Page 301 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

278 Chapter 9 Implementation of C Procedures

struct spiDevice {
unsigned int spie:l,spe:l,swom:l,mstr:l f cpol:l r cpha:l,ssoeil,
lsbf:l;
} *spiPtr = (struct spiDevice *)OxdO;
#define spi (*spiPtr)

raain() { spi.spe = 1; do ; while(spi.spe); }
a. A C Program

0000095B 3B PSHD
0000095C FE0800 LDX spiPtr
0000095F 6E80 STX 0,SP
00000961 OC0040 BSET 0,X,#64
00000964 EE80 LDX 0,SP
00000966 OE0040FA BRSET 0,X,#64,*-6 ;abs = 0964
0000096A 30 PULX
0000096B 3D RTS
b. Assembly Language developed from Part (a)

Figure 9.13. A Program Setting and Testing a Bit

The do while statement in Figure 9.13a tests the condition after the loop is
executed at least once, but it tests the result of the loop's activities. This is very useful
in I/O software because it lets you get data within the statement and test the result in the
conditional expression, which is not executed until the statement is executed at least
once. See Figure 9.13. The program sets a bit of a struct and tests it repeatedly until
it is cleared (by hardware). It is compiled into assembly language shown in Figure 9.13b.
The struct definition shown below merely defines accesses using the pointer spiPtr.
The elements can be accessed using "arrow" notation. For instance the bit spe can be
set using spiPtr->spe = 1; However, by declaring #define spi (*spiPtr), the
expression spi. spe = 1; can be used instead. This is encoded into assembly language
using the BSET instruction:

BSET 0, X, #6 4 ; set bit 6 of location pointed to by X (the spi port)

The statement do ; while (spi. spe); is implemented with

BRSET 0, X, #6 4, * - 6 ; wait while bit 6 of location OxDO is 1

Generally, if the bitfield is more than one bit, data to be inserted will have to be shifted
to the correct bit position, and masked parts of it are ORed with masked parts of bits in
other fields, to be written into the memory. This code looks like the code for statement
lui = (lui « 3) + (lui « 1) + Isc -'0' ; that we studied at the end of
Section 9.2. Data read from such a bitfield will have to be shifted and masked in like
manner.