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zero_gravi |
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:sectnums:
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==== Machine System Timer (MTIME)
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[cols="<3,<3,<4"]
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[frame="topbot",grid="none"]
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|=======================
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| Hardware source file(s): | neorv32_mtime.vhd |
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| Software driver file(s): | neorv32_mtime.c |
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| | neorv32_mtime.h |
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| Top entity port: | `mtime_i` | System time input from external MTIME
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| | `mtime_o` | System time output (64-bit) for SoC
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| Configuration generics: | _IO_MTIME_EN_ | implement MTIME when _true_
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| CPU interrupts: | `MTI` | machine timer interrupt (see <<_processor_interrupts>>)
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|=======================
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**Theory of Operation**
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The MTIME machine system timer implements the memory-mapped MTIME timer from the official RISC-V
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specifications. This unit features a 64-bit system timer incremented with the primary processor clock.
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The current system time can also be obtained using the `time[h]` CSRs and is made available for processor-external
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use via the top's `mtime_o` signal.
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[NOTE]
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If the processor-internal **MTIME unit is NOT implemented**, the top's `mtime_i` input signal is used to update the `time[h]` CSRs
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and the `MTI` machine timer interrupt) CPU interrupt is directly connected to the top's `mtime_irq_i` input.
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The 64-bit system time can be accessed via the `MTIME_LO` and `MTIME_HI` memory-mapped registers (read/write) and also via
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the CPU's `time[h]` CSRs (read-only). A 64-bit time compare register – accessible via memory-mapped `MTIMECMP_LO` and `MTIMECMP_HI`
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registers – are used to configure an interrupt to the CPU. The interrupt is triggered
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whenever `MTIME` (high & low part) >= `MTIMECMP` (high & low part) and is directly forwarded to the CPU's `MTI` interrupt.
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[TIP]
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The interrupt request is a single-shot signal,
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so the CPU is triggered once if the system time is greater than or equal to the compare time. Hence,
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another MTIME IRQ is only possible when updating `MTIMECMP`.
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The 64-bit counter and the 64-bit comparator are implemented as 2×32-bit counters and comparators with a
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registered carry to prevent a 64-bit carry chain and thus, to simplify timing closure.
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.MTIME register map
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[cols="<3,<3,^1,^1,<6"]
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[options="header",grid="all"]
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|=======================
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| Address | Name [C] | Bits | R/W | Function
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| `0xffffff90` | _MTIME_LO_ | 31:0 | r/w | machine system time, low word
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| `0xffffff94` | _MTIME_HI_ | 31:0 | r/w | machine system time, high word
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| `0xffffff98` | _MTIMECMP_LO_ | 31:0 | r/w | time compare, low word
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| `0xffffff9c` | _MTIMECMP_HI_ | 31:0 | r/w | time compare, high word
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|=======================
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