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Automatic Contrast Control Displays Using MC68HC708LN56 Microcontroller
Stellini Transportation Systems Group Design Engineering Austin, Texas
Introduction
This application note describes implement automatic contrast control (liquid crystal display) using MC68HC08LN56 microcontroller. applications where power supply voltage vary, such battery-powered systems, maintaining constant contrast display desirable. This achieved software MC68HC08LN56 using (analog-to-digital) converter conjunction with controller. This application note discusses factors such amount multiplexing, type bias, voltage levels affect contrast. This followed description system which converter board MC68HC708LN56 samples system power supply contrast control updated based result. Also included here source code implementing this system.
Freescale Semiconductor, Inc., 2004. rights reserved.
Motorola, Inc., 1998
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Freescale Semiconductor, Inc. Application Note Contrast Control
contrast characters which appear display controlled average voltage difference across segments pixels character. general, larger applied voltage causes pixels appear darker, smaller voltages cause pixels appear lighter, OFF. nature liquid crystal material, voltages applied across them will cause permanent damage. result, relative contrast characterized magnitude voltage across order pixel OFF, voltage across must below threshold voltage, VTH. voltage must above saturation voltage, VSAT. typical contrast versus voltage characteristic curve shown Figure
CONTRAST
VSAT VOLTAGE
Figure Typical Contrast Characteristic When selecting display particular application, voltages should used ensure that proper contrast achieved. voltage controller should greater than VSAT glass. voltage should less than glass.
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Application Note MC68HC708LN56 Microcontroller
MC68HC708LN56 Microcontroller
MC68HC708LN56 onboard controller/driver module which capable directly driving matrix display with backplanes frontplanes. Typically, this display composed four rows eight characters rows characters. necessary voltage levels waveforms generated onboard charge pump. voltage levels produced what called bias. levels adjustable scale fraction voltage, VLL7, described later. Integrating controller/driver with microcontroller reduces system cost user. waveforms have 1/32 duty, which means they multiplexed time intervals. Figure shows waveforms backplane frontplane with alternating data. voltage across pixel difference backplane frontplane waveform, shown waveform BP0-FPX.
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Freescale Semiconductor, Inc. Application Note
1/32 DUTY FRAME VLL7 VLL6 VLL1 VLL7 VLL5 VLL2 DATA 10101 1010101
VLL7 VLL5
BP0-FPX
VLL1 -VLL1
-VLL5 -VLL7
Figure Waveforms Example Data 01.10101 voltage across pixel calculated using this equation.
Since data during time period when active, waveform BP0-FPX waveform. voltage during active time VLL7 only VLL1 remainder time.
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Application Note MC68HC708LN56 Microcontroller
voltage this waveform calculated with this equation: RMSON VLL7 VLL7 VLL7
Figure shows same frontplane waveform along with backplane (BP1) their difference waveform BP1-FPX. period when active, data and, therefore, waveform BP1-FPX waveform. During active time, voltage swings between VLL5 between VLL1 when inactive. This gives voltage
RMSON
VLL7 VLL7 VLL7
actual values voltages depend voltage, VLL7. typical value VLL7 volts which case VRMSON =1.58 volts VRMSOFF 1.32 volts.
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Freescale Semiconductor, Inc. Application Note
1/32 DUTY FRAME VLL7 VLL6 VLL1 VLL7 VLL5 VLL2 DATA 10101 1010101
VLL7 VLL5
BP1-FPX
VLL1 -VLL1
-VLL5 -VLL7
Figure Waveforms Example Data 01.10101 MC68HC708LN56 allows user ability value VLL7 and, therefore, move operating points VRMSON VRMSOFF. desired value VLL7, contrast control register should value resulting from this equation: VLL7 LCDCCR 47.143 SUPV 94.286 SUPV value depending range which operating, SUPV 3-volt operation, SUPV 5-volt operation. This LCDCCR register value written time during operation, waveforms will scale proportion value VLL7.
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Application Note Sample Application
Sample Application
typical battery application, power supply gradually decays over time. Since voltage dependent VDD, maintain constant contrast display, contrast register must updated with value that will result same voltage times. This done using converter periodically measure then determine correct setting contrast control register. doing this continuously software, automatic contrast control achieved. this sample application, contrast display maintained constant value battery-powered supply ranging from volts down volts. Manual contrast control also implemented down contrast buttons which allows user manually increment decrement contrast setting. schematic diagram circuit this application shown Figure used input channel A/D. MAXIM 6125 voltage reference chip generates VREFH A/D. This reference 2.5-volt must greater than maximum input voltage. simple resistor divider used divide half, which allows maximum input voltage volts (max volts). display used this circuit custom display with backplanes frontplanes. Displays this size ordered readily from many manufacturers optimized user's specifications. software uses timebase module create periodic interrupt every interval time. this time, samples VDD. offset generated lookup table contrast control values from value. contrast control register written with this value which will keep VLL7 original value. buttons used manual contrast control connected PA1/KBD1 PA0/KBD0 when pushed, generate keyboard interrupt. These keyboard interrupts will either increase decrease conversion factor lookup table constant amount. this application, manual adjustments were designed change VLL7 ~100-mv steps. automatic contrast control adjustments then relative latest manual adjustment.
