Version 23 (Anthony Rowe, 06/08/2007 10:16 pm)

1 11 Anthony Rowe
= Signals =
2 1 Anthony Rowe
3 1 Anthony Rowe
A signal is a message that a task can use to wakeup one or more tasks waiting on an
4 1 Anthony Rowe
event or events. When waiting on an event, a task is suspended and does not consume
5 4 Anthony Rowe
CPU time. Nano-RK supports 32 unique signals. It is possible to wait on multiple
6 1 Anthony Rowe
signals such that any one of them can wake a task from sleep. All signals must be
7 6 Anthony Rowe
created using nrk_sig_create() before they can be used.  Any task that wishes to wakeup on a signal
8 6 Anthony Rowe
must register the signal using nrk_sig_register().  There are a few special case signals that are
9 6 Anthony Rowe
generated by the kernel used to support event timeouts and notification of special actions.  See the
10 15 Anthony Rowe
'''Special Kernel Signals''' subsection for more information.  For more information about signals, please refer
11 14 Anthony Rowe
to the basic_signals project that comes with Nano-RK: 
12 14 Anthony Rowe
13 1 Anthony Rowe
14 11 Anthony Rowe
15 5 Anthony Rowe
16 5 Anthony Rowe
17 5 Anthony Rowe
// This is a macro used to convert a signal into a bitmask for nrk_event_wait()
18 5 Anthony Rowe
// See nrk_event_wait() for an example.
19 5 Anthony Rowe
#define SIG(x)  ((uint32_t)1)<<x 
20 5 Anthony Rowe
21 5 Anthony Rowe
// These are typedefs used to represent signals and semaphores
22 5 Anthony Rowe
typedef int8_t nrk_sig_t;
23 5 Anthony Rowe
typedef uint32_t nrk_sig_mask_t;
24 5 Anthony Rowe
typedef int8_t nrk_sem_t;
25 5 Anthony Rowe
26 5 Anthony Rowe
27 5 Anthony Rowe
28 23 Anthony Rowe
'''nrk_sig_t nrk_signal_create();'''
29 1 Anthony Rowe
30 23 Anthony Rowe
This function creates a signal.  Upon failure, this function returns NRK_ERROR.  Upon success a positive value
31 23 Anthony Rowe
representing the signal is returned.
32 23 Anthony Rowe
33 1 Anthony Rowe
'''int8_t nrk_delete_signal(nrk_sig_t sig_id);'''
34 1 Anthony Rowe
35 23 Anthony Rowe
This function deletes a signal ''sig_id'' so that it can be reused by different tasks.  This function returns NRK_OK upon success and NRK_ERROR on failure.
36 1 Anthony Rowe
37 1 Anthony Rowe
'''int8_t nrk_event_signal(nrk_sig_t sig_id);'''
38 1 Anthony Rowe
39 1 Anthony Rowe
This function is used to signal tasks that are waiting on events using nrk_event_wait().
40 23 Anthony Rowe
''sig_id'' is the signal that is sent.  This function returns NRK_OK upon success and NRK_ERROR upon failure.
41 23 Anthony Rowe
If a lower priority task signals a higher priority task, the high priority task will begin to execute at 
42 23 Anthony Rowe
the next context swap.  Normally this happens when the low priority task suspends, but it is also possible
43 23 Anthony Rowe
that a medium priority task could preempt the low priority task causing a context swap that would then
44 23 Anthony Rowe
schedule the waiting high priority task.  For this reason, processing that needs to be complete before the signaled task
45 23 Anthony Rowe
executes should be done before the signals are sent. 
46 1 Anthony Rowe
47 4 Anthony Rowe
48 1 Anthony Rowe
'''nrk_sig_mask_t nrk_event_wait(nrk_sig_mask_t event_mask);'''
49 1 Anthony Rowe
50 1 Anthony Rowe
This function will wait for a set events. nano-RK supports up to 32 signals. Each signal
51 23 Anthony Rowe
represents a bit in a 32 bit number so it is possible to logically
52 1 Anthony Rowe
OR multiple signals together if you wish to wait on a combination of events. The 32
53 23 Anthony Rowe
bit number returned by nrk_event_wait() corresponds to the signal or signals that were returned.
54 23 Anthony Rowe
When waiting on multiple signals, the return value can be used to determine which signal
55 1 Anthony Rowe
triggered the wakeup. All signals need to be registered in order for a task to receive them.
56 1 Anthony Rowe
This function returns 0 upon failure if a signal is specified that does not exist, or is not
57 1 Anthony Rowe
58 23 Anthony Rowe
59 23 Anthony Rowe
60 23 Anthony Rowe
'''int8_t nrk_signal_register(nrk_sig_t sig_id);'''
61 23 Anthony Rowe
62 23 Anthony Rowe
This function registers a signal ''sig_id'' so that a task is able to receive it.  This function returns NRK_OK upon success and NRK_ERROR upon failure if the signal does not exist.  A signal only needs to be registered for reception and not transmission.
63 23 Anthony Rowe
64 23 Anthony Rowe
'''int8_t nrk_signal_unregister(nrk_sig_t sig_id);'''
65 23 Anthony Rowe
66 23 Anthony Rowe
This function unregisters a signal ''sig_id'' so that the task is no longer able to be unsuspended by that event.  This function returns NRK_OK upon success and NRK_ERROR uopn failure.
