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 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 Figure 179.
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 Figure 179):
• 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
segment lines change (see Figure 179). 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.
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