External VLCD bit in the Control A register (CTRLA.XVLCD); the external power supply is selected by
writing a '1' to CTRLA.XVLCD.
The LCD power supply block generates up to three intermediate voltage levels (in internal or external
supply mode), depending on the bias configuration bits in the CTRLA register (CTRLA.BIAS[1:0]).
Table 43-6. VLCD Voltages
CTRLA.BIAS[1:0]
Configuration
Voltages
00
Static
VLCD
01
1/2
VLCD, 1/2 VLCD
10
1/3
VLCD, 2/3 VLCD, 1/3 VLCD
11
1/4
VLCD, 3/4 VLCD, 1/2 VLCD, 1/4 VLCD
The VLCD Status bit in the STATUS register (STATUS.VLCDS) indicates the current relation of VLCD and
VDD. When VDD33 > Target VLCD, STATUS.VLCDS is set to '1'. Otherwise, STATUS.VLCDS is cleared
to '0'.
The voltage status of VLCD itself is indicated by STATUS.VLCDR: when the VLCD voltage is not well
regulated to the target voltage, indicated by STATUS.VLCDR=0, the display quality may be impaired
during the ongoing settling period. If the display quality is critical for the application, transient phenomena
can be avoided by first disabling the COM/SEG output by writing CTRLD.DISPEN=0, then enable
COM/SEG output again once the STATUS.VLCDR bit is set.
43.6.1.9. Contrast Adjustment
The contrast of the LCD is determined by the value of VLCD voltage. The higher the VLCD voltage, the
higher is the contrast. The software contrast adjustment is only possible in internal supply mode.
In internal supply mode, VLCD is in the range of 2.5V to 3.5V. VLCD can be adjusted in 16 steps of 60mV
by writing a value to the Contrast bits in the Control B register (CTRLB.CTST), see also the according
Electrical Characteristics section.
The contrast value can be written at any time, even if SLCD is enabled and running.
Related Links
on page 1176
43.6.1.10. Saving Power with LowR and Buffer
The bias generation block generates the bias voltage that the LCD waveform needs. For intermediate
bias levels between ground and VLCD, they are generated by an on-chip resistive voltage divider.
The voltage divider is made of two strings, one high resistance string and one low resistance string. The
high resistance string is always on when the LCD is enabled. The low resistance string can be turned on
for a configurable amount of time (defined in the SLCD controller) to increase the drive capability of the
bias, at the price of increased power consumption. The on-time is aligned at waveform transition so that
the best trade off can be found between power and waveform quality by tuning the ratio of the LowR on-
time versus the waveform period to find the best trade-off. The LowR on-time is selected by writing the
Low Resistance Enable Duration bits in the Control B register (CTRLB.LRD).
A buffer is also provided for each of the intermediate bias levels. The buffers have a fixed current
consumption, but provide dynamic current drive capability. Similar to the LowR driver, the buffer can be
turned on for only a portion of the waveform period after transition. This is tuned by writing to the Bias
Buffer Enable bits in the Control B register (CTRLB.BBD).
Atmel SAM L22G / L22J / L22N [DATASHEET]
Atmel-42402E-SAM L22G / L22J / L22N_Datasheet_Complete-07/2016
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