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0.1UF 0.1UF VCP1 VCP2 VCP3 VCP4 VLL1 VLL2 VLL5 VLL6 VLL7 VLLH CPFLT 0.1UF 0.22UF 0.1UF 0.1UF 0.1UF 0.1UF 0.1UF 0.1UF PWRTERM2 4.7UF TANTALUM 0.1UF 0.1UF 0.1UF
APPLICATION POWER
VDD1 VDD2 VDD3
BATT_SOCKET
FFFFFFFFFFFFFFFFFFFFFFFFF PPPPPPPPPPPPPPPPPPPPPPPPP 2222211111111119876543210 432109876543210
FFFFFFFFFFFFFFF PPPPPPPPPPPPPPPFFFFFFFFFF 4321098765432109876543210
FP[0.39] BP[0.31]
FP10 FP11 FP12 FP13 FP14 FP15 FP16 FP17 FP18 FP19 FP20 FP21 FP22 FP23 FP24 FP25 FP26 FP27 FP28 FP29 FP30 FP31 FP32 FP33 FP34 FP35 FP36 FP37 FP38 FP39 SW_MOMENTARY +CONTRAST SW_MOMENTARY -CONTRAST PA0/KBD0 PA1/KBD1 PA2/KBD2 PA3/KBD3 PA4/KBD4 PA5/KBD5 PA6/KBD6 PA7/KBD7
FP25 FP26 FP27 FP28 FP29 FP30 FP31 FP32 FP33 FP34 FP35 FP36 FP37 FP38 FP39 BP31 BP30 BP29 BP28 BP27 BP26 BP25 BP31 BP30 BP29 BP28 BP27 BP26 BP25
FP25 FP26 FP27 FP28 FP29 FP30 FP31 FP32 FP33 FP34 FP35 FP36 FP37 FP38 FP39
FP10 FP11 FP12 FP13 FP14 FP15 FP16 FP17 FP18 FP19 FP20 FP21 FP22 FP23 FP24 FP25 FP26 FP27 FP28 FP29 FP30 FP31 FP32 FP33 FP34 FP35 FP36 FP37 FP38 FP39
LCD_DISPLAY PPPPPPPPPPPPPPPPPPPPPPPPP 0123456789111111111122222 012345678901234
BBBBBBBBBBBBBBBBBBBBBBBBB PPPPPPPPPPPPPPPPPPPPPPPPP 0123456789111111111122222 012345678901234
PB0/AD0 PB1/AD1 PB2/AD2 PB3/AD3
330K
OSC1 OSC2 VDDA AVDD
32.768 0.47UF 20PF 0.1UF 0.1UF Title MC68HC708LN56 APPLICATION Size Document Number Date: June 1998 Sheet DESIGN ENGINEERING 20PF
IRQ2* IRQ1* RST* IRQ2 IRQ1 MAX6125 SW_MOMENTARY 0.1UF
BP10 BP11 BP12 BP13 BP14 BP15 BP16 BP17 BP18 BP19 BP20 BP21 BP22 BP23 BP24 BP25 BP26 BP27 BP28 BP29 BP30 BP31 BP10 BP11 BP12 BP13 BP14 BP15 BP16 BP17 BP18 BP19 BP20 BP21 BP22 BP23 BP24 BP25 BP26 BP27 BP28 BP29 BP30 BP31
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CGND VSS1 VSS2 VSS3 VSSA VREFH AVSSA MC68HC708LN56
Application Note Sample Application
Figure Schematic Diagram Sample Application
Freescale Semiconductor, Inc.
Application Note Sample Application
START
INITIALIZE 4-MHZ
LOAD WITH DATA DISPLAY MESSAGE
INITIALIZE TIMEBASE MODULE
INITIALIZE KEYBOARD MODULE
LOOP
Figure Main Program Flow
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Freescale Semiconductor, Inc. Application Note
ANOTHER
DELAYED DESIRED SECONDS?
CALL SAMPLE
INCREMENT SECOND COUNTER
START TIMEBASE AGAIN
RETURN
Figure Timebase Interrupt Routine
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Application Note Sample Application
SAMPLE
CONVERSION
SUBTRACT CONVERSION FACTOR LOOKUP TABLE OFFSET
RESULT BELOW BOTTOM LOOKUP TABLE?
LOAD BOTTOM LOOKUP TABLE VALUE
LOAD LOOKUP TABLE VALUE
RESULT ABOVE LOOKUP TABLE?
