MELT LCD display module use recommendations

Connecting LCD display modules using connectors

Certain connector types (e.g., pin connectors) provide no secure electrical contact where the electrical connector experiences mechanical stress. This can cause LCD display breakdown!

Mechanical stress can occur due to many reasons: misalignment between the connector and the display fasteners, connector skew, temperature variations, frequent display replacements, external mechanical effects on the display and/or the product as a whole.

Thus it is recommended to use soldered electrical connections for LCD displays. At least, avoid using connectors just in between the LCD display board and any other board. LCD display should be connected to the main board with the help of the loop line, which in turn can be connected using either the soldering technique or connectors. Any other method eliminating the occurrence of mechanical stress in connectors can be used as well.

4-bit mode in alphanumeric LCD display modules

1. When LCD display module is operated in a 4-bit mode, R/W and A0 signals status change is impermissible throughout the byte transfer cycle. The same refers to an inactive E signal between two nibble transfers. Upon any change of R/W and A0 signals the internal nibble count in the LCD display module is reset to the high nibble receipt status. That’s a characteristic property that distinguishes our LCD display modules from the imported analogues and oriented at the enhancement of LCD display reliability.
2. All display access cycles should be paired (transfer of both the low and the high nibbles is compulsory). The only exclusion is the first four instructions in the initialization procedure. Another option is to use the capability of internal nibble count reset (see Clause 1) prior to the high nibble transfer. In the latter case the compatibility with imported LCD display modules is lost.
3. 4 LSBs of the data bus can be left unconnected since the entire data bus is pulled up to Ucc through high-ohmic resistors.
4. And don’t forget to choose the right interface type (4- or 8-bit) when changing the character-generator code page.

What is the actual storage capacity of LCD display modules?

All alpha-numeric displays have built-in 80-byte memory. Memory addresses should be verified against the LCD display module documentation. A memory portion is displayed on the display, while the entire storage capacity is available both on writing and on reading. The memory saves its contents for as long as the LCD display module is energized, no matter whether the display is on or off.

Storage capacity of graphic LCD display modules:

• MT-6116 = 80 byte/line * 4 lines = 320 bytes (displayed as 61 byte/line * 2 lines);
• MT-6464 = 64 byte/line * 8 lines = 512 bytes (displayed as 64 byte/line * 8 lines).
• MT-12232 = 80 byte/line * 4 lines * 2 crystals = 640 bytes (displayed as 61 byte/line * 4 lines * 2 LCD halves);
• MT-12864 = 64 byte/line * 8 lines * 2 crystals = 1024 bytes (displayed as 64 byte/line * 8 lines * 2 LCD halves).

The memory size doesn’t depend on the LCD alphabetic suffix.

In segment parallel-interface displays (MT-10T7, MT-10T8, MT-10T9), the built-in memory cannot be read (memory size is 10 bytes + lockout trigger).

Are LCD display modules compatible with high-speed controllers? What is the maximum fill rate?

Yes, they are. But one should bear in mind the signal preset and hold times. The maximum write speed:

• MT-**S* (MT-10S1, MT-20S1, MT-16S2, MT-24S2, MT-20S4, ...) - 25-30 thousand characters/s;
• MT-6116, MT-12232 – 0.5-1 mln. byte/s (4-8 mln. dot/s);
• MT-12864, MT-6464 - 100-130 thousand byte/s (1 mln. dot/s).

Displays with two crystals (MT-12232, MT-12864) offer a possibility of successive writing to the right/left crystal, which allows almost twofold increasing the write speed (however through the complication of a program). High speeds (as specified above) are achieved at display readiness polling. Exceptions are MT-6116 and MT-12232 displays, for which holding a pause between E signals could be more efficient than polling the display readiness.

What is the right way to switch on the LCD backlight?

All LCD display modules are designed in such a way that the backlight is energized from the power supply of the display itself. That is the ‘+’ terminal of the backlight (A pin) is connected to Ucc pin, while the ‘—’ terminal of the backlight (K pin) is connected to GND pin. This is true for both 5V and 3V displays.

What is the method for LCD display contrast adjustment?

1. Segment parallel-interface LCD display modules (MT-10T7, MT-10T8, MT-10T9): contrast adjustment is performed by changing the rating of the resistor between Uo and GND as specified in PDF documentation on the display.
2. 3V alpha-numeric LCD display modules: no contrast adjustment option is available.
3. 5V alpha-numeric LCD display modules: contrast adjustment is performed by changing voltage on Uo within the range of -2V..+2V relatively to GND. Note that supplying Uo pin with the voltage close to the display supply voltage (Ucc) is impermissible! Uo voltage should be at least 1V lower than that of the display! Otherwise the LCD display module becomes inoperable.
4. MT-6464* graphic displays: contrast is adjusted by changing voltage on Uo pin (as specified in PDF documentation on the display).
5. MT-12232* graphic displays: no contrast adjustment option is available.
6. MT-12864* graphic displays: contrast adjustment is performed by changing the rating of the resistor between the Uo and Uee pins (as specified in PDF documentation on the display).

Anyway, one should better review documentation on the specific LCD display module. If you have any doubts about the accuracy of documentation, contact us directly or ask your question on the forum.

