Reference / Paper · 1986
Application note on VF display drive: the drive-voltage equation, filament bias by Zener on the centre tap, the 'rolling' beat between refresh and filament frequency, and the MM58341/MM58348 drivers.
AN-440 New CMOS Vacuum Fluorescent Drivers Enable Three Chip System to Provide Intelligent Control of Dot Matrix VF Display Literature Number: SNLA145 New CMOS Vacuum Fluorescent Drivers Enable Three Chip System to National Semiconductor New CMOS Vacuum Application Note 440 Fluorescent Drivers Enable Tom Markman February 1986 Three Chip System to Provide Intelligent Control of Dot Matrix VF Display INTRODUCTION Vacuum Fluorescent (VF) displays are becoming more and quired. In this case the messages could be stored in the more common in a variety of applications. Manufacturers of ROM of the COPS and the need for a host system would be everything from Automobiles to Video Recorders have tak- eliminated. en advantage of these easy to read displays. VF displays VF DISPLAY AND VF DISPLAY DRIVER are available in a wide variety of configurations; clock dis- REQUIREMENTS plays, calculator displays, multi-segment, and dot matrix dis- plays are readily available at a low cost. This application The display used in this application was an Itron note develops and covers in some detail a small CMOS ÝDC205G2. This 20 segment, 5 x 7 dot matrix, multiplexed system consisting of a single chip microcontroller and two display required a filament voltage of 5.7 Vac and a filament display drivers which control a 20 character, 5 x 7 dot matrix current of 37 mAac. The anode and grid voltages were sup- VF display. plied by the display drivers. The voltage and current require- ments vary considerably for different displays depending on Figure 1 shows the schematic of the system. The microcon- the size and number of characters, and the configuration troller, a COPSTM 424C, receives a character in ASCII form Provide Intelligent Control of Dot Matrix VF Display (dot matrix, 7 segment, 14 segment, etc.). To determine the from the host system, stores the ASCII value of the charac- voltage requirements for a particular display, a simple calcu- ter in its onboard RAM, converts the ASCII value to a 5 byte lation can be made. If maximum possible brightness of the data word suitable for the display drivers and displays it on display is desired, the following equation must be true: the VF display. The COPS also refreshes the display contin- uously while performing character update, much like a dumb Et t Eb a Ek a (Ib) (Ron) where: terminal. Not including the address decoding logic, this ap- Et is the total Voltage of the display drvier or lVdisl a Vdd plication requires only the onboard RAM and ROM of the Ek is the display Cathode Bias Voltage COPS424C, and National’s MM58341 and MM58348 VF Eb e Ec is the typical Anode or Grid Voltage (Vp-p) display drivers. If a steady message or a scrolling sentence is desired, only small changes in the COPS software are re- Ib is the typical anode current (mAp-p) Ron is the display driver output impedance (X) AN-440 TL/F/8683 – 1 FIGURE 1. System Diagram Showing the Basic 3-Chip Display Controller and the Interface to a Microprocessor System COPSTM is a trademark of National Semiconductor Corp. C1995 National Semiconductor Corporation TL/F/8683 RRD-B30M26/Printed in U. S. A. If the maximum brightness is not desired, the following eye begins to notice flicker at about 40 Hz, a display with a equation can be used: (Et)(1.2) t Eb a Ek a (Ib) (Ron). In refresh rate less than that will appear to be flashing on and this application, the calculated Et was 42.25V, however, the off. display was legible under normal lighting conditions, with an The second type of flicker occurs when the refresh rate is Et as low as 25V. If your display requires more than the 35V between 40 Hz and 90 Hz. In this case, the display will output of the MM58341 and MM58348, pin for pin compati- appear to be rolling rather than flashing. This condition oc- ble 60V VF Display Drivers (MM58241, MM58248) are avail- curs when the refresh rate and the filament frequency are able. close together. If a character is only on during the time Figure 2 shows the relationship between the required VF when the filament voltage is negative, it will appear to be display voltages. The cut-off voltage (Ek) is set by the Zener slightly brighter than the character next to it which may only diode on the center tap of the filament transformer. This be on during the positive cycle of the filament voltage. If this value is given in the VF display data sheet. is the case, as it was in this application, the simplest solu- tion is to increase the frequency of the filament. A DC oscil- Avoiding Flicker and Pulsing lator circuit, such as the one shown in Figure 3 , can be used There are two different conditions which may cause the dis- to replace the AC voltage source. The filament frequency play to appear to flicker. The first is the refresh rate. This is can be easily adjusted to eliminate this condition. particularly a problem on displays where the micro-control- ler must up-date more than 25 characters. Since the human TL/F/8683 – 2 FIGURE 2. Voltage Levels for VF Display TL/F/8683 – 3 FIGURE 3. Filament Oscillator Circuit 2 VF Display Drivers Two high voltage display drivers were needed to control the lowed by 35 bits of data, is received, the data is latched and VF display. A MM58341, was used to control the grids and a displayed. The chip is automatically reset upon power up. MM58348 was used to control the individual pixels or an- MULTIPLEXED DISPLAY REFRESH TIMING odes. Both of these drivers receive serial information and output 32 and 35 segments of data respectively. Considering first the digit driver (MM58341), it becomes clear that the digits must be enabled or refreshed sequen- The MM58341 has three control pins which make it ideal for tially and that this process must be continuous regardless if controlling the grids of a VF display. The blanking control the display data has changed. The data for the MM58341 is pin will turn off all segments of the display when a logic ‘1’ is simply a 1 followed by 19 zeroes where the 1 is shifted applied to this pin. This is particularly important for reducing through the internal registers of the MM58341. As each digit ghosting, and controlling brightness. Ghosting is a condition is enabled, the corresponding segment data is displayed. To where the last characters shadow appears behind the char- insure that no ghosting effects are seen during the transition acter being displayed. The enable pin acts as an envelope between digits, the blanking control is activiated just before for the input signal. Only while it is at a logic ‘1’ level will the the data is latched into the dot or anode driver and deacti- circuit accept clock inputs. When the pin goes low, all the vated just after the data has been latched. During this time data is latched and displayed. A data out pin is also provid- when the blanking