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# CH32V003 (SOP-8) Arduino Pin Remapping & Serial Guide
## Overview
The **CH32V003J4M6** comes in a tiny SOP-8 package. Due to the limited pin count (only 6 usable I/O pins), standard Arduino pin mappings often conflict with required peripherals like I2C or the SWIO programming interface.
![WCH LinkE Connection](images/WCH-LinkE-connection.jpg)
This guide details how to:
1. **Remap Hardware Serial** to different pins by modifying the Arduino Core.
2. **Manage Conflicts** (e.g., losing I2C to gain Serial RX).
3. **Share Pin 8 (SWIO)** for both programming and Serial TX.
4. **Unbrick/Erase the chip** using `minichlink` if configuration errors lock you out.
---
## The Pinout Constraints
On the SOP-8 package, several peripherals share the same physical pins. You cannot use all functions simultaneously.
![CH32V003J4M6 Pinout](images/CH32V003J4M6.webp)
| Pin # | Pin Name | Default Function | Conflict / Alt Function |
| :--- | :--- | :--- | :--- |
| **1** | **PD6** | UART1 RX (Default) | **Can be remapped to UART1 TX** |
| **5** | **PC1** | **I2C SDA** | Can be remapped to UART1 RX |
| **6** | **PC2** | **I2C SCL** | - |
| **8** | **PD1** | **SWIO (Programming)** | Can be remapped to UART1 TX |
---
## Method 1: Permanent Remap (Editing Core Files)
To use Hardware Serial on **Pin 1 (PD6)** as TX, you must modify the board definition files. This allows `Serial.print` to work natively on the new pins.
### 1. Locate the Core Files
Navigate to the directory where the WCH Arduino core is installed:
* **Windows:** `%LOCALAPPDATA%\Arduino15\packages\WCH\hardware\ch32v\1.0.4\variants\CH32V003F4`
* **Linux:** `~/.arduino15/packages/WCH/hardware/ch32v/1.0.4/variants/CH32V003F4`
* **Mac:** `~/Library/Arduino15/packages/WCH/hardware/ch32v/1.0.4/variants/CH32V003F4`
### 2. Modify `PeripheralPins.c`
Open `PeripheralPins.c`. You need to change the pin definitions in the `PinMap_UART_TX` and `PinMap_UART_RX` arrays.
**To set TX to Pin 1 (PD6) and RX to Pin 5 (PC1):**
*Note: This configuration sacrifices hardware I2C SDA on Pin 5.*
```c
//*** UART ***
#ifdef UART_MODULE_ENABLED
WEAK const PinMap PinMap_UART_TX[] = {
{PD_6, USART1, CH_PIN_DATA(CH_MODE_OUTPUT_50MHz, CH_CNF_OUTPUT_AFPP, 0, AFIO_NONE)}, // from PD_5
{NC, NP, 0}
};
#endif
#ifdef UART_MODULE_ENABLED
WEAK const PinMap PinMap_UART_RX[] = {
{PC_1, USART1, CH_PIN_DATA(CH_MODE_INPUT, CH_CNF_INPUT_PUPD, PULLUP, AFIO_NONE)}, // from PD_6
{NC, NP, 0}
};
#endif
```
### 3. Modify `variant_CH32V003F4.h`
Open `variant_CH32V003F4.h` and update the definitions to match your changes in `PeripheralPins.c`.
```c
// UART Definitions
#ifndef SERIAL_UART_INSTANCE
#define SERIAL_UART_INSTANCE 1
#endif
// Default pin used for generic 'Serial' instance
// Mandatory for Firmata
#ifndef PIN_SERIAL_RX
#define PIN_SERIAL_RX PC1 // from PD6
#endif
#ifndef PIN_SERIAL_TX
#define PIN_SERIAL_TX PD6 // from PD5
#endif
```
> **Warning:** These changes affect **all** sketches compiled for the CH32V003F4 board. If you switch projects, you may need to revert these files.
