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C按位OR返回的值不正确

zynq embedded arm c

jhe4x · 技术社区 · 3 年前

我有一个void函数,用来设置32位寄存器的前8位。但是,它不断返回不正确的值(只有2的幂)。

void ugvPwm_setSpeed(ugv_pwm *InstancePtr, u8 spd_val)
{
    u32 reg_read;
    u32 spd_sel;
    reg_read = MOTORPWM_mReadReg(InstancePtr->RegBaseAddress,
                                MOTORPWM_S00_AXI_SLV_REG0_OFFSET); // read original register value
    //
    xil_printf("\r\n\nugvPwm_setSpeed: reg0 = %x\r\n", reg_read);
    xil_printf("ugvPwm_setSpeed: duty = %x\r\n", (u32) spd_val);
    spd_sel = reg_read | (u32) spd_val;                        // bitwise OR in new value; PROBLEM HERE!
    xil_printf("ugvPwm_setSpeed: new reg0 (reg0 | duty) = %x\r\n\n", spd_sel);
    //
    InstancePtr->speedSelect = spd_val;
    MOTORPWM_mWriteReg(InstancePtr->RegBaseAddress, MOTORPWM_S00_AXI_SLV_REG0_OFFSET, spd_sel);
}

我的平台上的u32和u8分别对应于无符号int和无符号char。我尝试过使用onlinegdb,使用unsigned int和unsigned char,效果非常好。我的平台上的输出如下所示:

ugvPwm_setSpeed: reg0 = 300
ugvPwm_setSpeed: duty = 1
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 301
ugvPwm_setSpeed: expected               = 301

ugvPwm_setSpeed: reg0 = 301
ugvPwm_setSpeed: duty = 2
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 303
ugvPwm_setSpeed: expected               = 302

ugvPwm_setSpeed: reg0 = 303
ugvPwm_setSpeed: duty = 3
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 303
ugvPwm_setSpeed: expected               = 303

ugvPwm_setSpeed: reg0 = 303
ugvPwm_setSpeed: duty = 4
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 307
ugvPwm_setSpeed: expected               = 304

ugvPwm_setSpeed: reg0 = 307
ugvPwm_setSpeed: duty = 5
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 307
ugvPwm_setSpeed: expected               = 305

ugvPwm_setSpeed: reg0 = 307
ugvPwm_setSpeed: duty = 6
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 307
ugvPwm_setSpeed: expected               = 306

ugvPwm_setSpeed: reg0 = 307
ugvPwm_setSpeed: duty = 7
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 307
ugvPwm_setSpeed: expected               = 307

ugvPwm_setSpeed: reg0 = 307
ugvPwm_setSpeed: duty = 8
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 30F
ugvPwm_setSpeed: expected               = 308

ugvPwm_setSpeed: reg0 = 30F
ugvPwm_setSpeed: duty = 9
ugvPwm_setSpeed: new reg0 (reg0 | duty) = 30F
ugvPwm_setSpeed: expected               = 309

1 回复 | 直到 3 年前

Andrew Henle 3 年前

如果你想设置将最低阶8位转换为特定值(按位或按位),则不会执行此操作。按位or基本上是一个“一位累加器”——一旦一位被设置为一,所有按位or操作都将保留 1 位值。

你需要 清楚的 低阶8位:

u32 clearLowOrderBitsMask = ~( ( u32 ) 0xFF );
spd_sel = reg_read & clearLowOrderBitsMask;

这是通过使用位来实现的- 和通过按位执行and操作,保持高位24位与之前一样,并对高位24位执行all 1. 位,它通过对所有零位执行按位and来清除低位8字节。

然后可以设置位:

spd_sel = reg_read | (u32) spd_val;