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==== General Purpose Timer (GPTMR)

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|=======================
| Hardware source file(s): | neorv32_gptmr.vhd | 
| Software driver file(s): | neorv32_gptmr.c |
|                          | neorv32_gptmr.h |
| Top entity port:         | none | 
| Configuration generics:  | _IO_GPTMR_EN_ | implement timer when _true_
| CPU interrupts:          | fast IRQ channel 12 | transmission done interrupt (see <<_processor_interrupts>>)
|=======================


**Theory of Operation**

The general purpose timer module provides a simple yet universal 32-bit timer. The timer is implemented if
_IO_GPTMR_EN_ top generic is set _true_. It provides a 32-bit counter register (`COUNT`) and a 32-bit threshold
register (`THRES`). An interrupt is generated whenever the value of the counter registers matches the one from
threshold register.

The timer is enabled by setting the _GPTMR_CTRL_EN_ bit in the device's control register `CTRL`. The `COUNT`
register will start incrementing at a programmable rate, which scales the main processor clock. The
pre-scaler value is configured via the three _GPTMR_CTRL_PRSCx_ control register bits:

.GPTMR prescaler configuration
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|=======================
| **`GPTMR_CTRL_PRSCx`**      | `0b000` | `0b001` | `0b010` | `0b011` | `0b100` | `0b101` | `0b110` | `0b111`
| Resulting `clock_prescaler` |       2 |       4 |       8 |      64 |     128 |    1024 |    2048 |    4096
|=======================

The timer provides two operation modes that are configured by the _GPTMR_CTRL_MODE_ control register bit:
if _GPTMR_CTRL_MODE_ is cleared (`0`) the timer operates in _single-shot mode_. As soon as `COUNT` matches
`THRES` an interrupt request is generated and the timer stops operation (i.e. it stops incrementing). If
_GPTMR_CTRL_MODE_ is set (`1`) the timer operates in _continuous mode_. When `COUNT` matches `THRES` an interrupt
request is generated and `COUNT` is automatically reset to all-zero before continuing to increment.

[NOTE]
Disabling the timer will not clear the `COUNT` register. However, it can be manually reset at any time by
writing zero to it.


**Timer Interrupt**

The timer interrupt is triggered when the timer is enabled and `COUNT` matches `THRES`. The interrupt
remains pending until explicitly cleared by writing the according `mip` CSR bit.


.GPTMR register map (`struct NEORV32_GPTMR`)
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| Address | Name [C] | Bit(s), Name [C] | R/W | Function
.5+<| `0xffffff60` .5+<| `NEORV32_GPTMR.CTRL` <|`0` _GPTMR_CTRL_EN_    ^| r/w <| Timer enable flag
                                              <|`1` _GPTMR_CTRL_PRSC0_ ^| r/w .3+| 3-bit clock prescaler select
                                              <|`2` _GPTMR_CTRL_PRSC1_ ^| r/w 
                                              <|`3` _GPTMR_CTRL_PRSC2_ ^| r/w 
                                              <|`4` _GPTMR_CTRL_MODE_  ^| r/w <| Counter mode: `0`=single-shot, `1`=continuous
| `0xffffff64` | `NEORV32_GPTMR.THRES` |`31:0` | r/w | Threshold value register
| `0xffffff68` | `NEORV32_GPTMR.COUNT` |`31:0` | r/w | Counter register
|=======================