Page 267 - Embedded Microprocessor Systems Real World Design

Yield. Tells the RTOS that the task is finished for now and that the next task on
the list may be executed.
Define TimeSlice. Defines the number of time-slice intervals that the task will
be allowed to execute.
Allocate Memory. Requests a specified number of memory blocks.
Mailbox In. Receives a mailbox message. The parameters would include the task
number and the mailbox number.
Send Mail. Sends mail to a mailbox. The parameters could include the mailbox
number, destination task number, and priority of the message.
Wait On. Waits for the queue to fill, the semaphore to be active, or the mailbox
to receive mail.
Of course, this is not a comprehensive list of RTOS services, it is just an indication
of the kind of things an RTOS supports.
A few pointers if you are thinking about using an RTOS: Make sure that assump
tions about memory are correct. The Ethernet/RS232 system assumed that
transmit and receive were halfduplex. If this assumption turns out to be wrong; if
both buffers are ever needed simultaneously, then there will be a memory alloca-
tion problem. This may be a minor problem, as a task waits until the memory is
available. However, it can cause a lockup if the task that has the memory will not
release it until the task requesting memory can execute.
Make sure the RTOS does not bog down the system operation. While an RTOS
is deterministic, it still takes time to do things. Be sure this time is no problem. Also
make sure that task priorities do not prevent a low-priority but essential task from
ever executing.

Preemption Considerations

Two considerations you must keep in mind when using a preemptive RTOS are that
the RTOS manages the operation of the software and any RTOS function can
perform a task switch. The idea is to get maximum use of the CPU, but it means
you must take things into consideration that otherwise you need not. Say you have
an analog-todigital converter (ADC) that requires you to read the result within 100
microseconds (ms) of starting a conversion. Also say you have a solenoid that is
activated by the software, held for 20ms, and then turned off. The solenoid timing
is performed by counting interrupts from a 1 ms timer. The polling loop activates
the solenoid and sets a variable, SOLENOID, to 20. The 1ms ISR decrements
SOLENOID as long as it is nonzero. When it decrements to zero, the solenoid is
turned off. After the solenoid is turned off, a pump is started. The way this might
work in a polled environment is:

248 Embedded Microprocessor Systems