/**
@defgroup   boards_sipeed_longan_nano Sipeed Longan Nano board
@ingroup    boards
@brief      Support for the Sipeed Longan Nano board
@author     Gunar Schorcht <gunar@schorcht.net>

## Overview

The [Sipeed Longan Nano](https://longan.sipeed.com/en) is a
development board for the GigaDevice GD32VF103CBT6 MCU with the following
on-board components:

- GD32VF103CBT6 RISC-V MCU @108MHz
- USB Type C
- TF card slot
- 3 user LEDs

@image html "https://media-cdn.seeedstudio.com/media/catalog/product/cache/7f7f32ef807b8c2c2215b49801c56084/1/1/114992425_1.jpg" "Sipeed Longan Nano" width=600

## Hardware:

| MCU         | GD32VF103CBT6                          | Supported |
|:----------- |:-------------------------------------- | --------- |
| Family      | RISC-V with ECLIC                      |           |
| Vendor      | GigaDevice                             |           |
| RAM         | 32 kByte                               |           |
| Flash       | 128 KByte                              |           |
| Frequency   | 108 MHz                                |           |
| Power Modes | 3 (Sleep, Deep Sleep, Standby)         |   no      |
| GPIOs       | 37                                     |   yes     |
| Timers      | 5 x 16-bit timer                       |   yes     |
| RTC         | 1 x 32-bit counter, 20-bit prescaler   |   yes     |
| WDT         | 2 x 12-bit counter, 3-bit prescaler    |   yes     |
| ADC         | 2 x 12-bit units, 16 channels, 1 Msps  |   no      |
| DAC         | 2 x 12-bit channel                     |   no      |
| UART        | -                                      |   yes     |
| USART       | 3                                      |   yes     |
| SPI         | 3                                      |   no      |
| I2C         | 2 x Fast Mode 400 kHz                  |   no      |
| I2S         | 2                                      |   no      |
| CAN         | 2 x CAN 2.0B with up to 1 Mbps         |   no      |
| PWM         | 6 Channels                             |   no      |
| USB         | 1 x USB FS OTG                         |   no      |
| Vcc         | 3.0V - 3.6V                            |           |
| Datasheet   | [Datasheet](https://gd32mcu.com/data/documents/datasheet/GD32VF103_Datasheet_Rev1.6.pdf) | |
| Reference Manual | [Reference Manual](https://gd32mcu.com/download/down/document_id/222/path_type/1) | |
| Board Manual | [Board Manual](https://longan.sipeed.com/en/) | |
| Board Schematic | [Board Schematic](http://dl.sipeed.com/fileList/LONGAN/Nano/HDK/Longan%20Nano%203302/2_Schematic/Longan_nano_3302_Schematic.pdf) | |

## Pin Layout / Configuration

The general pin layout is shown below.

@image html "https://longan.sipeed.com/assets/longan_nano_pinout_v1.1.0_w5676_h4000_large.png" "Sipeed Longan Nano Pinout" width=800

The following table shows the connection of the on-board components with the
MCU pins and their configuration in RIOT.

| MCU Pin | MCU Peripheral | RIOT Peripheral     | Board Function        |
|:--------|:---------------|:--------------------|:----------------------|
| PA9     | USART0 TX      | UART_DEV(0) TX      | UART TX               |
| PA10    | USART0 RX      | UART_DEV(0) RX      | UART RX               |
| PB0     |                |                     | LED1                  |
| PB1     |                |                     | LED2                  |
| PB5     |                |                     | LED0                  |

## Flash the board

The board is flashed via a JTAG interface with OpenOCD (at least [release version 0.12.0]
(https://github.com/openocd-org/openocd/tree/9ea7f3d647c8ecf6b0f1424002dfc3f4504a162c)).
By default, an FTDI adapter according to the configuration defined in
[`interface/openocd-usb.cfg`]
(https://github.com/openocd-org/openocd/blob/9ea7f3d647c8ecf6b0f1424002dfc3f4504a162c/tcl/interface/ftdi/openocd-usb.cfg)
is assumed.
```
BOARD=seeedstudio-gd32 make -C examples/hello-world flash
```
To use an FTDI adapter with a different configuration, the configuration can be
defined using the variable `OPENOCD_FTDI_ADAPTER`, for example:
```
OPENOCD_FTDI_ADAPTER=tigar BOARD=seeedstudio-gd32 make -C examples/hello-world flash
```
If another adapter is used, it can be specified using variable
`OPENOCD_DEBUG_ADAPTER`, for example for a Segger J-Link adapter:
```
OPENOCD_DEBUG_ADAPTER=jlink BOARD=seeedstudio-gd32 make -C examples/hello-world flash
```

## Accessing STDIO via UART

The `stdio` is directly accessible through the first UART interface. If an
external USB-to-UART interface is used, this interface is mapped to
`/dev/ttyUSB<n>` on a Linux host, where `<n>` is the index of the UART
interface, which is 0 by default.

Use the `term` target to connect to the board using `/dev/ttyUSB0`:
```
BOARD=seeedstudio-gd32 make -C examples/hello-world term
```
If the UART interface index of board's USB to UART bridge is not 0, use
the following command to connect:
```
BOARD=seeedstudio-gd32 make -C examples/hello-world term PORT=/dev/ttyUSB<n>
```
 */