Page 313 - Programming Microcontrollers in C

298 Chapter 6 Large Microcontrollers

24-bit address bus, a three-line function control signal bus, control sig­
nals for dynamic bus sizing, and handshaking for external bus arbitration.

Interrupts
The CPU16 contains a three-wire, seven-level interrupt system.
These interrupts are interfaced to the SIM through the IMB. The
outside world is interfaced into the SIM as seven individual interrupt
lines which are multiplexed onto the three-wire line within the SIM.
There are also two sources of interrupt from within the SIM itself.
These interrupts are from the periodic interrupt timer and the soft­
ware watchdog timer.

Chip Selects

There are many signal lines on the microcontroller that might
not be needed with a typical system. For example, address lines 20
through 23 all follow the condition of address line 19. Perhaps not
all of the external interrupt lines are needed. Often the function con­
trol lines that can be used to decode the nature of a bus cycle—i.e.,
either data or program access—are not used. Altogether there are 12
signal lines that can be implemented as chip selects. This line will
assert when there is an access within a memory range specified.

Reset and System Initialization
The microcontroller has several sources of reset. The reset and
system initialization section directs the several resets to the proper
operation, and records the source of the resets. Also, when the sys­
tem is reset initially, the state of several pins on the system bus is
analyzed to determine the mode of the part when it exits the reset
sequence.

General Purpose I/O
There are 16 SIM pins that can be configured as general-purpose
input/output signals. These ports are ports E and F, and the pins are
all multiply assigned. Port E pins, for example, are bus control
pins, and Port F pins have a second use as the external interrupt
inputs to the system.