Monday, August 14, 2017

Debugging seek timing simulation


Testing resumed today, both with the oscilloscope and the logic analyzer. With what I have completed, I can check out everything related to basic operation, virtual disk spinning and seeking. This includes the signals FileReady, ReadyToSWR, AddrAck, LogAddrIntlk, Sect1, Sect2, Sect4, Sect8, and SectorMark.

It is important that they are the proper length and occur in the appropriate relationship to each other during operations such as seeking to a new cylinder. My test setup used a button on the fpga board to simulate the Strobe signal coming from the Alto, but alas it does not have a one-shot so that it repeatedly triggers the seek logic while depressed.

I added a stage in the seek logic that will not wrap up unless the Strobe line is logically 0. This does work although the button remains on long enough that I still could get additional cycles. Not with seeks that are valid, because my simulation of the time delay of movement adds many milliseconds to the seek process, long enough for my stab at the button to have switched it on and back off.

The time for a seek is not being properly set, thus I have to dig deeper into the logic that calculates the seek distance (number of cylinders to travel from current to new position), then waits 0.6 ms per cylinder moved. At the tail end of a movement there is a 14.4 ms delay while the heads settle into position.

I exposed the calculated number of cycles on a connector, hooked to the logic analyzer, and tried to capture it. Too, both the calculated distance and the calculated number of cycles are displayed on the seven segment displays as a quick aid. I found some flaws in how I was handling this and reconfigured the logic.

Somehow I get a delay of 200 ms minimum whereas I should have 30 us before an ack, 5 us for the ack, 600us for each cylinder moved and 14.4 ms for settle time at the end. That is, just over 15 ms not 200. Undoubtedly another case of VHDL that looks straightforward but doesn't work as intended.

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