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ATAR092 Datasheet(PDF) 8 Page - ATMEL Corporation |
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ATAR092 Datasheet(HTML) 8 Page - ATMEL Corporation |
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8 / 107 page  8 4535E–4BMCU–05/07 ATAR092/ATAR892 4.2.5 I/O Bus The I/O ports and the registers of the peripheral modules are I/O mapped. All communication between the core and the on-chip peripherals takes place via the I/O bus and the associated I/O control. With the MARC4 IN and OUT instructions the I/O bus allows a direct read or write access to one of the 16 primary I/O addresses. More about the I/O access to the on-chip periph- erals is described in the section “Peripheral Modules”. The I/O bus is internal and is not accessible by the customer on the final microcontroller device, but it is used as the interface for the MARC4 emulation (see also the section “Emulation”). 4.2.6 Instruction Set The MARC4 instruction set is optimized for the high level programming language qFORTH. Many MARC4 instructions are qFORTH words. This enables the compiler to generate a fast and compact program code. The CPU has an instruction pipeline allowing the controller to prefetch an instruction from ROM at the same time as the present instruction is being executed. The MARC4 is a zero-address machine, the instructions contain only the operation to be performed and no source or destination address fields. The operations are implicitly performed on the data placed on the stack. There are one and two byte instructions which are executed within 1 to 4 machine cycles. A MARC4 machine cycle is made up of two system clock cycles (SYSCL). Most of the instructions are only one byte long and are executed in a single machine cycle. For more information refer to the “MARC4 Programmer’s Guide”. 4.2.7 Interrupt Structure The MARC4 can handle interrupts with eight different priority levels. They can be generated from the internal and external interrupt sources or by a software interrupt from the CPU itself. Each interrupt level has a hard-wired priority and an associated vector for the service routine in the ROM (see Figure 4-2 on page 10). The programmer can postpone the processing of inter- rupts by resetting the interrupt enable flag (I) in the CCR. An interrupt occurrence will still be registered, but the interrupt routine only started after the I flag is set. All interrupts can be masked, and the priority individually software configured by programming the appropriate control register of the interrupting module (see section “Peripheral Modules”). 4.2.7.1 Interrupt Processing For processing the eight interrupt levels, the MARC4 includes an interrupt controller with two 8-bit wide interrupt pending and interrupt active registers. The interrupt controller samples all interrupt requests during every non-I/O instruction cycle and latches these in the interrupt pend- ing register. If no higher priority interrupt is present in the interrupt active register, it signals the CPU to interrupt the current program execution. If the interrupt enable bit is set, the processor enters an interrupt acknowledge cycle. During this cycle a short call (SCALL) instruction to the service routine is executed and the current PC is saved on the return stack. An interrupt service routine is completed with the RTI instruction. This instruction resets the cor- responding bits in the interrupt pending/active register and fetches the return address from the return stack to the program counter. When the interrupt enable flag is reset (triggering of inter- rupt routines are disabled), the execution of new interrupt service routines is inhibited but not the logging of the interrupt requests in the interrupt pending register. The execution of the interrupt is delayed until the interrupt enable flag is set again. Note that interrupts are only lost if an inter- rupt request occurs while the corresponding bit in the pending register is still set (i.e., the interrupt service routine is not yet finished). |
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