[docs] update GPTMR section

docs/datasheet/soc_gptmr.adoc

@@ -8,7 +8,7 @@
 | Hardware source files:  | neorv32_gptmr.vhd |
 | Software driver files:  | neorv32_gptmr.c |
 |                         | neorv32_gptmr.h |
-| Top entity ports:       | `gptmr_trig_i` | timer capture input
+| Top entity ports:       | none |
 | Configuration generics: | `IO_GPTMR_EN` | implement general purpose timer when `true`
 | CPU interrupts:         | fast IRQ channel 12 | timer interrupt (see <<_processor_interrupts>>)
 |=======================
@@ -18,14 +18,13 @@
 
 The general purpose timer module implements a simple yet universal 32-bit timer. It is implemented if the processor's
 `IO_GPTMR_EN` top generic is set `true`. The timer provides a pre-scaled counter register that can trigger an interrupt
-when reaching a programmable threshold value. Additionally, a timer-capture feature is implemented that copies the current
-counter value to a dedicated register if a programmable edge occurs at the `gptmr_trig_i` input signal.
+when reaching a programmable threshold value.l.
 
-Four interface registers are available: a control register (`CTRL`), a 32-bit counter register (`COUNT`), a 32-bit
-threshold register (`THRES`) and a 32-bit read-only capture register (`CAPTURE`). The timer is globally enabled by setting the
-`GPTMR_CTRL_EN` bit in the device's control register `CTRL`. When the timer is enable 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:
+The GPTMR provides three interface registers : a control register (`CTRL`), a 32-bit counter register (`COUNT`) and a
+32-bit threshold register (`THRES`). The timer is globally enabled by setting the `GPTMR_CTRL_EN` bit in the module's
+control register. When the timer is enable the `COUNT` register will start incrementing from zero at a programmable
+rate that scales the main processor clock. this pre-scaler is configured via the three `GPTMR_CTRL_PRSCx`
+control register bits:
 
 .GPTMR prescaler configuration
 [cols="<4,^1,^1,^1,^1,^1,^1,^1,^1"]
@@ -35,40 +34,24 @@ three `GPTMR_CTRL_PRSCx` control register bits:
 | Resulting `clock_prescaler` |       2 |       4 |       8 |      64 |     128 |    1024 |    2048 |    4096
 |=======================
 
-[NOTE]
-Disabling the timer will not clear the `COUNT` register. However, it can be manually reset at any time by
-writing zero to it.
-
-
-**Interval Timer**
-
-Whenever the counter register `COUNT` reaches the programmable threshold value `THRES` the counter register
-is reset to zero and the _timer-match_ flag `GPTMR_CTRL_TRIGM` gets set. This flag has to be cleared manually
-by writing zero to it. Optionally, an interrupt can be triggered if the `GPTMR_CTRL_IRQM` bit is set.
+Whenever the counter register `COUNT` equals the programmable threshold value `THRES` the module's interrupt
+signal becomes pending (indicated by `GPTMR_CTRL_IRQ_PND` being set). Note that a pending interrupt has to be
+cleared manually by writing a `1` to `GPTMR_CTRL_IRQ_CLR`.
 
+The control register's `GPTMR_CTRL_MODE` bit defines what will happen when `COUNT == THRES`.
 
-**Timer Capture**
+* `GPTMR_CTRL_MODE = 0`: **single-shot mode** - the `COUNT` register will stop incrementing
+* `GPTMR_CTRL_MODE = 1`: **continuous mode** - the `COUNT` register is automatically reset and restarts incrementing from zero
 
