Monochrome 128*64 OLED screen - SSD1306 driver questions

[fabienroyer]

Hi,

I ported the SSD1306 OLED display driver on github (https://github.com/adafruit/SSD1306) to C#, targeting the netduino micro-controller and I have a couple questions about the driver code. Please keep in mind that I have not yet been able to test my code with the actual display since it was shipped on Monday

So, here it goes...

Question #1:

Based on the functions in SSD1306.cpp and after reading the SSD1306's datasheet, you're using the 4-wire SPI interface of the controller (as opposed to the 3-wire SPI interface) and I was wondering if there was a specific reason why you chose to use Arduino's ShiftOut() function instead of using the hardware SPI support provided by the SPI Arduino library, combined with two other digital I/O lines controlling D/C and RST?

Question #2:

In ssd1306_command() and ssd1306_data() (SSD1306.cpp lines 369-383), the code pulses the CS line before spiwrite():

void SSD1306::ssd1306_command(uint8_t c) {
digitalWrite(cs, HIGH);
digitalWrite(dc, LOW);
digitalWrite(cs, LOW);
spiwrite(c);
digitalWrite(cs, HIGH);
}

Why is this necessary?

In the datasheet, "Figure 8-5 : Write procedure in 4-wire Serial interface mode" seems to show that CS is just kept low during the transmission and only brought back to high when it's complete.

Thanks,
-Fabien.

[adafruit]

1) so you could use any pins, not just the hardware SPI pins.

2) thats a typo but hey, it still works

[fabienroyer]

Thanks for the quick response

1) so you could use any pins, not just the hardware SPI pins.

Right, which is what the ShiftOut() function does in the driver today, bit-banging on the pins (http://www.arduino.cc/en/Reference/ShiftOut).

My question was: is there a reason why the SSD1306 driver isn't using hardware SPI (http://arduino.cc/en/Reference/SPI) instead in order to maximize throughput?

[adafruit]

yes, so that the user can use any pins they want

[fabienroyer]

Got it! Thanks

[adafruit]

(also, to make it easier to port!)

please post up a photo when you get it working, we like to promote netduino hacking!

[fabienroyer]

Will do!

[fabienroyer]

No dice just yet with the C# driver on the netduino... but while testing the OLED display, I removed the need for the level shifter

http://fabienroyer.wordpress.com/2011/0 ... l-shifter/

I'll post an update when I have figured out the issue on the netduino.

Cheers,
-Fabien.

[scout119]

I have ported the driver for FEZ Panda (SPI, .net microframework)

http://www.breakcontinue.com/archive/20 ... splay.aspx
http://www.tinyclr.com/forum/8/2211/

One issue I had with the device is that 2 pins were bridged on the back side of the board. Spend a lot of time trying to figure out why it didn't work. Almost got to the point of sending it back .
Since yours works with Arduino code it should be something else, anyways I am using SPI and it works on Panda. I have Netduino but haven't tried it on it. I will try it later and let you know if it works on Netduino.

[fabienroyer]

Hi Scout,

The only real difference that I see between your code and mine is in the SPI config:

Code: Select all

     
public AdaFruitSSD1306(SPI.SPI_module spiModule, Cpu.Pin DC, Cpu.Pin RST, Cpu.Pin CS) {
            dc = new OutputPort(DC, false);
            rst = new OutputPort(RST, false);

            var spiConfig = new SPI.Configuration(
                ChipSelect_Port: CS,
                ChipSelect_ActiveState: false, // CS active low
                ChipSelect_SetupTime: 0,
                ChipSelect_HoldTime: 0,
                Clock_IdleState: false, // <------------------------------- I have it set to false by default.
                Clock_Edge: true, // data clocked in on high edge
                Clock_RateKHz: 10000,
                SPI_mod: spiModule);
           
            Spi = new SPI(spiConfig);
        }

In your config, Clock_IdleState is set to true:

Code: Select all

       
public SSD1306(Cpu.Pin csPin, Cpu.Pin resetPin, Cpu.Pin dcPin, SPI.SPI_module spiModule)
        {
            SPI.Configuration spiConfig = new SPI.Configuration(csPin, false, 0, 0, true, true, 10000, spiModule);
            _spi = new SPI(spiConfig);

            _resetPin = new OutputPort(resetPin, false);
            _dcPin = new OutputPort(dcPin, false);

            Init();
        }

I'll try your driver later this weekend to see what happens on the netduino.

Just to confirm, the FEZ Panda sends SPI data with MSB first, right?

Cheers,
-Fabien.

[scout119]

Yes.

If MSB is an issue on Netduino you can try bitbang method.
You'll have to comment SPI initialization and pass 2 more pins for clock and data.

Code: Select all

public void Write(byte data)
{
    byte i = 8;
    int mask;

    while (i > 0)
    {
        mask = (1 << i - 1);
        clkPin.Write(false);
        if (((int)data & mask) == 0)
            dataPin.Write(false);
        else
            dataPin.Write(true);
        clkPin.Write(true);
        --i;
    }
}


[fabienroyer]

It turns out that sending data to the display using hardware SPI on the netduino does not work.
However, I was able to get the display working with the bit-banging method. This is very odd...

[scout119]

Great!

I am glad it worked for you.

[fabienroyer]

Thanks Scout.

I'll keep plugging at this issue with SPI as it would improve performance quite a bit, but at least, my netduino driver works now

Here's a link to the driver and sample code: http://fabienroyer.wordpress.com/2011/01/16/driving-an-adafruit-ssd1306-oled-display-with-a-netduino/

Cheers,
-Fabien.

[adafruit]

there is a max speed for the SPI. one thing that would make it faster is to update the library so it only refreshes the rectangle that has changed.
http://jeelabs.net/projects/cafe/reposi ... ow/GLCDlib has examples for this.