Driving the LCD - share your experience

I did a project, informally as part of the PiCasso design challenge, that used some I2C LCD displays. They were 2 rows of 16 and had the common Hitachi chip compatible registers. The datasheet was very messy, and muddled up the different variants in a really confusing way (the panel was available as either a I2C, SPI, or parallel build), so I had to do some experimenting to get the interface to work.

/challenges-projects/design-challenges/picasso/b/blog/posts/9-pieces-of-pi-slice-2-manifesto-for-art-electronic

That might give you some ideas, but be warned that the interface to your panel looks different to the one I had, so don't blindly copy what I did there.

One important aspect to be aware of is that the controller on the panel has an embedded microcontroller, but it runs very slowly, to minimise the power consumption, so you have to be careful to ensure that it can keep up with your much faster device. One of the registers has a status bit that can be polled to see if the last command is finished. That's the fastest way of talking to the panel, if you can read the registers. In the case of the panel I chose, the I2C interface was write-only, so I had to instead look at the timings given by the datasheet for the various commands and make sure I exceeded them. If you use a library, it's likely that it will make use of that register bit and would fail if the registers can't be read, but hopefully the panel you've been given will allow for reading back from the registers.

A second factor that may not be obvious, if you've not worked with the panels, is that the voltage for the LCD section has a separate pin to the logic supply for the controller. That's so that the contrast can be adjusted. The complication is, though, that internally the common voltage between the logic part and the segment drive part is normally the positive logic voltage, not the ground that you might expect. For the panel I used, the LCD supply (Vo) had to be about 6V, so ended up negative relative to the ground for good contrast (easy to achieve because the panel itself had a charge-pump generator on it). In the case of your panel, it looks like the LCD supply only needs to be about 4.5V, so your Vo will be approx 0.5V above ground and you can just use a pot between the logic supplies to supply it. Adjust it to the right sort of area initially with a meter, because if it's too far out the display will look blank rather than just having poor contrast.

 jc2048 A quick adaptation of your script. Works! Thanks for sharing!

from smbus2 import SMBus

LCD_CLEARDISPLAY = 0x01
LCD_FUNCTIONSET_4LINE_5x8DOTS_BITMODE = 0x38
LCD_DISPLAYCONTROL_ON = 0x0C
LCD_ENTRYMODESET_LEFT = 0x06
LCD_1LINE = 0x80
LCD_2LINE = 0xC0
LCD_3LINE = 0x94
LCD_4LINE = 0xD4


# initialise the LCD panels
# and write the static first line
bus = SMBus(1)

firstLineString = "#### ELEMENT14 #####"

displayBuffer = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
for i in range (0, 20):
  displayBuffer[i] = ord(firstLineString[i])

bus.write_i2c_block_data(0x3C , 0x00, [LCD_FUNCTIONSET_4LINE_5x8DOTS_BITMODE])
time.sleep(0.015)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_DISPLAYCONTROL_ON])
time.sleep(0.001)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_CLEARDISPLAY])
time.sleep(0.001)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_ENTRYMODESET_LEFT])
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_1LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_2LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_3LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_4LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)

 jc2048 A quick adaptation of your script. Works! Thanks for sharing!

from smbus2 import SMBus

LCD_CLEARDISPLAY = 0x01
LCD_FUNCTIONSET_4LINE_5x8DOTS_BITMODE = 0x38
LCD_DISPLAYCONTROL_ON = 0x0C
LCD_ENTRYMODESET_LEFT = 0x06
LCD_1LINE = 0x80
LCD_2LINE = 0xC0
LCD_3LINE = 0x94
LCD_4LINE = 0xD4


# initialise the LCD panels
# and write the static first line
bus = SMBus(1)

firstLineString = "#### ELEMENT14 #####"

displayBuffer = [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0]
for i in range (0, 20):
  displayBuffer[i] = ord(firstLineString[i])

bus.write_i2c_block_data(0x3C , 0x00, [LCD_FUNCTIONSET_4LINE_5x8DOTS_BITMODE])
time.sleep(0.015)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_DISPLAYCONTROL_ON])
time.sleep(0.001)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_CLEARDISPLAY])
time.sleep(0.001)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_ENTRYMODESET_LEFT])
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_1LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_2LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_3LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)
time.sleep(0.01)
bus.write_i2c_block_data(0x3C , 0x00, [LCD_4LINE])
bus.write_i2c_block_data(0x3C , 0x40, displayBuffer)