unsigned char spiTransfer(unsigned char out_data)
{
    // softwared SPI
    unsigned char temp=0x80,in_data=0;
    while(temp)
    {
        // set MOSI
        BIT_SET(ISP_PORT,ISP_MOSI,(out_data & temp));
        // delay 1/2 of period
        delay_us(duration);
        // read MISO
        if(BIT_TEST(ISP_PIN,ISP_MISO))
            in_data|=temp;
        // SCK high
        BIT_ON(ISP_PORT,ISP_SCK);
        // delay 1/2 of period
        delay_us(duration);
        // prepare for next bit
        temp>>=1;
        // SCK low
        BIT_OFF(ISP_PORT,ISP_SCK);
    }
    return in_data;
}