Strange memory protocol

I have a chip that should hold some calibration data. I expected to be a EEPROM but it looks like it is not a classic EEPROM. This is how it is connected.


I think it is clear that pins 4 and 8 (or 1) are power pins and pins 5 and 7 are for communication. But what communication? All I2C EEPROMs I have found datasheet of have I2C pins on pins 5 and 6. I have tried I2C protocol anyway but didn’t get any response. I have thought about SPI but just two data lines are strange. Morover why there may be the resistors?

So do you have any idea how it could communicate? Or any hint how I can I find the protocol (without hijacking the original communication)?

I will presume no markings on the IC. Any idea what the circuit came out of?

Can't you follow the "traces" and get some info from the other chips this one is connected to? Maybe post some hi-res pictures from the whole PCB.

Also, you could plug a logic analyser on the pins and let the circuit run. You might be able to get some insight about the communication protocol.

I believed there are no markings on the chip but I was wrong. There are some letters. Under “microscope” I managed to read
145 (or 146?)

I wanted to post the picture but it didn’t pass a security check. I don’t know why.

The chip comes from Datex Ohmeda Flow sensor. I din’t want to talk about it much because I guess it is prohibited to reverse engineer it. But since I don’t plan to manufacture replacement or use them for their intended purpose I hope noone will care. I can dissect and play with discarded ones but it is absolutely unimaginable to connect anything unauthorized to the anaesthetic machine they are used for so I cannot use logic analyser to watch the protocol. Since I didn’t find the datasheet is there a clever way how to try to force the chip to communicate? I thought about sending random pulses over the wires and look if and how the chip react but possibilities are nearly endless. So I tried if someone notices familiar connection I don’t know. BTW I measured the resistors and they are around 0.5 Ohm. What they are for? ESD protection?

After much research I have found there is some VESA E-EDID protocol which could be connected this way - except for pin 1 is NC.

BUT! I think I have read the markings wrong. If I am right it is:
ATTiny45! OMG.
It would explain 5V on pin 1 (reset pin)… It would explain why it didn’t response to any common protocol - it may use any protocol they programmed. It would explain 3mA consumption - too much for (idle) EEPROM but perfect for ATTiny @ 4MHz. The connected pins are SCL and SDA so hardware USI may be used. I wonder how they deal with “Atmel products are not intended, authorized, or warranted for use as components in applications intended to support or sustain life.”