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@@ -0,0 +1,42 @@
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+* OpenPIC and its interrupt numbers on Freescale's e500/e600 cores
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+
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+The OpenPIC specification does not specify which interrupt source has to
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+become which interrupt number. This is up to the software implementation
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+of the interrupt controller. The only requirement is that every
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+interrupt source has to have an unique interrupt number / vector number.
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+To accomplish this the current implementation assigns the number zero to
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+the first source, the number one to the second source and so on until
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+all interrupt sources have their unique number.
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+Usually the assigned vector number equals the interrupt number mentioned
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+in the documentation for a given core / CPU. This is however not true
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+for the e500 cores (MPC85XX CPUs) where the documentation distinguishes
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+between internal and external interrupt sources and starts counting at
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+zero for both of them.
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+
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+So what to write for external interrupt source X or internal interrupt
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+source Y into the device tree? Here is an example:
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+
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+The memory map for the interrupt controller in the MPC8544[0] shows,
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+that the first interrupt source starts at 0x5_0000 (PIC Register Address
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+Map-Interrupt Source Configuration Registers). This source becomes the
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+number zero therefore:
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+ External interrupt 0 = interrupt number 0
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+ External interrupt 1 = interrupt number 1
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+ External interrupt 2 = interrupt number 2
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+ ...
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+Every interrupt number allocates 0x20 bytes register space. So to get
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+its number it is sufficient to shift the lower 16bits to right by five.
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+So for the external interrupt 10 we have:
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+ 0x0140 >> 5 = 10
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+
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+After the external sources, the internal sources follow. The in core I2C
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+controller on the MPC8544 for instance has the internal source number
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+27. Oo obtain its interrupt number we take the lower 16bits of its memory
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+address (0x5_0560) and shift it right:
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+ 0x0560 >> 5 = 43
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+
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+Therefore the I2C device node for the MPC8544 CPU has to have the
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+interrupt number 43 specified in the device tree.
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+
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+[0] MPC8544E PowerQUICCTM III, Integrated Host Processor Family Reference Manual
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+ MPC8544ERM Rev. 1 10/2007
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Sebastian Andrzej Siewior