control is activated, the grid driver is ed for cascading. If the display has more than 32 grids, a clocked shifting the 1 to the next location. Figure 4 shows second grid driver can be cascaded by connecting the data the micro-controller waveforms and the resultant display out pin to the input data for the second grid driver. waveforms for the 20 character display. The MM58348 is a 35 bit shift register and latch which is used to control each pixel or dot. When a leading 1, fol- TL/F/8683 – 4 FIGURE 4. Timing Diagram 3 In between digit strobes, the segment data is updated. The first 34 bits of segment data are set up in the dot driver and the blanking signal is activated to disable all 20 digits. The 35th bit of data is clocked in, updating the segments. Since the MM58348 resets its internal shift register each time the data is latched, it can accept all but the final data bit while still displaying the previous digit. The digit driver is then clocked, shifting the digit strobe to the next position. The enable is then brought low, enabling the next digit. Finally blanking control is deactivated and the data displayed. During the time which the blanking control is high, the order in which the segments or the digits are updated is not crit- ical. Since this occurs while the display is blank. The digit driver may be clocked first, or the segments could be changed first. In general, the philosophy for the driving this VF multiplexed display is outlined in Figure 5 . HOST INTERFACE AND PROGRAMMING With a minimal amount of address decoding and an eight bit latch, COPS can be interfaced with a common microproces- sor bus. When a character has been input into the host to be displayed, the ASCII value of that character is latched into the eight bit latch (MM74HC373) and is read on the L port (L0 – 6) of the COPS. The MSB of the ASCII value must be a logic 1. This MSB is the signal to the COPS that a new character is being presented. Once the character has been stored, an interrupt is sent from the COPS to the host through the D-0 port. The COPS checks for a new character being input every 200 ms. If a character is being sent, 1 ms is required to store that character in the RAM of the COPS. With the COPS controlling the display, the host micro-proc- essor is not being tied down with character look-up and dis- play refresh. A simple flowchart of the host requirements is shown in Figure 6 . COPS SOFTWARE There are four main sections of the COPS software. The first section, the initialization of the RAM, sets up the RAM as shown in Figure 7 . A ‘0’ is stored in all of the LSB posi- TL/F/8683 – 5 tions and a ‘2’ is stored in all of the MSB positions. Since FIGURE 5. Flowchart for Display Drivers the COPS is in a constant display loop, this is necessary to insure a blank display. 20H is the ASCII value of a space. With the RAM set up in this way, a maximum of 28 charac- ters can be stored in RAM. Since the display in this applica- tion is only 20 characters long, RAM locations M1,4 to M1,11 and M3,4 to M3,11 are not used. RAM locations 1,12 to 1,15 and 3,12 to 3,15 are used as temporary storage throughout the program and cannot be used for character storage. The second part of the program, stores the new characters sent by the host CPU in RAM. Once a character has been sent, this section of the program checks the ASCII value of that character to see if it is a control character or a display character. If it is a display character, the character is stored in RAM and an