Simple Interrupt Controller
Author: Aditya Shevade <aditya.shevade@gmail.com>
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This is a simple 8 input interrupt controller.
Written in verilog.
Currently it supports two modes, polling and custom priority.
In polling, there are no priorities. All the interrupts are
checked periodically and if any one of them is active then it
is serviced.
In the custom priority mode, the priorities are set during the
initialization phase. The controller then polls the sources in
this custom order.
Operation
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After Reset, the controller waits for commands from the master
(processor) on the bus. The lowest 2 bits determine the mode of
operation.
01 - Polling mode
10 - Priority mode.
It keeps waiting for valid input mode.
Polling
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The bus is driven as xxxx_xx01 for exactly 1 clock by processor.
Controller knows this is the polling mode. It then enters the
polling state where it keeps checkig for all interrupt sources
in cycle.
If any interrupt is found active then intr_out is set to 1.
After that, the controller waits for an acknowledgement from
the processor.
Once the processor gives acknowledgement, exactly for 1 clock
cycle on intr_in by a high -> low -> high transition, the
controller starts driving the bus with the condition code of
01011_intrID. The intrID part is the ID of the source currently
being serviced.
Controller keeps this data on the bus it it receives another
acknowledgement from the processor on the intr_in pin in the
same mannter (High -> Low -> High).
After that, the controller waits for the confirmation from the
processor that the interrupt has been serviced. Processor, once
done, sends another acknowledgement on the intr_in pin in the
same mannter (High -> Low -> High) along with the condition
code of 10100_intrID on the bus. This interrupt ID must match
the one sent last by the controller.
If either the condition code or the intrID does not match then
the controller goes to reset state. Else it just checks (polls)
the next source and continues in this cycle unless reset.
Custom Priority
===============================================================================
Working of this mode is exactly similar to the polling exept
that it does not poll the sources in order.
After reset, if the input is xxxyyy10 then the controller is
in custom priority mode. xxx is the source ID of the highest
interrupt and yyy is the ID of the second to highest. The
processor needs to give 4 such cycles to and give the prio-
-rities for all 8 sources.
After that the controller goes on checking from the highest
to lowest source to check if any of them is active. If it is
then the operation continues similar to the polling mode.
The only things that change are the condition codes that are
sent to and from the processor. The controller sends 10011
and the processor acknowledges with 01100.
-------------------------------------------------------------------------------
TODO: Currently there is no way of checking if two (or more) of
the sources are same. This can be good if you want to disable
some interrupts. You can repeat the last priority till you
have reached the total count of 8.
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Timing Diagram
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intr_out __________________
____| |___________________________________________
intr_in __________________ ______________ ___________ _____
|____| |____| |____|
data_bus _______________________|||||||||||||||||||||___________||||||_____
-------------------------------------------------------------------------------
Note - The first time the data_bus is active is when the controller
drives the bus. Next time when it's active, the processor drives it.
-------------------------------------------------------------------------------
Note - The timing diagram remains the same on both polling and custom
priority modes. Only thing that changes is the ack data on the bus.
-------------------------------------------------------------------------------
Condition Codes
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Polling:
From Controller - 01011
From Processor - 10100
Custom Priority
From Controller - 10011
From Processor - 01100
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