OLED

Pmod OLED

Contents of Readme

1. About
2. Brief information about Pmod OLED
3. Interface Description
   1. oled
      1. Description
      2. Ports
      3. Character Mapping
      4. Utilization
   2. oled_bitmap
      1. Description
      2. Ports
      3. Utilization
4. Simulation
5. Tests
6. Status Information
7. Licence

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About

Simple interface for the Digilent Pmod OLED or any other module with Solomon Systech SSD1306.

Brief information about Pmod OLED

The Digilent Pmod OLED is 128 x 32 Pixel Monochromatic OLED Display module. The Pmod OLED utilizes a Solomon Systech SSD1306 display controller to receive information from the host board and display the desired information on the OLED screen. Module communicates with the host board via SPI protocol.

An alternative breakout board that can be used with modules here is Adafruit 128x32 SPI OLED display.

The Pmod OLED needs Start-up sequence. Implemented Start-up sequence follows as:

1. Power up VDD
2. 100 ms wait
3. Power up VBAT
4. 4µs Reset
5. 1 ms wait
6. Send display off command
7. 2 ms wait
8. Send initilization commands
   1. Charge pump enable (0x8D-0x14)
   2. Set pre-charge period (0xD9-0xF1)
   3. Column inversion enable (0xA1)
   4. Invert COM Output Scan Direction (0xC8)
   5. COM pins configuration (0xDA-0x22)
   6. Set addressing mode to Horizontal (0x20-0x00)
9. 100 ms wait

Similarly there is a shutdown sequance:

1. Power down VBAT
2. 100 ms wait
3. Power down VDD

Before refreshing screen contents following commands are send:

1. Set Column Address (0x21-0x00-0x7F)
2. Set Page Address (0x22-0x00-0x03)
3. Set High Column to 0 (0x10)

With 2,5 MHz SPI clock, screen refresh takes 1,66 ms.

Interface Description

oled

Module oled provides a basic screen interface with coded characters, easily selectable line count and a cursor. This modules rely on an external decoder, such as oled_decoder, to generate 8x8 bitmap from 8 bit code.

Ports of oled

Port	Type	Width	Description
clk	I	1	System Clock
rst	I	1	System Reset
ext_spi_clk	I	1	External SPI Clock
character_code	O	1	Character code to be decoded
current_bitmap	I	64	Decoded 8x8 bitmap
CS	O	1	SPI Chip Select
MOSI	O	1	SPI Data
SCK	O	1	SPI Clock
data_command_cntr	O	1	Data/~Command for Data
power_rst	O	1	Reset pin of the Display
vbat_c	O	1	Active low VBAT control
vdd_c	O	1	Active low VDD control
power_on	I	1	Power on module
display_reset	I	1	Reset display
display_off	I	1	Turn off display
update	I	1	Update display content
display_data	I	512	Display data
line_count	I	2	Number of display lines
contrast	I	8	DisplayContrast
cursor_enable	I	1	Enable cursor
cursor_flash	I	1	Cursor flashes
cursor_pos	I	6	Position of cursor

I: Input O: Output

ext_spi_clk frequency should be the double of desired SPI clock frequency. During testing 5 MHz ext_spi_clk is used.

line_count	Line Count
2'b11	4
2'b10	3
2'b01	2
2'b00	1

current_bitmap
[63]	...	[56]
...	...	...
[7]	...	[0]

display_data
[511:504]	...	[391:384]
...	...	...
[127:120]	...	[7:0]

Character Mapping for oled_decoder:

Full list is available at wiki.

Codes 0x00 - 0x5e corresponds to ASCII table 0x20 - 0x7e.

Unused codes will be displayed as empty.

Mapping can be edited easily via localparameters of oled_decoder. New characters can be appended by adding a new code as localparameter and bitmap to following case statement. Or compeletly new decoder module can be used.

Character Mapping for oled_decoder_ascii:

Decoder oled_decoder_ascii uses extended ASCII table for character mapping. Non printable characters and unimplemented chracters are displayed as a square, □ .