Can 5V LCD display module be connected to 3V controller?

Technically, it can. But the difference in logic signal levels should be considered: for some displays logic ‘1’ level can appear to be higher than the controller is capable of generating. This is true for RES pin of the MT-12864 display, whose log. ‘1’ level can be 3.75V (0.7*5.5V) min, while log. ‘1’ level of other pins is as little as 2.4V.

The problems can also occur during reading from LCD display module. During the read cycle the LCD display module will deliver log. ‘1’ voltage of up to 5V to the pins, and the current will flow through the protective diodes in the controller, which could result in the breakdown of both the display and the controller. It is necessary to make provisions for the measures such as level matching circuits, current limitation by pins, etc.

What is the right way to issue instructions to alphanumeric and graphic LCD display modules?

There are several options. You can either choose the most appropriate variant or invent the new one not contradicting the LCD display module documentation.

1. Before (or after) each access cycle hold the minimum specified pause time. This method is the simplest but the least optimal in terms of time consumption.
2. After each display access cycle, check the busy bit and wait for the indicator to execute the instruction issued. This method is better than the first one but still is quite non-optimal.
3. Wait for LCD display module readiness before each access cycle. This, probably, is the most convenient way to control the display from the main program (not from the interruptions). Though it doesn’t ensure minimum time consumption by the controller, but sets aside maximum time for other actions besides display operation.
4. One could write a program designed to issue instructions to the display in such a way that at least the minimum specified time passed between any two successive access cycles. This method is optimal in terms of the controller time consumption (no excessive operations) and information output speed but complicated in terms of creating and debugging the program.
5. If the display access cycles are generated in the interruption, the interruption frequency could be set so that at least the minimum specified pause time passed between the calls. Probably, this is the best method for the systems supporting that low interruption frequency and output speed.
6. For the systems that require high interruption frequency and output speed, the option is to check the display readiness status in the interruption and, if not ready, exit the interruption without generating the display access cycle.

Certainly, above listed is not the exhaustive list of available options, though in most cases they are just enough.

What is the right way to check the readiness of the LCD display module to exchange data?

In the most general case, it is necessary to execute the cycle of information reading from the LCD display module by setting control signals to the receipt of the status byte, and check the BUSY bit in the read-in byte. For alphanumeric LCD in 4-bit mode, one should remember to receive both nibbles whether the display is ready or not. In controllers offering the option of the data bus operation mode selection (input or output), remember to switch the data bus to input prior to E pulse generation (read strobe).

There is a faster way to check the BUSY flag in alphanumeric LCDs: start the read cycle but check the BUSY flag right at the data bus without resetting the ‘E’ strobe subject only to holding the delay time set for data output by the display. Meanwhile the ‘E’ strobe can be kept active until the BUSY flag reset is detected; whereupon the status byte read cycle can be terminated. The termination must always be performed correctly: e.g., for alphanumeric displays in 4-bit mode, the receipt of the low nibble of the status byte is compulsory, notwithstanding the fact that the BUSY bit is located in the high nibble and reading the low nibble could seem to be excessive. But it’s not!

Can LCD display modules be operated under the negative temperature conditions?

We offer several versions of LCD display modules, most of which are designed to be operated, in particular, at negative temperatures. Our products include commercial LCD display modules with operating temperatures of up to --30°C (with storage temperatures of up to -40°C). The max permissible operating t range is +50°C to +70°C (storage t range is +60°C to +80°C). But one should understand that the extended temperature range displays are: 1) more expensive; and 2) demonstrate considerable increase in the information refresh times under the negative temperature conditions (from 0.2s at +20°C to 7s at -20°C and 15s at -30°C). Refresh time is the time period between writing the new information to the display and termination (by sight) of transient processes in the LCD glass. If during the write process no information change occurs in RAM, no transient processes will take place either. That is, time for transient processes is required only where output information refresh occurs. This time has nothing to do with the internal RAM write time.

Readout of the changing information to the display more frequently than with the above intervals would not incur any damages, but one will be able to see something in between the old and the new information on the display.

Is there any way to change the LCD control interface?

Yes, there is. For MT-6116, MT-6116B, and MT-12232B LCD display modules, 68000 control interface can be replaced by 8080 interface. In this case the R/W signal will turn to /WR signal, and E signal – to /RD signal. Only one of those signals can be the active one. To choose 8080 interface, send the log.1’log.0 edge to the RES pin and maintain log.0 over the entire running time of the display.

For more detail see the documentation on КБ145ВГ4 crystal (ANGSTREM) or SED1520DOA or contact us. In MT-12232A, MT-12232C, and MT-12232D LCD display modules the change of interface type is also physically feasible. However, it can cause the display to become inoperable due to two crystal access decoder in its circuit.

