• 1/4 Multiplex (all COM0, COM1, COM2 and COM3
are used)
The number of active commons used is configured by
the LMUX<1:0> bits (LCDCON<1:0>), which deter-
mines the function of the PORTE<6:4> pins (see
Table 17-3 for details). If the pin is configured as a COM
drive, the port I/O function is disabled and the TRIS
setting of that pin is overridden.
TABLE 17-3: PORTE<6:4> FUNCTION
17.5 Segment Enables
The LCDSEx registers are used to select the pin
function for each segment pin. Setting a bit configures
the corresponding pin to function as a segment driver.
LCDSEx registers do not override the TRIS bit settings,
so the TRIS bits must be configured as input for that
pin.
17.6 Pixel Control
The LCDDATAx registers contain bits which define the
state of each pixel. Each bit defines one unique pixel.
Table 17-2 shows the correlation of each bit in the
LCDDATAx registers to the respective common and
segment signals. Any LCD pixel location not being
used for display can be used as general purpose RAM.
I = C x
dV
dT
Note: On a Power-on Reset, the LMUX<1:0>
bits are ‘00’.
LMUX<1:0> PORTE<6> PORTE<5> PORTE<4>
00 Digital I/O Digital I/O Digital I/O
01 Digital I/O Digital I/O COM1 Driver
10 Digital I/O COM2 Driver COM1 Driver
11 COM3 Driver COM2 Driver COM1 Driver
Note: On a Power-on Reset, these pins are
configured as digital I/O.