DAY 26. The LCD Driver

novice intermediate advanced expert

The TI-85 and higher-number calculators have a memory mapped screen. That means a section of the calculator's RAM that holds a bit image of the screen is constantly being monitored. Whenever a byte in this area is changed, the display changes immediately. It is also possible to change the RAM area the driver looks at with a single port output. By switching the buffer location back and forth rapidly, you could create Game Boy-style flickerless 4- or 8-level grayscale.

Unfortunately, the "crap series" (everything lower than a TI-85) uses a driver that stores the screen image in its own RAM. In order to change the display, you have to send each byte in the image to the driver. It is also a very slow driver which needs a delay every time it is accessed.
Hopefully you are sufficiently depressed now, so let's look at how to make the Toshiba T6A04 (that's the name of the driver) our bitch.

Control Ports

The two ports that control the LCD are
$10 Command Port Alters the driver's status.
$11 Data Port Let's you muck about with the driver's RAM.
In order to send a command to the driver, interrupts must be disabled. And since the driver is so slow, you need a 10µs delay between operations. This means that a 6Mhz TI-83 Plus needs about 60 T-States' worth of waste instructions, and A TI-83 Plus SE in fast mode needs about 100.

_LCD_BUSY_QUICK Creates a sufficient delay for the LCD driver, regardless of what model.
Location $000B
Remarks For this routine, you use a CALL instead of the normal b_call().

Driver RAM

The T6A04's RAM is a 960-byte bitmap mapped to 64 rows of 15 bytes, however the last three bytes in each row aren't used, giving an effective RAM space of a 768-byte bitmap mapped to 64 rows of 12 bytes (sound familiar?). To read the data or write to it, the driver has to know which byte to look at. You tell it by sending a row command and a column command through the command port. The only confusing thing is that the Cartesian plane is flipped: the column is called Y and the row is called X. The commands are:
Instruction Effect
$20 to $2E Set column: 0 (leftmost 8 pixels) to 14 (rightmost 8 pixels).
$80 to $BF Set row: 0 (top row) to 63 (bottom row).


So that life with the driver is not as grim as it could be, the driver will automatically update the LCD coordinates after a data read or write. There are four modes.

$04 X auto-decrement Driver moves back one byte along X.
$05 X auto-increment Driver moves forward one byte along X.
$06 Y auto-decrement Driver moves back one byte along Y.
$07 Y auto-increment Driver moves forward one byte along Y.

The TI-OS expects X auto-increment mode for all its routines, and must be set back to this mode if you change it. Although the results could be interesting if you don't.

Reading and Writing (But Thank God No 'Rithmetic)

After the X or Y coordinate is set, a dummy read must be done if you want to read data. To read:
    IN    A, ($11)
A dummy read is an intermediary between setting a coordinate and reading a byte. Therefore, you don't need a dummy read between setting coordinates, nor between two successive reads.

To write (which doesn't require a dummy):

    OUT    ($11), A

Program 26-1

Read the enitre LCD (or at least as much as you can stuff into a Pic variable) to Pic0
_LCD_BUSY_QUICK    .EQU    $000B

    LD     HL, Pic0Name
    RST    20h
    RST    10h
    JR     NC, ExistError

    EX     DE, HL
    INC    HL
    INC    HL

    CALL   ScanLCDToPic


    LD     A, $80           ; Set row 0
    OUT    ($10), A

    LD     C, $20-1         ; C will hold column

    INC    C
    LD     A, C
    CP     $2C             ; See if C exceeded maximum column value
    RET    Z

    OUT    ($10), A         ; Set column

    IN     A, ($11)         ; Dummy read
    LD     B, 63            ; 63 display rows to a picture
    LD     DE, 12           ; Because LCD is read in column-major order, 
                           ; and picture data is in row-major order.

    IN     A, ($11)         ; Read one byte
    LD     (HL), A          ; And put it to the picture
    ADD    HL, DE
    DJNZ   Column

    CALL   _LCD_BUSY_QUICK      ; Restart at row 0
    LD     A, $80
    OUT    ($10), A

    LD     DE, -(12 * 63) + 1    ; -(12*63) returns to the first row.
    ADD    HL, DE                ; + 1 moves one column over.
    JR     Row

    ; Display an error message if Pic0 already exists
    LD     HL, 0
    LD     (CurRow), HL
    LD     HL, ExistErrorMsg
    SET    TextInverse, (IY + TextFlags)
    RES    TextInverse, (IY + TextFlags)

    .DB    PictObj, tVarPict, tPic0, 0
    .DB    "ERR:  PIC EXISTS", 0
    .DB    "Press any key...", 0


There are 40 contrast settings: 0 to 39. To set the contrast, you output to the command port the desired contrast command.
command = contrast + (24 OR $C0)
Interrupts are allowed to be active.

Reading the command port does not tell you what the current contrast setting is, so the system's contrast value is held in (contrast). The value here is in the range 0 to 39.

Program 26-2

It's everyone's favorite cinematic! (Shut up you punk cross-dissolvers!).
_LCD_BUSY_QUICK    .EQU    $000B
#define    DEC_A_OP    $3D
#define    INC_A_OP    $3C


    LD     A, (contrast)
    LD     B, A             ; Number of times to decrease contrast
    ADD    A, $18 | $C0     ; "|": TASM command for bitwise OR

    PUSH   BC              ; Save current contrast so we can fade back

    LD     HL, FadePatch    ; SMC Fade routine to fade to white.
    LD     (HL), DEC_A_OP
    CALL   Fade

    ; Display a picture here. Make sure to keep A intact.

    POP    BC              ; Restore counter -- number of times to increase
                           ; contrast to restore the original setting.

