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Motorola 68HC11 SPI Interface

The Motorola 68HC11's SPI interface uses four pins on the MicroStamp11. These are pins 15-18. They correspond to bits PD2 and PD5 on PORTD. The clock line comes out of pin PD4 with the logical name SCK. This line is a 50 percent duty cycle clock whose rate can be controlled by the programmer. There are two data lines. The master-out slave-in (MOSI/PD3) is used to transmit data to the slave device. The master-in slave-out (MISO/PD2) is used to receive data from a slave device. In addition to the clock and data lines, the Motorola 68HC11 has an additional control line with the logical name SS on pin PD5. The so-called slave select pin (PD5/SS) is an optional logical control line that can be used when the channel is active. It is often used to signal the end or beginning of a transmission.

The top drawing in figure 28 shows how the SPI interface is arranged. The data (MOSI/MISO) pins are connected to an 8-bit data register SPDR. When a data transfer operation is performed, this 8 bit register is serially shifted eight positions and the data is transmitted or received from the slave. The SPI interface is controlled by three registers shown in the bottom half of figure 28. These registers are the control register SPCR, the status register SPSR and the data register SPDR. The figure also shows the logical names for specific bits in these registers.

Figure 28: SPI registers and connections
\begin{figure}\centerline{\psfig{file=figs/spi-buffers.eps,width=4in}} \end{figure}

As before, rather than discussing the detailed use of the SPI control registers, we'll introduce two kernel functions that use the SPI interface. These functions are shiftout() and shiftin(). The function shiftout shifts out an 8 bit frame at a specified rate over the MOSI line. The function shiftin shifts in an 8 bit frame at a specified rate over the MISO line.

A listing for the shiftout function is given below. This function takes two arguments, the data (a single byte) to be transmitted and the rate at which the communication is to take place. Upon completion the function returns nothing (void). But shiftout is a blocking function. This means that the function will not return until the data has actually been transmitted by the MicroStamp11.

void shiftout(unsigned char data,unsigned char rate){
    unsigned char dummy;
    DDRD |= 0x38;
    PORTD = 0x00;
    SPCR = 0x50;
    SPCR |= rate;
    SPDR = data;
    while((SPSR & SPIF)==0){};
    PORTD |= 0x20;
    PORTD &= ~0x20;
    SPCR = 0x00;
The first line in this function sets the direction state of the PORTD pins to output. It then zeros PORTD. The instruction SPCR=0x50 enables the SPI system and puts the micro-controller in master mode. The rate is then set and the data is loaded in SPDR. The function then waits until bit SPIF in the status register SPSR is set, indicating that the transfer is complete. The last two lines of code toggle the SS pin (to signal that the transmission is complete) and the SPI interface is then disabled.

A listing for the shiftin function is also given below. This function takes a single argument, the rate at which the data is to be clocked into the MicroStamp11. Upon completion the function returns the data to the main routine. Once called, the function does not return until the data has been received. So shiftin is also a blocking function call.

 unsigned char shiftin(unsigned char rate){
   unsigned char data;
   DDRD |= 0x38;
   PORTD |= 0x00;
   PORTD &= ~0x20;

   SPDR=0; //start SPI
   while((SPSR & SPIF));
   data = SPDR;

   return data;
The function shiftin() works as follows. We start by setting the SS line low to signal the slave to start its operation. In the case of the ADC we'll use in the next section, this signal initiates the conversion by the ADC0831 chip. We then start the SPI by loading the register with zero. The function then sits in a while loop until the SPIF bit in the status register SPSR has been set. Setting this bit indicates that the SPI's data register is full. In other words, the data transfer is complete. The contents of the data register SPDR are then transferred to a local variable (data) which is returned by the function.

next up previous
Next: SPI interface to ADC0831 Up: Serial Communication Previous: Synchronous Serial Communication
Bill Goodwine 2002-09-29