Add #ifdef support for CH3/CH4 in tim2_pwm example

-Implement CH3 and CH4 optionally via #define
-By default only CH1 and CH2 are enabled.
-Channels 3 and 4 can be enabled by uncommenting the appropriate line
-Choose whether default of PWM pins is HIGH (0xff) or LOW (0x00)

NOTES:
-Channel 3 is on a different port (GPIOC) which will be enabled if
needed.

-Channel 4 is on the NRST pin so not usable unless you use `minichlink -d`
to disable the NRST function so the pin can be used as a GPIO

-If your board has a bypass capacitor on PD7 for a reset button the
waveforms observed on CH4 will not be nice square waves.

examples/tim2_pwm/tim2_pwm.c

 /*
- * Example for using Advanced Control Timer (TIM2) for PWM generation
+ * Example for using General Purpose Timer (TIM2) for PWM generation
  * 03-28-2023 E. Brombaugh
  * 05-29-2023 recallmenot adapted from Timer1 to Timer2
+ * 07-29-2023 frosty implemented CH3 and CH4
  */
 
 /*
@@ -31,6 +32,21 @@ Timer 2 pin mappings by AFIO->PCFR1
 #include "ch32v003fun.h"
 #include <stdio.h>
 
+//By default this example only enables CH1(PD4) and CH2(PD3)
+//Uncomment this line to enable CH3 (PC0)
+//#define TIM2_EN_CH3
+
+//CH4 uses PD7 which is also the NRST pin
+//To use CH4 you must disable the NRST feature
+//by using `minichlink -d` and uncomment this line.
+#define TIM2_EN_CH4
+
+// mask for the CCxP bits
+// when set PWM outputs are held HIGH by default and pulled LOW
+// when zero PWM outputs are held LOW by default and pulled HIGH
+#define TIM2_DEFAULT 0xff
+//#define TIM2_DEFAULT 0x00
+
 /*
  * initialize TIM2 for PWM
  */
@@ -40,6 +56,11 @@ void t2pwm_init( void )
 	RCC->APB2PCENR |= RCC_APB2Periph_GPIOD;
 	RCC->APB1PCENR |= RCC_APB1Periph_TIM2;
 
+	#ifdef TIM2_EN_CH3
+	// If using T2CH3 must also enable GPIOC
+	RCC->APB2PCENR |= RCC_APB2Periph_GPIOC;
+	#endif
+
 	// PD4 is T2CH1, 10MHz Output alt func, push-pull
 	GPIOD->CFGLR &= ~(0xf<<(4*4));
 	GPIOD->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP_AF)<<(4*4);
@@ -47,7 +68,23 @@ void t2pwm_init( void )
 	// PD3 is T2CH2, 10MHz Output alt func, push-pull
 	GPIOD->CFGLR &= ~(0xf<<(4*3));
 	GPIOD->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP_AF)<<(4*3);
-		
+
+	#ifdef TIM2_EN_CH3
+	// PC0 is T2CH3, 10MHz Output alt func, push-pull
+	GPIOC->CFGLR &= ~(0xf<<(4*0));
+	GPIOC->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP_AF)<<(4*0);
+	#endif
+
+
+	#ifdef TIM2_EN_CH4
+	// PD7 is T2CH4, 10MHz Output alt func, push-pull
+	// PD7 is also the NRST (Reset) pin and you must use minichlink
+	// or the WCH utility to disable the NRST function so the pin
+	// can be used as a GPIO.
+	GPIOD->CFGLR &= ~(0xf<<(4*7));
+	GPIOD->CFGLR |= (GPIO_Speed_10MHz | GPIO_CNF_OUT_PP_AF)<<(4*7);
+	#endif
+
 	// Reset TIM2 to init all regs
 	RCC->APB1PRSTR |= RCC_APB1Periph_TIM2;
 	RCC->APB1PRSTR &= ~RCC_APB1Periph_TIM2;
@@ -62,15 +99,26 @@ void t2pwm_init( void )
 	// enabling preload causes the new pulse width in compare capture register only to come into effect when UG bit in SWEVGR is set (= initiate update) (auto-clears)
 	TIM2->CHCTLR1 |= TIM_OC1M_2 | TIM_OC1M_1 | TIM_OC1PE | TIM_OC2M_2 | TIM_OC2M_1 | TIM_OC2PE;
 
+	#ifdef TIM2_EN_CH3
+	TIM2->CHCTLR2 |= TIM_OC3M_2 | TIM_OC3M_1 | TIM_OC3PE;
+	#endif
+	#ifdef TIM2_EN_CH4
+	TIM2->CHCTLR2 |= TIM_OC4M_2 | TIM_OC4M_1 | TIM_OC4PE;
+	#endif
 	// CTLR1: default is up, events generated, edge align
 	// enable auto-reload of preload
 	TIM2->CTLR1 |= TIM_ARPE;
 
-	// Enable CH1 output, positive pol
-	TIM2->CCER |= TIM_CC1E | TIM_CC1P;
-	// Enable CH2 output, positive pol
-	TIM2->CCER |= TIM_CC2E | TIM_CC2P;
+	// Enable Channel outputs, set default state (based on TIM2_DEFAULT)
+	TIM2->CCER |= TIM_CC1E | (TIM_CC1P & TIM2_DEFAULT);
+	TIM2->CCER |= TIM_CC2E | (TIM_CC2P & TIM2_DEFAULT);
 
+	#ifdef TIM2_EN_CH3
+	TIM2->CCER |= TIM_CC3E | (TIM_CC3P & TIM2_DEFAULT);
+	#endif
+	#ifdef TIM2_EN_CH4
+	TIM2->CCER |= TIM_CC4E | (TIM_CC4P & TIM2_DEFAULT);
+	#endif
 	// initialize counter
 	TIM2->SWEVGR |= TIM_UG;
 
@@ -88,10 +136,14 @@ void t2pwm_setpw(uint8_t chl, uint16_t width)
 	{
 		case 0: TIM2->CH1CVR = width; break;
 		case 1: TIM2->CH2CVR = width; break;
+		#ifdef TIM2_EN_CH3
 		case 2: TIM2->CH3CVR = width; break;
+		#endif
+		#ifdef TIM2_EN_CH4
 		case 3: TIM2->CH4CVR = width; break;
+		#endif
 	}
-	TIM2->SWEVGR |= TIM_UG; // load new value in compare capture register
+	//TIM2->SWEVGR |= TIM_UG; // load new value in compare capture register
 }
 
 
@@ -116,8 +168,14 @@ int main()
 	printf("looping...\n\r");
 	while(1)
 	{
-		t2pwm_setpw(0, count);			// Chl 1
-		t2pwm_setpw(1, (count + 128)&255);	// Chl 2 180° out-of-phase
+		t2pwm_setpw(0, count);			// CH1
+		t2pwm_setpw(1, (count + 128)&255);	// CH2 180° out-of-phase
+		#ifdef TIM2_EN_CH3
+		t2pwm_setpw(2, count);			// CH3
+		#endif
+		#ifdef TIM2_EN_CH4
+		t2pwm_setpw(3, (count + 128)&255);	// CH4 180° out-of-phase
+		#endif
 		count++;
 		count &= 255;
 		Delay_Ms( 5 );