Board definition for CH32X033F8P6 TSSOP20 #171

At the current stage it works for me. Have fun!
Info below is also in the readme of the board definition directory.

+------v------+
MISO/A6 D6~ 1-+PA6 PA5+-20 D5 SCK/A5
MOSI/A8/TX4 D7 2-+PA7=PB0 PA4+-19 D4~ CS/A4
A9/RX4 D8 3-+PB1 PC19+-18 D17 SWCLK
/RST D9 4-+PB7 PA3+-17 D3~ RX2/A3*
USBDM D10 5-+PC16=PC11 PA2+-16 D2~ TX2/A2
USBDP D11 6-+PC17=PC10 PA1+-15 D1~ A1
GND 7-+VSS PA0+-14 D0~ A0
SWDIO D16 8-+PC18 PC3+-13 D15~ A13
VCC 9-+VDD PA10+-12 D14 SCL/TX1
D12 10-+PA9 PA11+-11 D13 SDA/RX1
+-------------+
*A3, VREF and hardware I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).

 
Tested features

digitalWrite()/digitalRead() - can use Arduino pin numbers or PAx notation.
analogWrite() - 12-bit resolution, pins marked with ~ in pinout above.
analogRead() - very stable 12-bit resolution, but issues with A3/A0/PADC_VREF
Serial.print() - Tested 115200 bps on Serial2 (PA2/PA3) and Serial1 (PA10/PA11), as set in variant_CH32X033F8P6.h
EEPROM library - may need improvement (updated to support 122 bytes).
Pin PB7 (Arduino pin 9) can be configured as hardware reset /RST using WCH Link Utility. (See issue Hardware reset pin doesn't work on CH32X033 or CH32X035 #123)
Known issues/limitations

Pins PA7/PB0, PC16/PC11 and PC17/PC10 cannot be used for output.
Any signal on A0 seems to show on other ADC pins when disconnected.
A3, PADC_VREF and I2C don't work on CH32X033F8P6 0-series (lot number with the penultimate bit 5 being 0).
To counter missing I2C the Software_I2C library by Seeed Studio is a good alternative, although it needs some changes to improve compatibility in I2C scanning.

openwch/arduino_core_ch32#171

Author: jobitjoseph

variants/CH32X035/CH32X033F8P/variant_CH32X033F8P.cpp

@@ -0,0 +1,58 @@
+/**
+ *******************************************************************************
+ * Copyright (c) 2021 Nanjing Qinheng Microelectronics Co., Ltd.
+ * All rights reserved.
+ *
+ * This software component is licensed by WCH under BSD 3-Clause license,
+ * the "License"; You may not use this file except in compliance with the
+ * License. You may obtain a copy of the License at:
+ *                        opensource.org/licenses/BSD-3-Clause
+ *
+ *******************************************************************************
+ */
+
+#include "pins_arduino.h"
+
+
+
+
+// Digital PinName array
+const PinName digitalPin[] = {
+  PA_0,   // D0  A0
+  PA_1,   // D1  A1
+  PA_2,   // D2  TX2/A2
+  PA_3,   // D3  RX2/A3
+  PA_4,   // D4  CS/A4
+  PA_5,   // D5  SCK/A5
+  PA_6,   // D6  MISO/A6
+  PB_0,   // D7  MOSI/A8  PA7 is tied to PB0 => input only 
+  PB_1,   // D8  A9
+  PB_7,   // D9  RST
+  PC_16,  // D10 USBDM  tied to PC11=input only 
+  PC_17,  // D11 USBPD  tied to PC10=input only 
+  PA_9,   // D12
+  PA_11,  // D13 SDA (not in 0-series)
+  PA_10,  // D14 SCL (not in 0-series)
+  PC_3,   // D15 A13
+  PC_18,  // D16 SWDIO
+  PC_19,  // D17 SWCLK
+};
+
+// Analog (Ax) pin number array, refers to PinName in digitalPin[]?
+const uint32_t analogInputPin[] = {
+  0,  // A0,  PA0 
+  1,  // A1,  PA1
+  2,  // A2,  PA2
+  3,  // A3,  PA3 (no ADC Ch3 in 0-series!)
+  4,  // A4,  PA4
+  5,  // A5,  PA5
+  6,  // A6,  PA6
+  7,  // A7,  PA7=PB0, input only
+  7,  // A8,  PB0=PA7, input only
+  8,  // A9,  PB1
+  10, // A10, none
+  11, // A11, none
+  12, // A12, none
+  15, // A13, PC3
+};
+