之前写过了mcu通过硬件spi接口向dac芯片ad5764的数据寄存器写值输出电压，ad5764的offset与gain寄存器的值也是可以通过硬件spi读出来的。
第一步：将待读取的芯片设置为读模式
回读的命令定义如下：
当要回读时，应向输入移位寄存器写入0x010000，因为要将REG2、REG1、REG0全部设置为0，将A2、A1设置为0，将A0设置为1，同时将DB0设置为0；
第二步与第三步：
可以看到在时序图中，回读发生了2次数据发送（spi是同步通信，发送几次意味着接收几次），第一次发送向芯片写入命令，告诉芯片准备读哪个通道的哪个寄存器，返回的数据是未定义的，可以不用管；第二次发送NOP，接收芯片返回的寄存器的值。
接下来就是编写代码实现功能了，因为板子上有2颗ad5762与2颗ad5764，还是需要使用菊花链通信。
数据定义：
//REGISTER MAP
#define AD5764_NOP 0x000000
#define READ 0x800000
#define WRITE 0x000000
#define REG_FUNCTION 0x000000
#define REG_DATA 0x100000
#define REG_COARSE_GAIN 0x180000
#define REG_FINE_GAIN 0x200000
#define REG_OFFSET 0x280000
#define DAC_A 0x000000
#define DAC_B 0x010000
#define DAC_C 0x020000
#define DAC_D 0x030000
#define DAC_ALL 0x040000
#define SDO_ENABLE 0x000000
#define SDO_DISABLE 0x000001
enum lcc_ch_num{
ch1 = 1, ch2, ch3, ch4, ch5, ch6
};
enum lcc_ch_abcd{
a = 1, b, c, d
};
typedef struct DAC_SEND_ARRAY
{
uint32_t send_array[4];//AB
uint32_t send_cd_array[4];//CD
uint32_t receive_array[4];
}ad_5764_spi;
//spi读写函数的实现过程在之前的博文中
void hw_spi_read_write(uint32_t *send_data, uint32_t *receice_data, uint8_t size)
{
SPI_SET_SS0_LOW(SPI1);
for (uint8_t i = 0; i < size; i++)
{
SPI_WRITE_TX0(SPI1, send_data[i]);
SPI_TRIGGER(SPI1);
while (SPI_IS_BUSY(SPI1));
receice_data[i] = SPI_READ_RX0(SPI1);
}
SPI_SET_SS0_HIGH(SPI1);
LDAC = PIN_LOW;
Delay(1);
LDAC = PIN_HIGH;
}
void daisy_chain_readregister(enum lcc_ch_num ch, enum lcc_ch_abcd abcd, uint32_t RegisterSelect)
{
memset(spi_ad5764.send_array, 0, sizeof(spi_ad5764.send_array));
// 设置ad5764为读模式
if (ch == ch1)
{
spi_ad5764.send_array[3] = 0x010000;
}
else if (ch == ch2)
{
spi_ad5764.send_array[2] = 0x010000;
}
else if (ch == ch3 || ch == ch4)
{
spi_ad5764.send_array[1] = 0x010000;
}
else if (ch == ch5 || ch == ch6)
{
spi_ad5764.send_array[0] = 0x010000;
}
hw_spi_read_write(spi_ad5764.send_array, spi_ad5764.receive_array, 4);
switch (ch)
{
case ch1:
if (abcd == a)
{
spi_ad5764.send_array[3] = READ + RegisterSelect + DAC_A;
}
else if (abcd == b)
{
spi_ad5764.send_array[3] = READ + RegisterSelect + DAC_B;
}
break;
case ch2:
if (abcd == a)
{
spi_ad5764.send_array[2] = READ + RegisterSelect + DAC_A;
}
else if (abcd == b)
{
spi_ad5764.send_array[2] = READ + RegisterSelect + DAC_B;
}
break;
case ch3:
if (abcd == a)
{
spi_ad5764.send_array[1] = READ + RegisterSelect + DAC_A;
}
else if (abcd == b)
{
spi_ad5764.send_array[1] = READ + RegisterSelect + DAC_B;
}
break;
case ch4:
if (abcd == c)
{
spi_ad5764.send_array[1] = READ + RegisterSelect + DAC_C;
}
else if (abcd == d)
{
spi_ad5764.send_array[1] = READ + RegisterSelect + DAC_D;
}
break;
case ch5:
if (abcd == a)
{
spi_ad5764.send_array[0] = READ + RegisterSelect + DAC_A;
}
else if (abcd == b)
{
spi_ad5764.send_array[0] = READ + RegisterSelect + DAC_B;
}
break;
case ch6:
if (abcd == c)
{
spi_ad5764.send_array[0] = READ + RegisterSelect + DAC_C;
}
else if (abcd == d)
{
spi_ad5764.send_array[0] = READ + RegisterSelect + DAC_D;
}
break;
default:
break;
}
//发送读哪个寄存器和哪个通道
hw_spi_read_write(spi_ad5764.send_array, spi_ad5764.receive_array, 4);
memset(spi_ad5764.send_array, 0, sizeof(spi_ad5764.send_array));
memset(spi_ad5764.receive_array, 0, sizeof(spi_ad5764.receive_array));
for (uint8_t i = 0; i < 4; i++)
{
spi_ad5764.send_array[i] = AD5764_NOP;
}
//发送接收命令
hw_spi_read_write(spi_ad5764.send_array, spi_ad5764.receive_array, 4);
}
至此回读功能就实现了。在测试过程中发现，在设置为读模式回读寄存器值之后，如果要向数据寄存器写入值输出电压，直接使用写命令即可，无需将芯片设置为写模式；另外我使用NUC123SD4AN0的SPI1接口可以将时钟设置为16MHz，写命令是正常的，但是回读的时候，每次都少了2个bit（第0位和第1位），当我把SPI的时钟降低为8MHz时，回读会少1个bit（第0位），当把时钟降低为4MHz时，回读的数据才正确无误了，不知为何。