Page 268 - Embedded Microprocessor Systems Real World Design

Activate solenoid
Start solenoid timer by setting SOLENOID to 20
Poll for 19ms, checking to see if SOLENOID went to 0
Polling loop Ands that it is time to start an ADC conversion.
Call ADC routine
Start ADC conversion
Wait for ADC to complete
*** Right here, the timer interrupt occurs, so the ISR decrements
SOLENOID. SOLENOID decrements to 0.
Read/store ADC result
Return to polljng loop
Polling loop checks SOLENOID, fhds it has rolled to 0, turns off
solenoid.

Now, in an RTOS environment, it might work like this:

Solenoid/pump driver task turns on solenoid and suspends for 20ms.
19ms go by, during which other tasks are executed
Some event tells RTOS that it is time to start an ADC conversion
RTOS starts ADC conversion task
ADC conversion is started
*** Again, the timer interrupt occurs, and the RTOS finds that
20ms has gone by.
RTOS reactivates solenoid/pump driver task
Solenoid is turned off
Pump is turned on
Other processing goes on until solenoid/pump drive task suspends
again.
RTOS reactivates ADC task. but now it is too late. ADC result is bad.

The result here is that, sometimes, the analog-todigital conversion will be bad.
There are a number of ways to fix this. The ADC task could be given higher
priority than the solenoid/pump driver task. Or, before starting the conversion, the
ADC task could tell the RTOS that it is about to begin a noninterruptible function
(if the RTOS supports that). Or, the ADC task could ask temporarily to have its
priority set higher than the solenoid task until the conversion is complete (again,
assuming the RTOS supports it). The point is that, in an RTOS environment, any
event that results in an RTOS function being executed can result in a task switch.
An ISR does not necessarily return to the task that was executing when the inter-
rupt occurred-at least not right away. You must take this into account in the
software. You do not know when interrupts will occur, so you must assume they will
occur at the worst possible time.

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