(Synthesized) Utilization of oled:

• On Artix-7:
  • Slice LUTs: 301 (as Logic)
  • Slice Registers: 117 (as Flip Flop)
  • F7 Muxes: 72
  • F8 Muxes: 32

(Synthesized) Utilization of oled_decoder:

• On Artix-7:
  • Slice LUTs: 234 (as Logic)

oled_bitmap

Module oled_bitmap provides a basic screen interface to display 128x32 bitmap on screen.

Ports of oled_bitmap

Port	Type	Width	Description
clk	I	1	System Clock
rst	I	1	System Reset
ext_spi_clk	I	1	External SPI Clock
character_code	O	1	Character code to be decoded
current_bitmap	I	64	Decoded 8x8 bitmap
CS	O	1	SPI Chip Select
MOSI	O	1	SPI Data
SCK	O	1	SPI Clock
data_command_cntr	O	1	Data/~Command for Data
power_rst	O	1	Reset pin of the Display
vbat_c	O	1	Active low VBAT control
vdd_c	O	1	Active low VDD control
power_on	I	1	Power on module
display_reset	I	1	Reset display
display_off	I	1	Turn off display
update	I	1	Update display content
contrast	I	8	Display Contrast
bitmap	I	4096	Bitmap to display

I: Input O: Output

ext_spi_clk frequency should be the double of desired SPI clock frequency. During testing 5 MHz ext_spi_clk is used.

bitmap
[4095]	...	[3968]
...	...	...
[127]	...	[0]

(Synthesized) Utilization of oled_bitmap:

• On Artix-7:
  • Slice LUTs: 1252 (as Logic)
  • Slice Registers: 99 (as Flip Flop)
  • F7 Muxes: 546
  • F8 Muxes: 268

Simulation

Module oled simulated on oled_sim.v. In simulation delay times reduced to 1/1000th. Only a simple update and power up sequence is simulated.

Test

oled Test

Module oled is tested on Digilent Basys 3 with oled_test.v and Basys3.xdc. Block diagram for this test can be generated via design_1.tcl. Missing utility modules can be found under Utils directory of this repository. Module oled is tested with the Digilent Pmod OLED and Adafruit 128x32 SPI OLED display.

Testing module in oled_test.v handles the board connections. Display signals are connected to both JB and JC header. Leftmost switch is used as power on switch, second leftmost switch is used as display on switch. Switches 9 and 8 is used to determine line count. 8 right most switches are used to get data. Depending on which button is pressed, data handled diffrently. Up button configures the cursor. Left button changes contrast. Right button saves the current data and shifts it into the next position in display. Down button is used to reset display. Display signals are monitored with Digital Discovery.

oled Demo

A demo module for oled and oled_decoder is on oled_demo.v. Demo module is tested on Digilent Arty 7 with Arty-A7-100.xdc. TCL design_oled_demo.tcl generates the demo hardware. The Pmod OLED can be connected either to JA or JD header.

Demo module automatically displays all characters on display. In every half minute next character appears and other characters shift. Line can be changed with button 0. Contrast can be changed with button 1. Cursor position can be increased with button 2. And button 3 reset the display. Switch 3 enables cursor flash and switch 2 enables cursor itself. Switch 1 turns off the display and switch 0 power on the display module.

oled_bitmap Test

Module oled_bitmap is tested on Digilent Arty 7 with oled_bitmap_test.v and Arty-A7-100.xdc. The Pmod OLED can be connected either to JA or JD header. Switch 3 power ups the module. Remainin switches control the display contrast. Four bitmaps are hardcoded to tester module. Displayed bitmap can be changed via push buttons.

Status Information

Last simulation: 13 May 2021, with Vivado Simulator.

Last test: 20 May 2021, on Digilent Arty A7.

License

CERN Open Hardware Licence Version 2 - Weakly Reciprocal