
DocID024597 Rev 5
769/1830
RM0351
Liquid crystal display controller (LCD)
787
In case the internal step-up converter is used (capacitor C
EXT
on VLCD pin is required):
•
Configure the VLCD pin as alternate function LCD in the GPIO_AFR register.
•
Wait for the external capacitor C
EXT
to be charged (C
EXT
connected to the VLCD pin,
approximately 2 ms for C
EXT
= 1
μ
F)
•
Set voltage source to internal source by resetting VSEL bit in the LCD_CR register
•
Enable the LCD controller by setting LCDEN bit in the LCD_CR register
In case of LCD external power source is used:
•
Set voltage source to external source by setting VSEL bit in the LCD_CR register
•
Configure the VLCD pin as alternate function LCD in the GPIO_AFR register
•
Enable the LCD controller by setting LCDEN bit in the LCD_CR register
LCD intermediate voltages
The LCD intermediate voltage levels are generated through an internal resistor divider
network as shown in
The LCD voltage generator issues intermediate voltage levels between V
SS
and V
LCD
:
•
1/3 V
LCD
and 2/3 V
LCD
in case of 1/3 bias
•
1/4 V
LCD
, 2/4 V
LCD
and 3/4 V
LCD
in case of 1/4 bias
•
only 1/2 V
LCD
in case of 1/2 bias.
LCD drive selection
Two resistive networks, one with low value resistors (R
L
) and one with high value resistors
(R
H
) are respectively used to increase the current during transitions and reduce power
consumption in static state.
The EN switch follows the rules described below (see
•
If LCDEN bit in the LCD_CR register is set, the EN switch is closed.
•
When clearing the LCDEN bit in the LCD_CR register, the EN switch is open at the end
of the even frame in order to avoid a medium voltage level different from 0 considering
the entire frame odd plus even.
The PON[2:0] (Pulse ON duration) bits in the LCD_FCR register configure the time during
which R
L
is enabled through the HD (high drive) switch when the levels of the common and
). A short drive time will lead to lower power
consumption, but displays with high internal resistance may need a longer drive time to
achieve satisfactory contrast.