### 4. Modify your sketch
Add the following lines to your setup() code in your Arduino IDE sketch:
```c
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOD | RCC_APB2Periph_USART1 | RCC_APB2Periph_AFIO, ENABLE);
GPIO_PinRemapConfig(GPIO_PartialRemap2_USART1, ENABLE);
Serial.begin(115200);
```
### 5. Full code
```c
#define ledpin PC4
int counter = 0;
void setup() {
pinMode(ledpin, OUTPUT);
// Must enable clock for AFIO
RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOD | RCC_APB2Periph_USART1 | RCC_APB2Periph_AFIO, ENABLE);
GPIO_PinRemapConfig(GPIO_PartialRemap2_USART1, ENABLE);
Serial.begin(115200);
}
void loop() {
digitalWrite(ledpin, HIGH);
delay(80);
counter++;
digitalWrite(ledpin, LOW);
delay(300);
Serial.print("blink ");
Serial.println(counter);
}
```
### 6. Switch the new RX/TX connections
Do not forget to switch to the new RX/TX pins. Then you can reprogram and use serial communication as well.
![Breadboard remap](images/Breadboard-remap.jpg)
---
## Method 2: Shared Serial on Pin 8 (SWIO)
If you need Pin 1 and Pin 5 for other uses (like sensors), you can use **Pin 8 (PD1)** for Serial TX. However, **PD1 is the SWIO programming pin**.
To use this safely, you must dynamically initialize and de-initialize the Serial port in your code. This ensures the pin is only driving data when you specifically print, leaving it free for the programmer (WCH-Link) the rest of the time.
### Code Implementation
Do **not** put `Serial.begin()` in `setup()`. Instead, wrap your print statements like this:
```cpp
void setup() {
// Do not initialize Serial here!
}
void loop() {
// 1. Initialize Serial (Claims Pin 8)
Serial.begin(115200);
// 2. Send Data
Serial.println("Battery Status: OK");
// 3. Wait for transmission to complete
Serial.flush();
// 4. End Serial (Releases Pin 8 back to High-Z/Input)
Serial.end();
// Pin 8 is now free for SWIO programming
delay(1000);
}
```
**Pros:** Keeps Pins 1, 5, and 6 free for sensors.
**Cons:** You cannot debug continuously; using the Serial Monitor constantly will make it difficult to upload new code.
---
## Unbricking / Full Erasure
If you configure the pins incorrectly (e.g., setting SWIO to output low permanently) or disable the debug interface, the WCH-Link might fail to detect the chip.
To recover the chip, you need to perform a full erasure using `minichlink`.
### Prerequisites
1. Clone the [cnlohr/ch32fun](https://github.com/cnlohr/ch32fun) or [openwch/ch32v003](https://github.com/openwch/ch32v003) repository.
```bash
cd ~
git clone --depth 1 https://github.com/cnlohr/ch32fun.git
```
3. Go to the directory
```bash
cd ch32fun/minichlink
```
4. Run compile
```bash
make
```
In case of an error
```bash
/usr/bin/ld: cannot find -lusb-1.0: No such file or directory
collect2: error: ld returned 1 exit status
make: *** [Makefile:47: minichlink] Error 1
```
Install dependencies
```bash
sudo apt update
sudo apt install libusb-1.0-0-dev libudev-dev
```
5. Install the binary
```bash
sudo cp minichlink /usr/local/bin/
```
Check if succesfull
```bash
which minichlink
```
Should output
```bash
/usr/local/bin/minichlink
```
6. Connect the WCH-LinkE to the chip using a single wire interface (SWD).
### The Unbrick Command
Run the following command in your terminal:
```bash
minichlink -u
```
* **`-u`**: This flag commands the programmer to perform a chip erase and re-enable the SWIO interface.
* If this fails, try holding the chip in "Reset" (connect NRST to GND if available, or power cycle rapidly) while issuing the command.
---
## References & Credits
This guide describes advanced configuration techniques derived from the following resources:
* **YouTube:** [Re-mapping the pins on a CH32v003](https://www.youtube.com/watch?v=jhtcRypzWq4) – Visual guide on locating and editing the Arduino Core files for pin redefinition.
* **Blog:** [CH32v003 Serial and SWIO Communication Clash](https://onecircuit.blogspot.com/2024/10/0000-0000-1111-0100.html) – Detailed analysis of pin conflicts on the SOP-8 package and methods for resolving SWIO/Serial overlaps.