LOAD CONTRAST CONTROL REGISTER WITH VALUE FROM LOOKUP TABLE
RESET COUNT SECONDS TIMEBASE
RETURN
Figure Sample Subroutine
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Freescale Semiconductor, Inc. Application Note
KEYBOARD
INTERRUPT FROM PA0? DOWN CONTRAST
REDUCE LOOKUP TABLE CONVERSION CONSTANT
INTERRUPT FROM PTA0? CONTRAST
INCREASE LOOKUP TABLE CONVERSION CONSTANT
CALL SAMPLE
RETURN
Figure Keyboard Interrupt Routine
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Application Note Code Listings
Code Listings
ADCLCD.ASM Stellini, 06/06/98 Applications Engineering This software demonstrates automatic contrast control display using MC68HC708LN56 microcontroller. Memory Equates RAMSPACE $0050 ROMSPACE $2000 LCDRAM1 $0E00 LCDRAM2 $0E80 LCDRAM3 $0F00 LCDRAM4 $0F80 Register Equates Registers $0000 DDRA $0004 Registers LCDFL0 $0033 LCDFL1 $0034 LCDFL2 $0035 LCDFL3 $0036 LCDFL4 $0037 LCDCR $0038 LCDCCR $0039 LCDDIV $003A LCDFR $003B Registers ADSCR $0040 $0041 ADCLKR $0042 Registers PCTL $004A PBWC $004B PMSH $004C PMSL $004D
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Freescale Semiconductor, Inc. Application Note
PVRS PRDS Time Base Registers TBCR Keyboard Registers KBSCR KBIER Register Interrupt Vectors RESET $004E $004F $003F $001A $001B $001F $FFDA $FFDC $FFD8 $FFFE
Constant Equates ADJUST !153 ;initial lookup conv factor NUMSECS ;number between samples Variables RAMSPACE TBMCNT ADCTOCCR Start Program Code ROMSPACE #$01,MOR ;Disable BEGIN CLRX CLRH #ADJUST,ADCTOCCR
;store conversion factor variable
Initialize Program from 32.768KHz crystal #$01,PCTL #$80,PBWC #$01,PMSH #$E8,PMSL #$D0,PVRS #$01,PRDS #$21,PCTL BRCLR 6,PBWC,* #$31,PCTL
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Application Note Code Listings
Initialize loading each bank with part message. LOOP1 LCDBANK1,X LCDRAM1,X INCX LOOP1 CLRX LOOP2 INCX CLRX INCX CLRX INCX LCDBANK2,X LCDRAM2,X LOOP2
LOOP3
LCDBANK3,X LCDRAM3,X LOOP3
LOOP4
LCDBANK4,X LCDRAM4,X LOOP4 #$01,LCDDIV #$04,LCDFR #$17,LCDCCR #$80,LCDCR 32KHz ;for frame rate ;starting value ;turn
Initialize Time Base Module ;clear interrupts #$4A,TBCR ;cgmxclk 32768 ;start #!01,TBMCNT ;initialize count Initialize Keyboard Module #$02,KBSCR ;mask interrupts #$03,KBIER ;enable interrupts bits #$06,KBSCR ;acknowledge false interrupts #$00,KBSCR ;unmask interrupts, edge sens AN1762 More Information This Product, www.freescale.com
Freescale Semiconductor, Inc. Application Note
Main Loop ;wait here timer interrupt Time Base Interrupt Routine ANOTHERSEC TBMCNT ;load count #NUMSECS ;have delayed enough seconds? INCREMENT ;don't take sample ADCSAMPLE take sample RETURN INCREMENT TBMCNT ;increment second counter RETURN #$4A,TBCR ;ack start time base again ;delay some more Take Sample Update ADCSAMPLE #$50,ADCLKR ;use (1MHz) #$01,ADSCR ;PTB1 input WAIT BRCLR 7,ADSCR,WAIT ;stay here conversion done CONVDONE ;load accum with value ADCTOCCR ;convert offset from table BOTTOM ;went below zero #!56 ;check over ;went above CORRECTED ;still within valid range BOTTOM #!00 ;don't past bottom CORRECTED ;return #!56 ;don't past CORRECTED ;put index Table,X ;get value from lookup table LCDCCR ;put table value #!01,TBMCNT ;start count over ;return Keyboard Interrupt Routine KEYBOARD #$00,DDRA ;make inputs read BRCLR 0,PTA,DOWN ;down contrast button pushed BRCLR 1,PTA,UP contrast button pushed DOWN ADCTOCCR ;load conversion constant #!03 ;subtract from constant KEYDONE ;return ADCTOCCR ;load conversion constant #!03 ;add constant KEYDONE ADCTOCCR ;write value back variable AN1762 More Information This Product, www.freescale.com
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Application Note Code Listings
ADCSAMPLE
update LCDCCR value ;return
Lookup table contrast control register values. Table AN1762 More Information This Product, www.freescale.com
Freescale Semiconductor, Inc. Application Note
Data writing display say: Freescale PRESENTS HC08 CHINOOK HC08 (backwards) LCDBANK1 CHINOOK LCDBANK2 (backwards) (forwards)
FREESCALE LCDBANK3
(forward) RESET ROMSPACE ANOTHERSEC KEYBOARD beginning reset ;time base interrupt routine ;keyboard interrupt routine
PRESENTS LCDBANK4
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Application Note Code Listings
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Freescale Semiconductor, Inc. Application Note
Reach
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