PWM控制参数寄存器结构图

备注:

• 标志位 PWM_CFG_ENALWAYS 和 PWM_CFG_ONESHOT的优先级顺序是en always高于one shot
• 标志位 PWM_CFG_ZEROCMP必须置1

• PWM占空比的计算比较复杂：

  

  左对齐脉宽占空比

  

  中央对齐脉宽占空比

寄存器 PWM0_REG(PWM_S)、PWM0_REG(PWM_CMP0)~PWM0_REG(PWM_CMP3)

// 计数器
PWM0_REG(PWM_COUNT) = 0;

// CFG控制字
PWM0_REG(PWM_CFG) = 0;
PWM0_REG(PWM_CFG) = (PWM_CFG_ENALWAYS) | (PWM_CFG_ZEROCMP) | (PWM_CFG_DEGLITCH);

int main (void)
{// 115200 Baud Rate at (65 / 2) MHz//UART0_REG(UART_REG_DIV) = 282;//UART0_REG(UART_REG_TXCTRL) = UART_TXEN;//UART0_REG(UART_REG_RXCTRL) = UART_RXEN;// Wait a bit because we were changing the GPIOsvolatile int i=0;while(i < 10000){i++;}//_puts(sifive_msg);//_puts(welcome_msg);uint16_t r=0x3F;uint16_t g=0;uint16_t b=0;// Set up RGB PWMPWM0_REG(PWM_CFG)   = 0;PWM0_REG(PWM_CFG)   = (PWM_CFG_ENALWAYS) | (PWM_CFG_ZEROCMP) | (PWM_CFG_DEGLITCH);PWM0_REG(PWM_COUNT) = 0;// The LEDs are intentionally left somewhat dim. PWM0_REG(PWM_CMP0)  = 0xFE;while(1){volatile uint64_t *  now = (volatile uint64_t*)(CLINT_CTRL_ADDR + CLINT_MTIME);volatile uint64_t then = *now + 400;while (*now < then) { }if(r > 0 && b == 0){r--;g++;}if(g > 0 && r == 0){g--;b++;}if(b > 0 && g == 0){r++;b--;}PWM0_REG(PWM_CMP1)  = 0xFF - (r >> 2);PWM0_REG(PWM_CMP2)  = 0xFF - (g >> 2);PWM0_REG(PWM_CMP3)  = 0xFF - (b >> 2);}// While (1)
}

// See LICENSE for license details.#ifndef _SIFIVE_PWM_H
#define _SIFIVE_PWM_H/* Register offsets */#define PWM_CFG   0x00
#define PWM_COUNT 0x08
#define PWM_S     0x10
#define PWM_CMP0  0x20
#define PWM_CMP1  0x24
#define PWM_CMP2  0x28
#define PWM_CMP3  0x2C/* Constants */#define PWM_CFG_SCALE       0x0000000F // 4bit掩码
#define PWM_CFG_STICKY      0x00000100
#define PWM_CFG_ZEROCMP     0x00000200
#define PWM_CFG_DEGLITCH    0x00000400
#define PWM_CFG_ENALWAYS    0x00001000
#define PWM_CFG_ONESHOT     0x00002000
#define PWM_CFG_CMP0CENTER  0x00010000
#define PWM_CFG_CMP1CENTER  0x00020000
#define PWM_CFG_CMP2CENTER  0x00040000
#define PWM_CFG_CMP3CENTER  0x00080000
#define PWM_CFG_CMP0GANG    0x01000000
#define PWM_CFG_CMP1GANG    0x02000000
#define PWM_CFG_CMP2GANG    0x04000000
#define PWM_CFG_CMP3GANG    0x08000000
#define PWM_CFG_CMP0IP      0x10000000
#define PWM_CFG_CMP1IP      0x20000000
#define PWM_CFG_CMP2IP      0x40000000
#define PWM_CFG_CMP3IP      0x80000000#endif /* _SIFIVE_PWM_H */

scale 缩放倍率

A value of 0 in pwmscale indicates no scaling, and pwms would then be equal to the low 16 bits of pwmcount. The maximum value of 15 in pwmscale corresponds to dividing the clock rate by 2^15, so for an input bus clock of 16 MHz, the LSB of pwms will increment at 488.3 Hz.

摘录自
https://sifive.cdn.prismic.io/sifive%2F4d063bf8-3ae6-4db6-9843-ee9076ebadf7_fe310-g000.pdf