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LCD Application Note 1

LCD Application Note

Memory LCD Programming

INTRODUCTION

Sharp’s Memory LCDs represent a step forward in

power-saving monochrome displays. However,

because the module’s onboard controller is geared for

power-saving functions, programming the module is

done in a serial manner instead of the more familiar

parallel manner in current use with color displays.

This Application Note will explain some general

steps to programming the module, as well as give

some practical examples.

VCOM

When designing with a memory LCD panel, a deci-

sion must be made as to how VCOM will be generated.

VCOM is an alternating signal that prevents a DC bias

from being built up within the panel. If this DC bias is

allowed to accumulate, it will eventually reach the point

where the state of the liquid crystals in the panel cells

cannot be changed by the panel electronics and the

cells will no longer change state.

Memory LCDs do not generate this signal internally.

It must be supplied using one of two methods: soft-

ware, or external clock. The mode is selected by the

EXTMODE pin on the interface.

If external clock is selected [EXTMODE = H], the

clock should be supplied on the EXTCOMM pin. See

Figure 1. For further information on the requirements

for amplitude and frequency for this clock signal,

please see your particular part’s Specifications sheet.

When the software clock is selected [EXTMODE =

L], bit V of the command bit string sets the state of

VCOM. See Figure 2. This bit must toggle (by writing to

the panel) at least once per second.

Any command string can be used to change the state

of the VCOM bit. If no update to the pixel memory is

needed within a second, ‘Change VCOM’ can be used

to toggle the bit state. In the descriptions and Figures,

the V-bit represents the desired VCOM state when soft-

ware VCOM control is selected. When the external

clock is selected, then the V-bit state doesn’t matter.

Figure 1. Selecting External Clock

Figure 2. Selecting Internal Clock

SCLK

SCS

EXTCOMIN

DISP

VDDA

VDD

EXTMODE

VSS

VSSA

External Clock

EXTMODE = H

LCD45-2

SCLK

SCS

EXTCOMIN

DISP

VDDA

VDD

EXTMODE

VSS

VSSA

Software Clock

EXTMODE = L

LCD45-1

Programming Sharp’s Memory LCDs

Ken Green, Sharp Microelectronics of the Americas

Summary of content (8 pages)

PAGE 1
LCD Application Note Memory LCD Programming Programming Sharp’s Memory LCDs Ken Green, Sharp Microelectronics of the Americas INTRODUCTION Sharp’s Memory LCDs represent a step forward in power-saving monochrome displays. However, because the module’s onboard controller is geared for power-saving functions, programming the module is done in a serial manner instead of the more familiar parallel manner in current use with color displays.
PAGE 2
Memory LCD Programming LCD COMMANDS Write Line There are four commands that can be sent to a memory LCD panel: The minimum amount of data that can be written to the panel with the Write Line comand is one line. The actual width of the data written depends on the horizontal resolution of the panel itself. Therefore, a panel with a resolution of 400 × 240 will require a minimum of 400 bits of data (plus overhead). See Figure 3.
PAGE 3
Memory LCD Programming After the data bits follow 8 trailer bits (instead of 16), then the address of the next line (8 bits) to be written, followed by the data bits for that line, and so on until all of the desired lines are written. See Figure 5. For the last line to be written, use 16 trailer bits (see Figure 6).
PAGE 4
Memory LCD Programming SOFTWARE IMPLEMENTATION This section contains code for interfacing with the panel. The SPI port should be set up so that when in the idle state, the serial chip select (SCS) and the serial clock (SC) are LOW. Data is clocked on the rising edge of SC. Care should be taken to meet all of the timing such as Setup and Hold times and SC clock speed as given in your part’s Specifications. It is important to note which bit (MSB or LSB) gets shifted out of the SPI port first.
PAGE 5
Memory LCD Programming //////////////////////////////////////////////////////////////// // // This routine transmits the contents of the pixel memory in // a frame buffer to the memory LCD. It uses DMA to transfer // each individual line. The address of the frame buffer to // transfer is in the global variable show_frm. // // NOTE: this routine also acts as the interrupt handler for SPI // interrupts.
PAGE 6
Memory LCD Programming case 1: //Sending a line (other than the first line). At this //point the DMA transfer for the previous line is done //and the channel disabled.
PAGE 7
Memory LCD Programming POWER CONSIDERATIONS Power Scenario 2 One of the prime attributes of the memory LCD is low power operation. The actual power usage is directly related to how often data is written to the panel, and how often VCOM is toggled. We’ll look at two scenarios and calculate power draw. Here, we write all the pixels to the panel only every 30 seconds. The current to write to the panel “averaged” over that time is approximately 0.1 µA.
PAGE 8
Memory LCD Programming SPECIFICATIONS ARE SUBJECT TO CHANGE WITHOUT NOTICE. Suggested applications (if any) are for standard use; See Important Restrictions for limitations on special applications. See Limited Warranty for SHARP’s product warranty. The Limited Warranty is in lieu, and exclusive of, all other warranties, express or implied. ALL EXPRESS AND IMPLIED WARRANTIES, INCLUDING THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR USE AND FITNESS FOR A PARTICULAR PURPOSE, ARE SPECIFICALLY EXCLUDED.