interrupt is sent to the host. There are three control characters which the COPS program will recognize. Cursor forward (ASCII value 08H) moves the cursor forward without destroying the data, cursor backwards (ASCII value 0CH) moves the cursor backwards without destroying the data, and return (ASCII value 0DH) will clear the display and put the cursor at the beginning of the display. To recognize and store a character, 1 ms is required. The third part of the program, the display loop, is the heart of the program. Unless a new character has been detected, TL/F/8683 – 6 the program is always in this loop. This section does the FIGURE 6. Host System Flowchart 4 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB LSB M0 Chr 1 Chr 2 Chr 3 Chr 4 Chr 5 Chr 6 Chr 7 Chr 8 Chr 9 Chr 10 Chr 11 Chr 12 Chr 13 Chr 14 Chr 15 Chr 16 MSB LSB Temp. ASCII LSB LSB LSB LSB M1 Pointer Pointer STORAGE Chr 17 Chr 18 Chr 19 Chr 20 MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB MSB M2 Chr 1 Chr 2 Chr 3 Chr 4 Chr 5 Chr 6 Chr 7 Chr 8 Chr 9 Chr 10 Chr 11 Chr 12 Chr 13 Chr 14 Chr 15 Chr 16 MSB MSB MSB MSB Temp. Storage of Pointer M3 Chr 17 Chr 18 Chr 19 Chr 20 FIGURE 7. COPS RAM Map Matrix PAD Column 1 Column 2 Column 3 Column 4 Column 5 PAD Binary 0 001001111101010001001000010100000111110 Hex. 13 EA 24 28 3E FIGURE 8 character font look-up, shifts the character data out the second byte. In this case, the third byte of data would be COPS serial port to the MM58348, and controls the stored at 2E1H. The fourth byte of data is stored at 300H MM58341 through the four bit parallel port (G0–4). Because plus the ASCII value of the character or at 341H for the the most significant nibble of the program counter is used letter ‘A’. The final byte of data is stored 40H from the fourth as part of some COPS instructions, it is important that parts byte or at 381H. Remember the LSB of each byte is stored of the program are located at specific locations in ROM. first. Table I shows the locations in ROM and the values The final part of the program is the data. Each character is stored in them for the letter ‘A’. represented by a 5 byte data word. Each byte of the data This application shows a VF display controller designed word is stored at a different location in ROM. Fonts for char- with a minimum number of IC’s. If additional information acters with the ASCII values from 20H–5AH have already about VF displays or VF display drivers is required, been stored in ROM. These characters can be changed or refer to Application Note AN-371 (The MM58348/ more characters can be added. The only limitation to the 342/341/248/242/241 direct drive Vacuum Fluorescent number of characters is the amount of available ROM. (VF) Displays. CREATING THE 5 BYTE DATA WORD TABLE I. Character Data of ‘A’ Any number or combination of pixels or dots can be turned and Its Locations in ROM on at a time. To create a new character, it is easiest to first create a binary string which represents the character. A ‘1’ Address Data in the binary string will turn on the pixel, a ‘0’ will turn it off. In ROM Stored To create this string, start in the upper left corner of the matrix and go down the columns. 