Electrical – Passive reset circuit for multi-chip game design

Yes, you can use the analog to digital converter on pin 1 even if the external reset is still enabled on that pin as long as the voltage on that pin does not drop below the reset threshold.

Here is a simpler version of your circuit that works reliably...

^{simulate this circuit – Schematic created using CircuitLab}

...

When the button SW1 is pressed, the resistors R2 and R1 form a voltage divider that puts about 4.54 volts onto pin RST.

Here is an Arduino test program that will light the onboard LED when ever the button is pressed...

int led = 1;    // Onboard led on trinket

setup() {
    pinMode( led, OUTPUT );
}

loop() {
    if (analogRead(0)) > 900 ) {   // reset pin is near Vcc
        digitalWrite( led , 0 );   // turn led off
    } else {                       // reset pin is less than 900/1024 * 5 vcc
        digitalWrite( led , 1 );   // turn led on
    }
}

Notes:

A Trinket is basically just an ATTINY85 that has a USB connector and a preloaded bootloader so it is easy to get code into it using the Arduino IDE. Everything said here about the Trinket goes for a bare ATTINY85 and vice versa.

The datasheet for the ATTINY85 specifies the minimum voltage on the Reset pin that will interpreted as "1" as 0.9 volts * (V_IH2), which comes out to 4.5 volts. Since the ~4.54 volts output by out voltage divider is higer than that, it should be seen as a "1" and not reset the chip.

There is 30k-60k built in pullup on the reset pin, so we could in principal just use, say, a 30k ohm R2 and not need R1 but I've found that you can get spurious resets from switching noise with only the internal pullup.

I picked 10k & 1k ohm resistors because they are common and provide enough current reliably work, but only waste about 2mA when the button is pushed. Very little current is used when the button is not pressed.

I picked 1000 as the threshold value on the analogRead because it gives plenty of headroom on both side for inaccuracies in the values of the two resistors. When the button is not pushed, the reset pin should be very near Vcc and so the analogRead() should be very near 1023. When the button is pushed, the voltage on reset will be pulled lower by R2, but exactly how much lower depends on the exact values of R1 and R2 (resistors' actual value can be quite different from their market value due to tolerance) and the internal pull-up, so I picked a higher value than just the midpoint.

I assume you understand how to create the I2C byte sequence for the SSD1306 but I'll repeat it anyway: The SSD1306 distinguishes between commands (incl. command parameters) and data (pixel data). With SPI, it uses a dedicated input pin to distinguish commands and data.

With I2C, 0x80 needs to be prepended to each command byte. 0x40 switch to data mode. The data mode continues until the end of the I2C transaction (indicated by a STOP condition).

To update a part of the screen, the start address of the top left corner has to be set and then the data can be sent. A valid byte sequence for starting at the coordinates (20, 16) for x and y looks like this:

0x80, 0xb1,  // page start address: 0xb0 | (y >> 3)
0x80, 0x04,  // lower nibble of column: 0x00 | (x & 0x0f)
0x80, 0x11,  // upper nibble of column: 0x10 | ((x >> 4) & 0x0f)
0x40, // switch to data mode
0x01, 0x02, 0x03, 0x04, 0x05, 0x06, ... // pixel data

The memory is divided into pages. Each page covers 8 pixel rows. So you can only update stripes of 8 vertical pixels and the stripes must be aligned to multiples of 8. As you can see in the first line of the byte sequence, the lower 3 bits of y are simply discarded.

The horizontal start position is provided in two parts: the upper and lower nibble. A nibble is four bits, i.e. half a byte. See lines 2 and 3 above.

The remaining two lines switch to data mode and send the pixel data. With each byte, the address advances by 1, i.e. it advances horizontally from left to right and each byte written affects a vertical piece of 8 pixels.

With the different addressing modes (command 0x20 to 0x22), you can determine how the address advances at the end of page, at the end of your update area etc. The simplest approach is to write to each page separately and to explicitly set the address the beginning of each page.

Note that there are clones of the SSD1306 chips that do not support the different addressing modes.