2020-08-06 23:02:37 +00:00
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# adb
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This directory contains the firmware for the ADB (Apple Desktop Bus)
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chip, which is actually an Apple-branded PIC16CR54 microcontroller.
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2021-04-24 01:30:16 +00:00
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# Functioning of the ADB
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See Guide to the Macintosh Family Hardware, page 312.
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* Automatically polls the last device that has sent data, the _active
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device_. Then shifts it out to the VIA, that causes an interrupt.
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* A non-active device asserts a _Service Request_ signal to the ADB
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when it has data to send. The ADB asserts an interrupt request
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signal to the VIA, which sets bit 3 in data register 3 to 0.
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Respond to requests of the ADB Manager to poll each device to find
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which one asserted the Service Request.
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Data bits are encoded as follows:
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* Bit-cell time: 100 us
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* 0: low 65 us
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* 1: low 35 us
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