0241H 31 The letter ‘A’ (Figure 9 ) would have a binary string shown in 02A1H AE Figure 8. The data must be padded to make it an even 5 02E1H 42 bytes in length. The pad at the beginning of the data (0001) 0341H 82 is used as the leading 1 for the MM58348. The one bit pad 0381H E3 at the end of the binary string must be a 0. If a 1 were sent as the pad, it would be used as the start bit for the next character. The 5 byte data word that would be stored in ROM and represent the letter ‘A’ would then be 13EA24283E. STORING THE DATA IN ROM The 5 bytes of data are stored in 5 different locations in ROM. The first byte of data will be stored, LSB first, at loca- tion 200H plus the ASCII value of the character. For exam- ple, the ASCII value of the letter ‘A’ is 41H. The first byte of data for the letter ‘A’ would be stored, least significant bit first, at 241H. The second byte of data is stored at the loca- tion of the first data byte plus 60H or in this case at 2A1H. TL/F/8683 – 7 The location of the third byte is 40H plus the location of the FIGURE 9. 5 x 7 Character as Stored in ROM 5 Section 1 of COPS Software .CHIP 424C ;DEFINES COPS CHIP ;THIS SECTION INITIALIZES THE RAM IN THE COPS BY LOADING A ;2 IN THE MSB AND A 0 IN THE LSB LOCATIONS OF EACH CHARACTER. ;IT ALSO STOPS THE CLOCK AND SETS THE POINTER AT THE FIRST ;CHARACTER OF THE DISPLAY. RESET: CLRA LBI 3,15 ;LOADS A 2 IN ALL JSR CLEAR2 ;MSB LOCATIONS LBI 2,15 ;LOADS A 2 IN ALL JSR CLEAR2 ;MSB LOCATIONS LBI 1,15 ;LOADS A 0 IN ALL JSR CLEAR ;LSB LOCATIONS LBI 0,15 ;LOADS A 0 IN ALL JSR CLEAR ;LSB LOCATIONS CLRA ;LOADS POINTER IN RAM XAD 1,15 ;MSB IN 1,OF CLRA AISC 15 ;LSB IN 1,0E XAD 1,14 RC ;RESETS CARRY TO XAS ; STOP CLOCK JMP START CLEAR: CLRA ;CLEARS REGISTORS XDS 0 JMP CLEAR RET CLEAR2: CLRA ;PUTS A 2 IN REGISTORS AISC 02 XDS 0 JMP CLEAR2 RET Section 2 of COPS Software ;THIS SECTION OF CODE IS ONLY EXECUTED WHEN A NEW ;CHARACTER HAS BEEN ENTERED. IF THE CHARACTER IS ;A CONTROL CHARACTER, THE CURSOR IS MOVED ACCORDINGLY, ;OTHERWISE THE CHARACTER IS STORED IN THE RAM OF THE COPS. ;NEW CHARACTER HAS BEEN ENTERED NEW: LBI 1,0C ;DUMMY POINTER INL ;READS ASCII FROM XIS 0 ;DATA BUS X 0 LDD 1,0D RC ;CHAR. MSB40 THEN YES AISC 15 ;MSBkl0 THEN NO JMP SPECIAL AISC 01 LDD 1,0E ;STORE ASCII IN RAM CAB LDD 1,0F XABR LDD 1,0C ;MSB IN 1,0C X 2 LDD 1,0D ;LSB IN 1, 0D X 0 6 Section 2 of COPS Software (Continued) JSR CURFOR LBI 0,01 ;SENDS INTERRUPT TO OBD ; HOST. CHAR. IS LBI 0,0 ; STORED IN RAM OBD JMP START ;SPECIAL CHARS. (CR, LF, CLEAR DISPLAY) CURFOR: LDD 1,0E ;MOVES CURSOR FORWARD ONE COMP ;SPACE. IF CURSOR IS AISC 01 ;MOVED BEYOND THE END OF JMP OK ;DISPLAY, IT WRAPS AROUND AISC OF ;TO THE OTHER END. DATA IS XAD 1,0E ;NOT DESTROYED BY MOVING CLRA ;CURSOR AISC 01 LBI 1,0F XOR JMP SKIP OK: COMP SKIP: LBI 1,0E X 0 RET CURBAC: LDD 1,0F ;MOVES CURSOR BACK ONE AISC 01 ;CHARACTER. DOES NOT JMP GOOD ;DESTROY DATA AS IT IS MOVED LBI 1,0E ;IF MOVED BEYOND THE CLRA ;END OF THE DISPLAY IT AISC 01 ;WRAPS AROUND TO THE OTHER XOR ;END X 0 JMP START GOOD: XAD 1,0F JMP START SPECIAL: LDD 1,0C ;CONTROL CHAR. HAS BEEN AISC 03 ;DETECTED JMP NOTRET JMP RESET ;RETURN CLEARS DISPLAY,STARTS ;PROGRAM OVER NOTRET: AISC 01 ;NOT RETURN, CHECK FOR CURSOR JMP CFOR ;FORWARD JMP CURBAC ;BY DEFAULT, CURSOR BACKWARDS CFOR: JSR CURFOR JMP START ;DISPLAY LOOP 7 Section 3 of COPS Software ;THIS IS THE DISPLAY LOOP OF THE PROGRAM. UNLESS A NEW CHARACTER ;HAS BEEN ENTERED AND IS BEING STORED, THE PROGRAM IS ALWAYS IN ;THIS DISPLAY LOOP. IT LOOKS UP THE CHARACTER FONT, SHIFTS THE ;CHARACTER DATA OUT THE SERIAL PORT AND CONTROLS THE GRID DRIVER. START: LBI 2,15 ;DISPLAY LOOP POINTER JSR HERE ;GOTO DISPLAY LOOP LBI 3,03 ;SECOND DISPLAY LOOP POINTER JSR HERE ;GOTO DISPLAY LOOP OGI 09 ;LOADS A 1 IN GRID DRIVER OGI 0D OGI 09 JMP START ;CHECKS FOR NEW CHAR HERE: RC ININ AISC 