    LD     HL, FadePatch    ; SMC Fade routine to fade in.
    LD     (HL), INC_A_OP

    OUT    ($10), A
    HALT                   ; Delay for approx. 1/20th second
    DEC    A
    DJNZ   Fade

Test Mode

This is a pretty useless feature, but you can really scare some people with it! By sending an instruction in the range $1C to $1F, you put the LCD driver into something called test mode, affectionately known as The Blue Lines of Death. In a nutshell, during test mode the liquid crystals are receiving an abnormal amount of energy. So much in fact that you get blue horizontal lines across the screen. Even weirder, the lines overflow out of each pixel cell (there are normally thin areas of blank space separating each pixel). By sending multiple test mode commands you can make more blue lines appear, potentially you can make the entire display blue.
To cancel test mode, send command $18. This will also set the contrast to its darkest.

Now that you know about test mode, please, don't use it. There is no way to make the BLODs appear on a specific row, nor can they be constrained. If you had any hopes of making some kind of waterfall animation, you can just as well forget about it.
As well, test mode is dangerous. If you leave the calculator in test mode for more than a minute, you risk damaging the LCD. Even a few seconds of BLODs may leave an imprint on the screen (not unlike phosphor burn-in when you forget your screen saver).


Command $02 will "unhook" the LCD from its internal RAM, and $03 will relink it. In other words, these turn the LCD off and on. Keep in mind that these commands will only turn off the LCD, not the calculator (actually, the only way to truly shut off the calculator is to remove all five batteries).


Commands $40 to $7F change how the driver's RAM is mapped to the screen. $40 is the default, the first 12 bytes of driver RAM are displayed on the top row of the LCD. Each successive number moves the screen up one pixel (there is vertical wrapping). This is still pretty pointless; I could see it used in a game for an earthquake effect, but aside from that, its only use is for this demo.

Program 26-3

    LD     B, 63
    LD     A, $41
    OUT    ($10), A
    INC    A
    LD     C, 5       ;Use a bigger number for slower scrolling.
    DEC    C
    JR     NZ, Delay

    DJNZ   Scroll

    LD     A, $40
    OUT    ($10), A

Word Size

The last feature of the T6A04 is the ability to change the word size from 8-bit to 6-bit. Technically, a word is defined as a string of bits that can occupy a single addressable location. This kind of conflicts with the popular definition of a word being a size of 16 bits, which is due to the fact that Intel made some computers with 16-bit words that were very popular. But this is the official Toshiba name, and I can't think of a better one, so oh well.

The word size is changed with two commands:
$00 Configure six bits per word
$01 Configure eight bits per word
What can we use this for? Well, Toshiba thought it would be nice if computers could have two font sizes, and in fact the 6x8 character routines do use 6-bit word mode to display characters. Maybe we could make a custom large font routine?

Program 26-4

    LD     HL, text
    CALL   CustomStr

text:    .DB    "Hello ", 1, 0

    LD     A, (HL)
    OR     A
    RET    Z

    CP     1
    JR     NZ, NormalChar    ; Trap for char $01 (custom)

    PUSH   AF

    XOR    A                ; Configure word size
    OUT    ($10), A

    LD     A, $05            ; Configure X auto-increment
    CALL   $000B
    OUT    ($10), A

    LD     A, (CurCol)       ; Set LCD Row
    ADD    A, $20
    CALL   $000B
    OUT    ($10), A

    LD     A, (CurRow)       ; Set LCD Row
    ADD    A, A
    ADD    A, A
    ADD    A, A
    ADD    A, $80
    OUT    ($10), A

    LD     DE, Smilie
    LD     B, 8
    LD     A, (DE)
    CALL   $000B
    OUT    ($11), A
    INC    DE
    DJNZ   FontLoop

    LD     A, (CurCol)       ; Advance cursor position
    INC    A
    AND    %00001111
    LD     (CurCol), A
    JR     NZ, DoneCustomFont

    LD     A, (CurRow)       ; Advance row. This doesn't check for a bad
    INC    A                ; position or scroll. Do that on your own time.
    LD     (CurRow), A

    POP    AF
    JR     DoneChar


    INC    HL
    JR     CustomStr

; Our custom character!
    .DB    %00011110
    .DB    %00101101
    .DB    %00101101
    .DB    %00111111
    .DB    %00101101
    .DB    %00110011
    .DB    %00011110
    .DB    %00000000

LCD Command Port Cheat Sheet

Command Function
$00 Configure six bits per word
$01 Configure eight bits per word
$02 Turn off
$03 Turn on
$04 X auto-decrement mode
$05 X auto-increment mode
$06 Y auto-decrement mode
$07 Y auto-increment mode
$08 – $0B Power supply enhancement. $08 is lowest.
$10 – $13 Power supply level. $10 is lowest.
$14 – $17 Unknown
$18 Exit test mode
$19 – $1B Unknown
$1C – $1F Enter test mode
$20 – $2E Set column in 8-bit word mode
$20 – $33 Set column in 6-bit word mode
$34 – $3F Unknown
$40 – $7F Set Z-address
$80 – $BF Set row
$C0 – $FF Set contrast

Bit State
7 1: LCD is busy
0: LCD can accept a command
6 1: 8 bits-per-word
0: 6 bits-per-word
5 1: Display is on
0: Display is off
4 1: In reset state
0: In operating state
3 Not defined
2 Not defined
1 1: Y-Auto mode
0: X-Auto mode
0 1: Auto increment mode
0: Auto decrement mode

This is part of Learn TI-83 Plus Assembly In 28 Days
Copyright (c) 2002, 2003, 2004 Sean McLaughlin
See the file gfdl.html for copying conditions