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TABLE 13.3
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INTERRUPTS SUPPORTED IN RTOS INTERRUPT SOURCE REGISTER
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INTERRUPT NUMBER
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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TMR1 TMR2 CCP1 SSP TX RC AD PSP CCP2 TMR3 LVD BCL RB INT0 TMR0
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PIR1 PIR1 PIR1 PIR1 PIR1 PIR1 PIR1 PIR1 PIR2 PIR2 PIR2 PIR2 INTCON INTCON INTCON Special purpose
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TMR0IE never should be reset by a task. The purpose of the TMR0 interrupt is to stop an application from taking up too many processor cycles and starving the other tasks. If you have a task with a large amount of data processing or input-output (I/O) polling, I suggest that you place it at as low a priority as possible to allow other tasks the chance to execute. During a task s execution, the INTCON GIE bit can be reset for critically timed code or a high-priority operation. I do not recommend disabling interrupts for more than 20 instruction cycles because the longer interrupts are disabled, the better chance there is that an interrupt will be missed or overwritten. The RTOS code only responds to one interrupt at a time and responds to them in the order given earlier. Any delays in acknowledging an interrupt can result in problems later in the application. The RTOS requests are listed in Table 13.4 along with the registers (and bits) that are changed. Each request is actually a macro, which was done to eliminate the need for keeping a reference for how to call the RTOS correctly. When the RTOS is booted, it will start AllTask. This task has two purposes. The rst is to start up the application code. To do this, it invokes the TaskStart macro to start up Task1 at a priority of 1. AllTask has a priority of 0, and once Task1 is
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PIC18 RTOS DESIGN
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TABLE 13.4 RTOS TASK REQUEST
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RTOS TASK REQUEST DEFINITION
INPUT PARAMETERS
OUTPUT PARAMETERS
INVOKING MACRO
Start task
Starting address Priority (0 3) le register requirements None Interrupt number
Carry reset, OK, and new task # in WREG Carry set, NO available or more than 63 task variables None None
TaskStart Address, Priority, Size Address, Priority, and Size are all constants. TaskNext IntWait Interrupt Interrupt is a constant. MsgSend TaskNumber TaskNumber is the address of two consecutive le registers. MsgWait
Execute next task Wait for interrupt
Send message
Task number, 2-byte message in FSR0L/FSR1L
None
Wait for message
None
2-byte message in FSR0L/FSR1L TaskNumber in WREG Carry reset, 2-byte message in FSR0L/FSR1L Carry set, no message to get Carry reset, operation complete Carry set, no message to acknowledge
Read message
Task number
MsgRead TaskNumber TaskNumber is the address of a le register. MsgAck TaskNumber TaskNumber is the address of a le register.
Acknowledge message
Task number
executing, it will just loop as the lowest-priority task in the RTOS, providing a lowest level of functionality for the RTOS. The code I used is
_AllTask: TaskStart Task1, 1, 63 _AllTask_Loop: TaskNext bra _AllTask_Loop ; Jump to Next Active Task ; ; Always Return Here Start Application Code
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Vcc PIC18C452 0.1 uF Tantalum Gnd Vcc 10 K _MCLR Osc 1 4 MHz Gnd Gnd Osc 2 Vss PORTB 8 Bits 8 + Vdd Vcc
220 8
Figure 13.2 This simple circuit, using the parts listed in Table 13.5, will allow you to demonstrate the operation of a basic RTOS.
The AllTask looping may not seem like an important function, but it is because when the RTOS code is active, all interrupt requests are disabled. If all the other tasks in the PIC microcontroller are waiting on interrupt requests or messages, then AllTask will be the only one able to execute, and when the branch always instruction executes, interrupts are enabled. In other operating systems, this function is known as system idle and executes when all other tasks are waiting on some kind of event. Task1 is the rst task of the application code. This is a standard label that is used to indicate the application code. Note that I have given it a priority of 1 and the full 63 le registers for variables. 1 is the lowest priority that your application should have to ensure that AllTask performs its function properly. When I created the RTOS, I wanted to use it with a simple application. As I mentioned earlier, this application simply waits on the timer and updates LED values once per second. The circuit that I used is shown in Fig. 13.2 and is part of the rtos01.asm application that is found in the code\rtos subdirectory of your PC s PIC microcontroller directory. The bill of materials for the project is listed in Table 13.5.
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