15 JMP OLDCHR JMP NEW ;DISPLAY LOOP FOR OLD CHAR AND ; LOOK UP OLDCHR: LD 2 ;LOOKS UP FIRST BYTE OF CHR.FONT JSR DATA4 ; 200H0ASCII VALUE AISC 06 ;ADDS 06H TO MSB OF ASCII JSR DATA2 ;LOOKS UP SECOND BYTE OF CHR FONT AISC 0A ;ADDS 0AH TO MSB OF ASCII JSR DATA2 ;LOOKS UP THIRD BYTE OF CHR. FONT JSR DATA3 ;LOOKS UP THIRD BYTE OF CHR. FONT ; AT 300H0ASCII VALUE AISC 06 ;ADDS 06H TO MSB OF ASCII VALUE OGI 02 ;TURNS ON BLANKING CONTROL JSR DATA3 ;LOOKS UP LAST BYTE OF CHR. FONT ;CLOCKS A 0 IN GRID DRIVER OGI 0A ;ENABLE,BLANKING CONTROL OGI 0E ;ENABLE,BLANKING CONTROL,CLOCK OGI 0A ;ENABLE,BLANKING CONTROL OGI 00 ;A 0 SHIFTED IN LD 0 XDS 2 JMP HERE RET RIGHT: LBI 3,15 CQMA JSR SHIFT ;OUTPUTS A X 0 ;NEW DATA JSR SHIFT ;OUTPUTS A LEI 01 ;COUNTER MODE LDD 3,14 ;1,0 IN A XABR ;A IN BR LDD 3,13 ;1,1 IN A CAB ;A IN BD LD 2 RET 8 Section 3 of COPS Software (Continued) POINTER: LEI 01 ;COUNTER MODE XAS ;A IN SIO XABR ;BR IN A AISC 02 ;ADD 2 XAD 3,14 ;A IN 1,0 CBA ;BD IN A XAD 3,13 ;A IN 1,1 LBI 3,15 XAS ;SIO IN A LEI 08 ;SERIAL MODE JMP RIGHT ;SHIFTS OUT SERIAL PORT SHIFT: LEI 08 ;THIS ROUTINE SHIFTS THE DATA SC ;FROM THE SI/O REGISTER OUT XAS ;THE SERIAL PORT WITH EACH NOP ;CLOCK CYCLE NOP RC XAS RET .40200 DATA3: LQID JMP RIGHT DATA4: LQID JMP POINTER .40300 DATA3: LQID JMP RIGHT Section 4 of COPS Software ;THE CHARACTER FONTS FOR THE CHARACTERS WITH ASCII VALUES BETWEEN 20H AND 5AH HAVE BEEN STORED IN THIS SECTION OF THE PROGRAM. ;DATA FOR FIRST 2 BYTES OF EACH ; CHAR. .40220 .WORD 001, 001, 001, 021, 021, 0C1, 061, 001 .WORD 031, 001, 041, 011, 001, 011, 001, 001 .WORD 071, 001, 041, 081, 011, 0E1, 031, 081 .WORD 061, 061, 001, 001, 001, 021, 001, 041 .WORD 071, 031, 081, 071, 081, 0F1, 0F1, 071 .WORD 0F1, 081, 081, 0F1, 0F1, 0F1, 0F1, 071 .WORD 0F1, 071, 0F1, 061, 081, 0F1, 0F1, 0F1 .WORD 0C1, 0C1, 081 9 Section 4 of COPS Software (Continued) ;DATA FOR SECOND 2 BYTES OF EACH ; CHAR. .40280 .WORD 000, 000, 0C1, 0F9, 0A4, 095, 02D, 000 .WORD 088, 000, 054, 020, 000, 020, 000, 014 .WORD 01D, 082, 003, 005, 058, 045, 0AC, 001 .WORD 02D, 023, 000, 000, 020, 058, 001, 001 .WORD 00D, 0AE, 0F3, 00D, 0F3, 02F, 02F, 00D .WORD 02E, 003, 00D, 02E, 00E, 08E, 08E, 00D .WORD 02F, 00D, 02F, 025, 001, 00C, 008, 00C .WORD 056, 040, 017 ;THIRD 2 BYTES OF DATA FOR EACH CHAR. .402C0 .WORD 000, 0E3, 000, 0AC, 0FB, 040, 0A5, 083 .WORD 00A, 002, 0F3, 0F1, 034, 040, 008, 040 .WORD 046, 0F7, 02E, 046, 021, 086, 046, 02E .WORD 046, 046, 0A0, 0B4, 0A0, 0A0, 015, 022 .WORD 0E6, 042, 04E, 006, 00E, 046, 042, 046 .WORD 040, 0F7, 006, 0A0, 004, 080, 0E0, 006 .WORD 042, 026, 062, 046, 0F3, 004, 008, 034 .WORD 040, 070, 046 ;FOURTH TWO BYTES OF DATA FOR EACH CHAR. .40320 .WORD 000, 008, 007, 0F7, 0AA, 031, 028, 000 .WORD 008, 02A, 049, 080, 000, 080, 000, 001 .WORD 01D, 018, 09C, 09D, 0F7, 01D, 09C, 084 .WORD 09C, 0AC, 000, 000, 022, 041, 041, 08C .WORD 0DC, 082, 09C, 01C, 01C, 09C, 084, 09C .WORD 080, 01C, 0EF, 022, 018, 002, 020, 01C .WORD 084, 02C, 0A4, 09C, 00C, 018, 028, 010 .WORD 041, 009, 01D ;LAST BYTES OF DATA FOR EACH CHAR. .40380 .WORD 000, 000, 000, 082, 084, 064, 0A0, 000 .WORD 000, 083, 044, 001, 000, 001, 000, 004 .WORD 0C7, 020, 026, 0CC, 080, 0C9, 0C8, 00E .WORD 0C6, 087, 000, 000, 028, 082, 001, 006 .WORD 027, 0E3, 0C6, 044, 0C7, 028, 008, 0C5 .WORD 0EF, 028, 008, 028, 020, 0EF, 0EF, 0C7 .WORD 006, 0A7, 026, 0C4, 008, 0CF, 08F, 0CF .WORD 06C, 00C, 02C .END 10 11 New CMOS Vacuum Fluorescent Drivers Enable Three Chip System to Lit. 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