ST20-C1 datasheet(54/205 Pages) STMICROELECTRONICS

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4.11 Peripherals and I/O

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The CPU saves the task descriptor of the current process in the exception

vector table at the exception level for the peripheral.

The CPU tells the peripheral the number of bytes to be read or written, the

address of the start of the data or input buffer.

The CPU signals to the peripheral that the job can start.

The CPU deschedules the process, using the

stop instruction, to wait for the

peripheral to complete the job.

The CPU executes other processes while the peripheral performs the job.

The peripheral completes the job and sends a micro-interrupt to the CPU, with

the exception level.

The CPU reads the exception vector table and recognises the entry as a user

process. The process is added to the back of the scheduling queue, or if the

schedule execution trap is enabled then the trap is taken.

The code to execute steps 3 to 4 must not be interrupted, since otherwise the periph-

eral job may be completed before the process is descheduled by a

stop instruction,

which may crash the processor. Interrupts may be temporarily disabled by clearing the

local_interrupt_enable bit of the status register, which is set automatically when the

process is descheduled. Steps 5 to 7 happen automatically and need no coding.

The code to drive the peripheral will depend on the peripheral and the interface to it.

Typically the parameters (e.g. the byte count and the buffer address) would be written

to memory mapped registers and then a further write would be needed to start the

peripheral job.

The code to perform a DMA to transmit

param_count bytes to or from param_buffer

using

exception level

except_level where

the DMA

registers

periph_count,

periph_buffer and periph_start are in a block at peripheral would be similar to the

following:

ldtdesc; ld ExceptionBase; stnl except_level;

ld param_count; ld peripheral; stnl periph_count;

rev; ld param_buffer; stnl periph_buffer;

ldc local_interrupt_enable; bitmask; statusclr;

rot; ldc start_value; arot; stnl periph_star t;

stop;

The process will resume at the next instruction after the

stop when the peripheral job

is complete.

Single tasking

For programs which are not multi-tasking, it is not desirable to deschedule the

program while the peripheral is busy. The main program should continue with other

jobs while the peripheral is busy. An interrupt handler can be written to signal to the

main program that the peripheral job is complete. The descriptor of the interrupt

handler exception control block is placed in the exception vector table. The descriptor

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