86 lines
4.2 KiB
Plaintext
86 lines
4.2 KiB
Plaintext
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:sectnums:
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==== Pulse-Width Modulation Controller (PWM)
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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_pwm.vhd |
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| Software driver file(s): | neorv32_pwm.c |
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| | neorv32_pwm.h |
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| Top entity port: | `pwm_o` | PWM output channels (12-bit)
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| Configuration generics: | `IO_PWM_NUM_CH` | number of PWM channels to implement (0..12)
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| CPU interrupts: | none |
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|=======================
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**Overview**
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**Overview**
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The PWM module implements a pulse-width modulation controller with up to 12 independent channels providing
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8-bit resolution per channel. The actual number of implemented channels is defined by the `IO_PWM_NUM_CH` generic.
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Setting this generic to zero will completely remove the PWM controller from the design.
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[NOTE]
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The `pwm_o` has a static size of 12-bit. If less than 12 PWM channels are configured, only the LSB-aligned channel
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bits are used while the remaining bits are hardwired to zero.
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**Theory of Operation**
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The PWM controller is activated by setting the `PWM_CTRL_EN` bit in the module's control register `CTRL`. When this
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bit is cleared, the unit is reset and all PWM output channels are set to zero. The module
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provides three duty cycle registers `DC[0]` to `DC[2]`. Each register contains the duty cycle configuration for four
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consecutive channels. For example, the duty cycle of channel 0 is defined via bits 7:0 in `DC[0]`. The duty cycle of
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channel 2 is defined via bits 15:0 in `DC[0]` and so on.
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[NOTE]
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Regardless of the configuration of `IO_PWM_NUM_CH` all module registers can be accessed without raising an exception.
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Software can discover the number of available channels by writing 0xff to all duty cycle configuration bytes and
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reading those values back. The duty-cycle of channels that were not implemented always reads as zero.
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Based on the configured duty cycle the according intensity of the channel can be computed by the following formula:
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_**Intensity~x~**_ = `DC[y](i*8+7 downto i*8)` / (2^8^)
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The base frequency of the generated PWM signals is defined by the PWM core clock. This clock is derived
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from the main processor clock and divided by a prescaler via the 3-bit `PWM_CTRL_PRSCx` in the unit's control
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register.
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.PWM prescaler configuration
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[cols="<4,^1,^1,^1,^1,^1,^1,^1,^1"]
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[options="header",grid="rows"]
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|=======================
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| **`PWM_CTRL_PRSCx`** | `0b000` | `0b001` | `0b010` | `0b011` | `0b100` | `0b101` | `0b110` | `0b111`
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| Resulting `clock_prescaler` | 2 | 4 | 8 | 64 | 128 | 1024 | 2048 | 4096
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|=======================
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The resulting PWM carrier frequency is defined by:
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_**f~PWM~**_ = _f~main~[Hz]_ / (2^8^ * `clock_prescaler`)
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**Register Map**
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.PWM register map (`struct neorv32_pwm_t`)
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[cols="<4,<2,<6,^2,<8"]
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[options="header",grid="all"]
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|=======================
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| Address | Name [C] | Bit(s), Name [C] | R/W | Function
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.3+<| `0xfffff000` .3+<| `CTRL` <|`0` `PWM_CTRL_EN` ^| r/w <| PWM enable
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<|`3:1` `PWM_CTRL_PRSC2 : PWM_CTRL_PRSC0` ^| r/w <| 3-bit clock prescaler select
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<|`31:4` - ^| r/- <| _reserved_, read as zero
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.4+<| `0xfffff004` .4+<| `DC[0]` <|`7:0` ^| r/w <| 8-bit duty cycle for channel 0
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<|`15:8` ^| r/w <| 8-bit duty cycle for channel 1
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<|`23:16` ^| r/w <| 8-bit duty cycle for channel 2
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<|`31:24` ^| r/w <| 8-bit duty cycle for channel 3
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.4+<| `0xfffff008` .4+<| `DC[1]` <|`7:0` ^| r/w <| 8-bit duty cycle for channel 4
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<|`15:8` ^| r/w <| 8-bit duty cycle for channel 5
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<|`23:16` ^| r/w <| 8-bit duty cycle for channel 6
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<|`31:24` ^| r/w <| 8-bit duty cycle for channel 7
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.4+<| `0xfffff00c` .4+<| `DC[2]` <|`7:0` ^| r/w <| 8-bit duty cycle for channel 8
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<|`15:8` ^| r/w <| 8-bit duty cycle for channel 9
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<|`23:16` ^| r/w <| 8-bit duty cycle for channel 10
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<|`31:24` ^| r/w <| 8-bit duty cycle for channel 11
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|=======================
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