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==== Two-Wire Serial Interface Controller (TWI)

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|=======================
| Hardware source file(s): | neorv32_twi.vhd | 
| Software driver file(s): | neorv32_twi.c |
|                          | neorv32_twi.h |
| Top entity port:         | `twi_sda_io` | 1-bit bi-directional serial data
|                          | `twi_scl_io` | 1-bit bi-directional serial clock
| Configuration generics:  | _IO_TWI_EN_ | implement TWI controller when _true_
| CPU interrupts:          | fast IRQ channel 7 | transmission done interrupt (see <<_processor_interrupts>>)
|=======================

**Theory of Operation**

The two wire interface – also called "I²C" – is a quite famous interface for connecting several on-board
components. Since this interface only needs two signals (the serial data line `twi_sda_io` and the serial
clock line `twi_scl_io`) – despite of the number of connected devices – it allows easy interconnections of
several peripheral nodes.

The NEORV32 TWI implements a **TWI controller**. It features "clock stretching" (if enabled via the control
register), so a slow peripheral can halt the transmission by pulling the SCL line low. Currently, **no multi-controller
support** is available. Also, the NEORV32 TWI unit cannot operate in peripheral mode.

The TWI is enabled via the _TWI_CT_EN_ bit in the _TWI_CT_ control register. The user program can start / stop a
transmission by issuing a START or STOP condition. These conditions are generated by setting the
according bits (_TWI_CT_START_ or _TWI_CT_STOP_) in the control register.

Data is send by writing a byte to the _TWI_DATA_ register. Received data can also be read from this
register. The TWI controller is busy (transmitting data or performing a START or STOP condition) as long as the
_TWI_CT_BUSY_ bit in the control register is set.

An accessed peripheral has to acknowledge each transferred byte. When the _TWI_CT_ACK_ bit is set after a
completed transmission, the accessed peripheral has send an acknowledge. If it is cleared after a
transmission, the peripheral has send a not-acknowledge (NACK). The NEORV32 TWI controller can also
send an ACK by itself ("controller acknowledge _MACK_") after a transmission by pulling SDA low during the
ACK time slot. Set the _TWI_CT_MACK_ bit to activate this feature. If this bit is cleared, the ACK/NACK of the
peripheral is sampled in this time slot instead (normal mode).

In summary, the following independent TWI operations can be triggered by the application program:

* send START condition (also as REPEATED START condition)
* send STOP condition
* send (at least) one byte while also sampling one byte from the bus

[IMPORTANT]
The serial clock (SCL) and the serial data (SDA) lines can only be actively driven low by the
controller. Hence, external pull-up resistors are required for these lines.

The TWI clock frequency is defined via the 3-bit _TWI_CT_PRSCx_ clock prescaler. The following prescalers
are available:

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

Based on the _TWI_CT_PRSCx_ configuration, the actual TWI clock frequency f~SCL~ is derived from the processor main clock f~main~ and is determined by:

_**f~SCL~**_ = _f~main~[Hz]_ / (4 * `clock_prescaler`)

.TWI register map
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|=======================
| Address | Name [C] | Bit(s), Name [C] | R/W | Function
.10+<| `0xffffffb0` .10+<| _TWI_CT_ <|`0` _TWI_CT_EN_     ^| r/w <| TWI enable
                                    <|`1` _TWI_CT_START_  ^| r/w <| generate START condition
                                    <|`2` _TWI_CT_STOP_   ^| r/w <| generate STOP condition
                                    <|`3` _TWI_CT_PRSC0_  ^| r/w .3+<| 3-bit clock prescaler select
                                    <|`4` _TWI_CT_PRSC1_  ^| r/w
                                    <|`5` _TWI_CT_PRSC2_  ^| r/w
                                    <|`6` _TWI_CT_MACK_   ^| r/w <| generate controller ACK for each transmission ("MACK")
                                    <|`7` _TWI_CT_CKSTEN_ ^| r/w <| allow clock-stretching by peripherals when set
                                    <|`30` _TWI_CT_ACK_   ^| r/- <| ACK received when set
                                    <|`31` _TWI_CT_BUSY_  ^| r/- <| transfer/START/STOP in progress when set
| `0xffffffb4` | _TWI_DATA_ |`7:0` _TWI_DATA_MSB_ : TWI_DATA_LSB_ | r/w | receive/transmit data
|=======================