Microchip PIC24FJ SPI写入正常但读取始终为0的问题求助

问题现象 #

• PIC24FJ256GA702通过SPI与AD5724RBREZ四通道DAC通信，写入功能正常，但读取控制寄存器时始终返回0。
• 示波器显示PIC引脚波形符合预期（时钟为蓝色，DAC输出到PIC的数据为黄色），波形非全0，与4通道及内部基准使能的状态匹配（波形振铃疑似长接地导致，实际芯片间距约25mm）。
• 已排除输入引脚为模拟引脚的可能：已正确配置为数字输入；将该引脚连接到Timer1计数器，读取DAC时计数器能正常计数，说明PPS配置有效、引脚无损坏、输入信号清晰。
• 推测问题出在代码或SPI模块解码时序，但时钟周期内数据稳定，无法定位。
• 查阅论坛资料，多数同类问题源于模拟功能启用，但本次并非此情况。

相关代码 #

初始化函数 #

void AOUT_init(void)
{
    //assume PPS is unlocked (it is on reset) - see note below
    
    //setup SPI1 itself
    SPI1CON1L   =   0x0160;     //TX work read 0
    //SPI1CON1L   =   0x0060;   //TX not work
    //SPI1CON1L   =   0x0120;   //TX work, read 0
    //SPI1CON1L   =   0x0020;   //not tried
    
    SPI1CON1H   =   0x3000;
    SPI1CON2L   =   0x0017;     //word length (17 hex = 24 bits)
    //BRG
    SPI1BRGL    =   0x0000;     //default = no divisor
    //PPS - assume unlocked at this point
    ANSBbits.ANSB13 = 0; 
    TRISBbits.TRISB13 = TRIS_INPUT;
    //##########################################################################
    RPINR20bits.SDI1R = 13;     //set SDI1 data input to PIC to RB13    
    //##########################################################################
    
    TRISBbits.TRISB15 = TRIS_OUTPUT;
    RPOR7bits.RP15R = 8;        //RB15 to SDI1 clock out from PIC
    TRISBbits.TRISB14 = TRIS_OUTPUT;    
    RPOR7bits.RP14R = 7;        //RB14 to SDI1 data out from PIC
    //AD5724R has additional lines - not all used in practice
    //setup and set initial level here
    //AOUT-/LDAC
    TRISBbits.TRISB6 = TRIS_OUTPUT;
    LATBbits.LATB6 = 1;
    //AOUT-/SYNC
    TRISBbits.TRISB7 = TRIS_OUTPUT;
    LATBbits.LATB7 = 1;
    //AOUT-/CLR
    TRISBbits.TRISB12 = TRIS_OUTPUT;
    LATBbits.LATB12 = 1;

    //turn SPI on
    SPI1CON1Lbits.SPIEN = 1;
    SPI1CON1Lbits.MSTEN = 1; //included in definition above 
    
    //now setup the AD chip
    //output range set
    AOUT_TX(0x0C00,0x0100); //all channels to 10V
    //control
    AOUT_TX(0x1900,0x0500);
    //power control - enable DACs
    AOUT_TX(0x1000,0x1F00); 
}

通信控制函数 #

static void AOUT_Comms(bool bSync, bool bLDAC, bool bClr)
{
    //AOUT-/LDAC
    LATBbits.LATB6 = bLDAC;
    //AOUT-/SYNC
    LATBbits.LATB7 = bSync;
    //AOUT-/CLR
    LATBbits.LATB12 = bClr;
            
}

写入函数（功能正常） #

void AOUT_TX(uint16_t dataH, uint16_t dataL)
{
    //AOUT uses 24 bit data
    //this routine handles /SYNC line
    //relies on AD chip having much faster response than chip cycle time 
    //AD chip limits at about 38MHz so much quicker than PIC.
    AOUT_Comms(0,1,1);
    SPI1BUFL = dataL; 
    SPI1BUFH = dataH;
    while(SPI1STATLbits.SPIBUSY) //wait until sent
    {
        ;
    }
    AOUT_Comms(1,1,1);
    //
     
}

读取函数 #

void AOUT_read_control(uint16_t ReadH, uint16_t ReadL)
{
    uint16_t temp;
    //to read, transmit the register to be read, then transmit a dummy command "NOP" to clock the data out.
    //send register
    AOUT_TX(0x9000,0x0000);
    //read out- this is similar to write but reads the received buffer at the end.
    //clear buffer
    while (SPI1STATLbits.SPIRBF)
    {
        temp = SPI1BUFL;
        temp = SPI1BUFH;
    }

    
    AOUT_Comms(0,1,1);
    SPI1BUFL = 0; 
    SPI1BUFH = 0x1800; //nop operation via control register
    while(SPI1STATLbits.SPIBUSY) //wait until sent
    {
        ;
    }
    while (!SPI1STATLbits.SPIRBF)
    {
        ; //hold until something received
    }
    ReadH = SPI1BUFH;
    ReadL = SPI1BUFL;  //these read zero     
  

    AOUT_Comms(1,1,1);
    //
    
    //dummy so can check counter also connected to same pin
    temp = TMR1;
    temp = TMR1;   
    
}

更新信息 #

• 将接收数据直接输出到显示器，结果均为0000；
• 用逻辑分析仪检测PIC引脚，读写解码均正确（通道1/MOSI写入0x180000，通道2/MISO读取0x00001F）。

请求帮助 #

• 提供排查方向；
• 分享可用的SPI读取代码以验证现有代码是否存在问题。

内容的提问来源于stack exchange，提问作者REPuzzle