LCD Type MCU
The nature of Liquid Crystal Displays require that only AC voltages can be applied to their pixels
as the application of DC voltages to LCD pixels will cause permanent damage. For this reason the
relative contrast of an LCD display is controlled by the actual RMS voltage applied to each pixel,
which is equal to the RMS value of the voltage on the COM pin minus the voltage applied to the
SEG pin. This differential RMS voltage must be greater than the LCD saturation voltage for the
pixel to be on and less than the threshold voltage for the pixel to be off. The requirement to limit the
DC voltage to zero and to control as many pixels as possible with a minimum number of connec-
tions, requires that both a time and amplitude signal is generated and applied to the application
LCD. These time and amplitude varying signals are automatically generated by the LCD driver cir-
cuits in the microcontroller. What is known as the duty determines the number of common lines
used, which are also known as backplanes or COMs. The duty, which is chosen by a configuration
option to have a value of 1/2, 1/3 or 1/4 and which equates to a COM number of 2, 3 and 4 respec-
tively, therefore defines the number of time divisions within each LCD signal frame. The following
timing diagrams depict the LCD signals generated by the microcontroller for various values of duty
and bias.
D u r in g R e s e t o r in H A L T M o d e
C O M 0 , C O M 1 , C O M 2
A ll s e g m e n t o u tp u ts
N o r m a l O p e r a tio n M o d e
C O M 0
C O M 1
C O M 2
A ll s e g m e n ts O F F
C O M 0 s e g m e n ts O N
C O M 1 s e g m e n ts O N
C O M 2 s e g m e n ts O N
C O M 0 , 1 s e g m e n ts O N
C O M 0 , 2 s e g m e n ts O N
C O M 1 , 2 s e g m e n ts O N
A ll s e g m e n ts O N
1 F ra m e
V B
V S S
V A
V B
V S S
V A
V A
V B
V S S
V B
V S
V A
V B
V S
V A
V B
V S
V A
V B
V S
S
S
S
S
V A
V A
V B
V S S
V B
V S S
V A
V B
V S S
V B
V S S
V B
V S S
V B
V S S
V A
V A
V A
V A
LCD Driver Output (1/3 Duty, 1/2 Bias)
52
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