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I am trying to read a microwire 93C66 EEprom which is connected to a Hitachi SH7055 MCU. The device is a complete automotive ECU. I need to be able to do this in circuit.

Currently I get an error when I try to read so I assume I need to hold the MCU in either reset or hardware standby mode.

The Eeprom is connected to the MCU as follows:

Pin1 - PIN193 PJ15/TI9F

Pin2 - PIN192 PJ14/TI9E

Pin3 - PIN191 PJ13/TI9D

Pin4 - PIN190 PJ12/TI9C

Can I just tie the HSTBY pin to GND or would I have to power the whole board?

Datasheet for the MCU

Datasheet for the Eeprom

What else would I have to do in order to read this EEprom in circuit?

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Keep the entire board powered. You will need to pull the HSTBY and the RES pins low. This will put the external processor bus in a high impedance condition. You should then be able to exercise the EEPROM provided there are no other driver ICs on the EEPROM lines and that the EEPROM is not tied to either of the above pins.

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Can I just tie the HSTBY pin to GND or would I have to power the whole board?

The MCU would probaby be reverse powered by the data lines, so powering the whole board is a much better idea.

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Can I just tie the HSTBY pin to GND or would I have to power the whole board?

Well, you have to power the EEPROM, and when you do that, it's very likely that you also supply the µC.
Setting it into Hardware Standby mode will set the pins you mentioned to high impedance state. It should be possible to access the EEPROM then with external wires.

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