Page 305 - Programming Microcontrollers in C

290 Chapter 6 Large Microcontrollers

index registers (IX, IY, and IZ), and the EK register. All these regis­
ters except for the EK are 16 bits wide. The EK register is an extension
register that is used with the extended addressing mode. Any ex­
tended address calculation will result in a 16-bit number. The result
will be concatenated with the EK register to create a 20-bit address.
(The EK register has nothing to do with the E accumulator found in
the device.) The extension registers are named PK, SK, XK, etc. The
extension registers are usually set during initialization of the device,
and there is no need to change them during program operation. The
content of an extension register is a page pointer. The pages in this
case are each 65536 bytes long. Transition from one page to another
is automatic. For example, if an address is calculated for a jmp which
will pass program control to code in another page, the proper ad­
dress will be calculated for both base and extension register
automatically. The calculation will alter the contents of both the base
and the extension register without programmer concern.
The MC68HC16 family currently has the several internal modules
mentioned above, and it is planned that future versions of the part will
have more modules. This complexity has suggested that the arrange­
ment of the header file for the part be broken into several different
files: One file for the main processor, and several different header files,
one for each of the peripheral modules in the individual part. This
approach is shown in the HEADER/HC16HEADERS directory on
the CD-ROM. There you will find a header file named hc16.h along
with files named adc.h, gpt.h, sim.h, sram.h, and qsm.h.
When writing code for any specific module, you should include the
hc16.h file along with the proper files for the peripheral portions
needed. The hc16.h file must be included first because there are
items defined in this file needed by the other headers.
One major difference between the large and small devices is the
way the exception vector table is handled. Recall that, in the MC68HC11
family, all interrupt vectors are placed at the top of memory. With the
MC68HC16 family, the vector table is contained within the first 512
bytes of memory. Upon reset, the CPU16 core processor reads the first
four words of memory where it must find certain data for the opera­
tion of the program. The address 0 must contain a word whose least
significant 12 bits are the contents of the ZK, the SK and the PK reg­
isters when the part comes out of reset. The address 2 contains the
initial program counter, and the address 4 must contain the initial stack