Distinctive features of MT-6116 and MT-12232 LCD display modules

MT-6116 and MT-12232 displays are based on the same crystal. They feature certain peculiarities to be considered in designing products containing these displays:

1. Though the display has the circuit for initial reset upon power on, it is often insufficient to ensure correct operation of the display, and the external reset signal may be needed. These displays can be reset with any edge on the RES pin (both 0’1 and 1’0). Besides, this very pin selects the control interface type. Thus it is desirable to apply the external LCD reset signal to RES output while maintaining RES=log.0 for at least 10 јs after the voltage supply to LCD, and then apply the log.0’log.1 edge with the front duration of 10јs max. Prior to 0’1 edge application the LCD display module can output random information to the data bus (depending on R/W, A0, and E control signals), thus it is necessary to provide for the input mode (or Z status) via the data bus in the controller for that time. Where the reset pulse is generated not only upon power-on but in the course of operation as well, the controller data bus should be switched to the input mode (or Z status) for the entire time of log.0 at RES output, to eliminate the bus conflict.
2. To speed-up the display update the special Read-Modify-Write mode is provided. In this mode the column address is incremented only after writing (RMW flag). When this mode is set, the byte from the display can be read, modified (if necessary), and written back to the display without the need to add the column address set instructions. Without this mode the sequence would be as follows: set the column address, read the data, set the same column address once again, and write the new data. This sequence is two operations longer (for modification of several byte sequences).
3. On the other hand, with the Read-Modify-Write mode on the LCD display module cannot process many instructions (e.g., the page set instruction doesn’t work). Thus remember to reset this mode when not used. In our documentation, the reset of this mode is not included in the initialization procedure for some displays. Therefore this mode can appear to be set upon power on. In this case the LCD display module will demonstrate incorrect operation. Thus the RMW mode reset instruction should better be added to the initialization procedure.
4. When reading information from the internal display memory, a “do-nothing” read cycle should be executed – after the column address set instructions the first read cycle won’t output any useful information. The real data will be output only starting with the second read cycle. To read the status byte, no extra read cycles are needed.
5. Since the crystals within the display are independent, both status bytes need to be checked (or at least the status byte of the crystal to be accessed).
6. For the same reason (crystal independency), to ensure correct operation of the LCD display module the initialization should be performed for both display crystals. If initialization is performed for one crystal only, the LCD will display the patter incorrectly (even on the half of the display).

Distinctive features of MT-6464 and MT-12864 displays.

The minimum pause time between the display access cycles (10јs) is not specified in our documentation. There are two options: to hold the above pause or to check the display busy flag.

It is also not mentioned that when reading information from the internal display memory a “do-nothing” read cycle should be executed – after the column address set instruction the first read cycle won’t output any useful information (the real data will be output only starting with the second read cycle).

No extra read cycles are required to read the status byte.

Since the crystals within the display are independent, both status bytes need to be checked (or at least for the crystal to be accessed).

For the same reason (crystal independency), to ensure correct operation of the LCD display module, the initialization should be performed for both display crystals.

No input or output current values for the displays are specified in the documentation.

The output voltages specified in the documentation are ensured at the following maximum output current values:

1. All alphanumeric displays: (MT-**S*): Ioh=0.4mA, Iol=1.2mA.
2. MT-6116*: Ioh=0.4mA, Iol=0.4mA.
3. MT-6464*: Ioh=0.2mA, Iol=1.6mA.
4. MT-12232*: Ioh=0.4mA, Iol=0.4mA.
5. MT-12864*: Ioh=0.2mA, Iol=1.6mA.

Input current values for the displays are specified in the appropriate display documentation, except for MT-6116* and MT-12232* displays:

1. MT-6116*: Iih=Iil=3јA.
2. MT-12232*: Iih=Iil=3јA.

Is there any fast way to test LCD functionality?

Yes, there is, but not for all displays. Below is the breakdown by display types:

1. Segment displays (MT-10T7, MT-10T8, and MT-10T9): after power on the display should show a random pattern.
2. Alphanumeric displays: to be able to operate this type of displays an initialization procedure must be performed first.
3. Graphic displays: sending a reset pulse (RES output) and ‘Display ON’ instruction is enough.

For online display testing you can use this program (920 Kb). It outputs a test pattern to the display connected to the computer via the LPT port. The connection diagram for each LCD is shown right in the program window.

The program screen view when MT-20S4A is selected

No pattern is displayed at all. What is there to do?

Incorrect contrast adjustment is the most probable reason for no information is displayed on LCD (virtually the display runs, the pattern exists, but it is invisible). This could be checked by reading the previously stored information from the LCD (inapplicable for segment displays).

Given any defects suspected, it is recommended:

• to check if LCD is energized,
• to check the control signal levels,
• to check contrast adjustment,
• to make sure the control pins and LCD power terminals are noise-free,
• to check the control signal waveform (especially where the long connection cable is used),
• to check observance of time parameters in controlling the display,
• to check correctness of the display initialization procedure,
• to connect the display to the computer via the LPT port and check the display operability with the help of the program referred to in the previous clause,
• turn on another similar LCD display module, contact us.

Do you have any sample programs for output to LCD?

Yes, we do. Study the archive containing sample programs for output to our LCDs. The programs have been created using a C-like language and are designed to demonstrate the LCD operation algorithm. The programs are accompanied with detailed comments but cannot be compiled (the time delay function and LCD control signal names need to be determined). The examples of application of our LCD display modules in various devices can be found here .