File content as of revision 9:07de3af99031:

#include "mbed.h"
#include "encoder.h"

DigitalOut encoder_cs(D9);
//SPI spi_encoder(D11, D12, D13); // mosi, miso, sclk
SPI spi_encoder(D4, D5, D3); // mosi, miso, sclk

unsigned short encoder_value;
unsigned short angle = 0;
unsigned short angle_old;
int angle_dif;
int a_dif;
int Angle = 0;
unsigned short k = 0;

void init_SPI_encoder()
{
    spi_encoder.format(16,3);
    spi_encoder.frequency(1000000); // 1MHz clock rate
}

void init_encoder()
{
    encoder_cs = 1;  // high：disable the device
}

void angle_measure()
{
    encoder_cs = 0; // Select the device by seting chip select low
    encoder_value = spi_encoder.write(0x00);
    angle = encoder_value >> 3;
    encoder_cs = 1; // Deselect the device

    if (k == 0) {
        Angle = 0;
        angle_old = angle;
        k++;
    } else {
        angle_dif = angle_count(angle, angle_old);
        Angle = Angle + angle_dif;
        angle_old = angle;
    }
}

int angle_count(unsigned short now,unsigned short old)
{
    a_dif = now - old;
    if (a_dif > 4096/2) {
        a_dif = -(4096 - a_dif);
    } else if (a_dif < -4096/2) {
        a_dif = 4096 + a_dif;
    } else {
        a_dif = a_dif;
    }
    return a_dif;
}