Macintosh-HW/macsehw
1
0
Fork 0
mirror of https://github.com/quorten/macsehw.git synced 2024-06-11 17:29:29 +00:00
Code Issues Projects Releases Wiki Activity
master
macsehw/firmware/adb/README.md
Andrew Makousky 5fc5d3489d Work-in-progress improvements to BBU.
2021-04-23 20:30:16 -05:00

766 B
Raw Permalink Blame History

adb

This directory contains the firmware for the ADB (Apple Desktop Bus) chip, which is actually an Apple-branded PIC16CR54 microcontroller.

Functioning of the ADB

See Guide to the Macintosh Family Hardware, page 312.

  • Automatically polls the last device that has sent data, the active device. Then shifts it out to the VIA, that causes an interrupt.

  • A non-active device asserts a Service Request signal to the ADB when it has data to send. The ADB asserts an interrupt request signal to the VIA, which sets bit 3 in data register 3 to 0. Respond to requests of the ADB Manager to poll each device to find which one asserted the Service Request.

Data bits are encoded as follows:

  • Bit-cell time: 100 us

  • 0: low 65 us

  • 1: low 35 us