-In addition to the the internal timer, the GPTMR provides a timer-capture feature. Whenever an edge is detected
-at the `gptmr_trig_i` input signal the current `COUNT` value is copied to the read-only `CAPTURE` register and the
-_capture-trigger_ flag `GPTMR_CTRL_TRIGC` gets set. This flag has to be cleared manually by writing zero to it.
-Optionally, an interrupt can be triggered if the `GPTMR_CTRL_IRQC` bit is set.
-
-The triggering edge can be a rising-edge (if `GPTMR_CTRL_RISE` is set), a falling-edge (if `GPTMR_CTRL_FALL` is
-set) or even both. By default, the `gptmr_trig_i` is sampled two times at the processor clock for checking for
-edges. This simple edge detection is sufficient for trigger signals that are generated by (on-chip) digital logic.
-
-For sampling chip-external signals an optional filtering mode is available that can be enabled by the
-`GPTMR_CTRL_FILTER` bit. If this bit is set, the `gptmr_trig_i` is sampled at a reduced clock speed (1/4 of the
-processor clock) and the signal has to be stable for at lest 4 sample clock in order to be considered high or low.
-This stabilized signal is then fed to the edge detection logic.
+.Resetting the Counter
+[NOTE]
+Disabling the GPTMR will also clear the `COUNT` register.
 
 
 **Interrupt**
 
-The GPTRM provides a single interrupt line that can be trigger by a timer-match event and/or by a timer-compare
-event. Once triggered, the interrupt will stay active until explicitly cleared by writing zero to the according
-interrupt flag (`GPTMR_CTRL_TRIGM` or `GPTMR_CTRL_TRIGC`).
+The GPTRM provides a single interrupt line is triggered whenever `COUNT` equals `THRES`. Once triggered, the interrupt will
+stay pending until explicitly cleared by writing a 1 to `GPTMR_CTRL_IRQ_CLR`.
 
 
 **Register Map**
@@ -78,17 +61,12 @@ interrupt flag (`GPTMR_CTRL_TRIGM` or `GPTMR_CTRL_TRIGC`).
 [options="header",grid="all"]
 |=======================
 | Address | Name [C] | Bit(s), Name [C] | R/W | Function
-.10+<| `0xfffff100` .10+<| `CTRL` <|`0`   `GPTMR_CTRL_EN`                       ^| r/w <| Timer enable flag
-                                  <|`3:1` `GPTMR_CTRL_PRSC2 : GPTMR_CTRL_PRSC0` ^| r/w <| 3-bit clock prescaler select
-                                  <|`4`   `GPTMR_CTRL_IRQM`                     ^| r/w <| Enable interrupt on timer-match
-                                  <|`5`   `GPTMR_CTRL_IRQC`                     ^| r/w <| Enable interrupt on capture-trigger
-                                  <|`6`   `GPTMR_CTRL_RISE`                     ^| r/w <| Capture on rising edge
-                                  <|`7`   `GPTMR_CTRL_FALL`                     ^| r/w <| Capture on falling edge
-                                  <|`8`   `GPTMR_CTRL_FILTER`                   ^| r/w <| Filter capture input
-                                  <|`29:9` -                                    ^| r/- <| _reserved_, read as zero
-                                  <|`30`  `GPTMR_CTRL_TRIGM`                    ^| r/c <| Timer-match has fired, cleared by writing 0
-                                  <|`31`  `GPTMR_CTRL_TRIGC`                    ^| r/c <| Capture-trigger has fired, cleared by writing 0
+.6+<| `0xfffff100` .6+<| `CTRL` <|`0`   `GPTMR_CTRL_EN`                       ^| r/w <| Timer enable flag
+                                <|`3:1` `GPTMR_CTRL_PRSC2 : GPTMR_CTRL_PRSC0` ^| r/w <| 3-bit clock prescaler select
+                                <|`4`   `GPTMR_CTRL_MODE`                     ^| r/w <| Operation mode (0=single-shot, 1=continuous)
+                                <|`29:5` -                                    ^| r/- <| _reserved_, read as zero
+                                <|`30`  `GPTMR_CTRL_IRQ_CLR`                  ^| -/w <| Write `1` to clear timer-match interrupt; auto-clears
+                                <|`31`  `GPTMR_CTRL_IRQ_PND`                  ^| r/- <| Timer-match interrupt pending
 | `0xfffff104` | `THRES`   |`31:0` | r/w | Threshold value register
-| `0xfffff108` | `COUNT`   |`31:0` | r/w | Counter register
-| `0xfffff10C` | `CAPTURE` |`31:0` | r/- | Capture register
+| `0xfffff108` | `COUNT`   |`31:0` | r/- | Counter register
 |=======================