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==== Machine System Timer (MTIME)

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| Hardware source file(s): | neorv32_mtime.vhd | 
| Software driver file(s): | neorv32_mtime.c |
|                          | neorv32_mtime.h |
| Top entity port:         | `mtime_i` | System time input from external MTIME
|                          | `mtime_o` | System time output (64-bit) for SoC
| Configuration generics:  | _IO_MTIME_EN_ | implement MTIME when _true_
| CPU interrupts:          | `MTI` | machine timer interrupt (see <<_processor_interrupts>>)
|=======================

**Theory of Operation**

The MTIME machine system timer implements the memory-mapped MTIME timer from the official RISC-V
specifications. This unit features a 64-bit system timer incremented with the primary processor clock.
The current system time can also be obtained using the `time[h]` CSRs and is made available for processor-external
use via the top's `mtime_o` signal.

[NOTE]
If the processor-internal **MTIME unit is NOT implemented**, the top's `mtime_i` input signal is used to update the `time[h]` CSRs
and the `MTI` machine timer CPU interrupt (`MTI`) is directly connected to the top's `mtime_irq_i` input.

The 64-bit system time can be accessed via the `TIME_LO` and `TIME_HI` memory-mapped registers (read/write) and also via
the CPU's `time[h]` CSRs (read-only). A 64-bit time compare register - accessible via memory-mapped `TIMECMP_LO` and `TIMECMP_HI`
registers - is used to configure an interrupt to the CPU. The interrupt is triggered
whenever `TIME` (high & low part) >= `TIMECMP` (high & low part) and is directly forwarded to the CPU's `MTI` interrupt.
The interrupt remain active (=pending) until `TIME` < `TIMECMP` (either by modifying `TIME` or `TIMECMP`).

.MTIME register map (`struct NEORV32_MTIME`)
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| Address      | Name [C]      | Bits | R/W | Function
| `0xffffff90` | `NEORV32_MTIME.TIME_LO`    | 31:0 | r/w | machine system time, low word
| `0xffffff94` | `NEORV32_MTIME.TIME_HI`    | 31:0 | r/w | machine system time, high word
| `0xffffff98` | `NEORV32_MTIME.TIMECMP_LO` | 31:0 | r/w | time compare, low word
| `0xffffff9c` | `NEORV32_MTIME.TIMECMP_HI` | 31:0 | r/w | time compare, high word
|=======================