参考v1.7.0版本例程，利用CR5513BEND，借用了ADC单端采样的实例，通过MSIO1管脚来检测电压，以下是测试代码，实际测量值应为1.8V左右，现测的电压在0.87V左右，请问是什么原因

#define TEST_CONV_LENGTH (16UL)

adc_handle_t g_adc_handle;

uint16_t conversion[TEST_CONV_LENGTH];

double voltage[TEST_CONV_LENGTH];

void adc_single(void)

{

g_adc_handle.init.channel_p = ADC_INPUT_SRC_IO1;

g_adc_handle.init.channel_n = ADC_INPUT_SRC_IO1;

g_adc_handle.init.input_mode = ADC_INPUT_SINGLE;

g_adc_handle.init.ref_source = ADC_REF_SRC_BUF_INT;

g_adc_handle.init.ref_value = ADC_REF_VALUE_1P6;

g_adc_handle.init.clock = ADC_CLK_1P6M;

hal_adc_deinit(&g_adc_handle);

hal_adc_init(&g_adc_handle);

memset(conversion, 0, sizeof(conversion));

printf("Start single sampling.\r\n");

hal_adc_poll_for_conversion(&g_adc_handle, conversion, TEST_CONV_LENGTH);

while (g_adc_handle.state == HAL_ADC_STATE_BUSY);

hal_gr551x_adc_voltage_intern(&g_adc_handle, conversion, voltage, TEST_CONV_LENGTH);

printf("Conversion value:\r\n");

}