User Manual

Jennic
24 JN-DS-JN5148-001 1v2 © Jennic 2009
Preliminary
7 Interrupt System
The interrupt system on the JN5148 is a hardware-vectored interrupt system. The JN5148 provides several interrupt
sources, some associated with CPU operations (CPU exceptions) and others which are used by hardware in the
device. When an interrupt occurs, the CPU stops executing the current program and loads its program counter with a
fixed hardware address specific to that interrupt. The interrupt handler or interrupt service routine is stored at this
location and is run on the next CPU cycle. Execution of interrupt service routines is always performed in supervisor
mode. Interrupt sources and their vector locations are listed in
Table 2 below:
Interrupt Source Vector Location Interrupt Definition
Bus error 0x08 Typically cause by an attempt to access an invalid address or a
disabled peripheral
Tick timer 0x0e Tick timer interrupt asserted
Alignment error 0x14 Load/store address to non-naturally-aligned location
Illegal instruction 0x1a Attempt to execute an unrecognised instruction
Hardware interrupt 0x20 interrupt asserted
System call 0x26
System call initiated by b.sys instruction
Trap 0x2c caused by the b.trap instruction or the debug unit
Reset 0x38 Caused by software or hardware reset.
Stack Overflow 0x3e Stack overflow
Table 2: Interrupt Vectors
7.1 System Calls
The b.trap and b.sys instructions allow processor exceptions to be generated by software.
A system call exception will be generated when the b.sys instruction is executed. This exception can, for example, be
used to enable a task to switch the processor into supervisor mode when a real time operating system is in use. (See
section 3 for further details.)
The b.trap instruction is commonly used for trapping errors and for debugging.
7.2 Processor Exceptions
7.2.1 Bus Error
A bus error exception is generated when software attempts to access a memory address that does not exist, or is not
populated with memory or peripheral registers or when writing to ROM.
7.2.2 Alignment
Alignment exceptions are generated when software attempts to access objects that are not aligned to natural word
boundaries. 16-bit objects must be stored on even byte boundaries, while 32-bit objects must be stored on quad byte
boundaries. For instance, attempting to read a 16-bit object from address 0xFFF1 will trigger an alignment exception
as will a read of a 32-bit object from 0xFFF1, 0xFFF2 or 0xFFF3. Examples of legal 32-bit object addresses are
0xFFF0, 0xFFF4, 0xFFF8 etc.
7.2.3 Illegal Instruction
If the CPU reads an unrecognised instruction from memory as part of its instruction fetch, it will cause an illegal
instruction exception.
7.2.4 Stack Overflow
When enabled, a stack overflow exception occurs if the stack pointer reaches a programmable location.