67 23 Anthony Rowe
68 23 Anthony Rowe
69 23 Anthony Rowe
'''nrk_sig_mask_t nrk_get_active_signal_mask();'''
70 23 Anthony Rowe
71 23 Anthony Rowe
This function returns the current registered signal mask for a task.
72 11 Anthony Rowe
73 11 Anthony Rowe
= Semaphores =
74 11 Anthony Rowe
75 11 Anthony Rowe
A semaphore is a protected variable and constitutes the classic method for restricting
76 11 Anthony Rowe
access to shared resources (e.g. storage,actuators etc) in a multiprogramming environment.
77 11 Anthony Rowe
Nano-RK implements semaphores and signals such that
78 12 Anthony Rowe
tasks that are suspended on an event or waiting for access to a semaphore will not be
79 12 Anthony Rowe
scheduled until the corresponding signal is sent or semaphore becomes available. For more information
80 12 Anthony Rowe
on using semaphores please refer to the basic_sem project that comes with the Nano-RK distribution: 
81 11 Anthony Rowe
82 16 Anthony Rowe
83 16 Anthony Rowe
''Note that the Atmel ISA does not have a test-and-set instruction, so we provide a best effort implementation by disabling interrupts.'' 
84 10 Anthony Rowe
85 1 Anthony Rowe
'''nrk_sem_t* nrk_sem_create(uint8_t count, uint8_t ceiling_priority);'''
86 10 Anthony Rowe
87 10 Anthony Rowe
This function creates a semaphore resource, with a priority ceiling value used by the task when accessing the resource. This facilitates the Priority Ceiling Protocol Emulation (PCPE) algorithm used in Nano-RK to avoid priority inversion. ''count'' specifies the number of entries allowed into the critical section. ''ceiling_priority'' sets the ceiling value for the task; Note if
88 10 Anthony Rowe
using PCPE this should be the highest priority task accessing the critical section.  Below is an example of declaring and creating a semaphore:
89 10 Anthony Rowe
90 10 Anthony Rowe
91 10 Anthony Rowe
92 10 Anthony Rowe
   nrk_sem_t *my_semaphore;
93 10 Anthony Rowe
94 10 Anthony Rowe
   my_semaphore = nrk_sem_create(1,4);
95 10 Anthony Rowe
   if(my_semaphore==NULL) nrk_kprintf( PSTR("Error creating Semaphore\r\n" ));
96 10 Anthony Rowe
97 10 Anthony Rowe
98 1 Anthony Rowe
This created a semaphore with a count of 1 (also called a mutex or binary semaphore) with a priority ceiling value of 4.
99 1 Anthony Rowe
100 1 Anthony Rowe
'''int8_t nrk_sem_pend(nrk_sem_t *rsrc );''' 
101 8 Anthony Rowe
102 8 Anthony Rowe
Semaphore pend takes the address of the created semaphore and attempts to access the resource.  If the resource is available,
103 8 Anthony Rowe
pend will decrement the resource counter and allow the program to continue, otherwise pend will suspend until the resource is
104 1 Anthony Rowe
posted by another task. This can be used to protect critical sections of code. Below is an example of a task pending on a semaphore:
105 1 Anthony Rowe
106 1 Anthony Rowe
107 1 Anthony Rowe
108 1 Anthony Rowe
   nrk_kprintf( PSTR("Task accessing semaphore\r\n"));
109 1 Anthony Rowe
   v = nrk_sem_pend(my_semaphore);
110 1 Anthony Rowe
   if(v==NRK_ERROR) nrk_kprintf( PSTR("Error calling pend\r\n"));
111 9 Anthony Rowe
   nrk_kprintf( PSTR("Task is now holding semaphore\r\n"));
112 9 Anthony Rowe
113 9 Anthony Rowe
114 9 Anthony Rowe
'''int8_t nrk_sem_post(nrk_sem_t* rsrc);'''
115 9 Anthony Rowe
116 9 Anthony Rowe
Semaphore post takes the address of a created semaphore and releases access to the resource.
117 9 Anthony Rowe
This should be called after exiting a critical section that was pended.  Below is an example of a task posting a semaphore:
118 1 Anthony Rowe
119 9 Anthony Rowe
120 9 Anthony Rowe
121 9 Anthony Rowe
   v = nrk_sem_post(my_semaphore);
122 9 Anthony Rowe
   if(v==NRK_ERROR) nrk_kprintf( PSTR("Error calling post\r\n"));
123 8 Anthony Rowe
   nrk_kprintf( PSTR("Task released semaphore\r\n"));
124 8 Anthony Rowe
125 11 Anthony Rowe
126 8 Anthony Rowe
= Special Kernel Signals =
127 18 Anthony Rowe
128 20 Anthony Rowe
129 19 Anthony Rowe
130 1 Anthony Rowe
Sometimes it might be convenient to have a timeout associated with an nrk_event_wait() call.   This can be achieved using the kernel generated nrk_wakeup_signal.  This signal is sent on a per-task basis (not globally) when the task’s internal next-wakeup timer expires.  Normally, a task’s next wakeup is set for its next period, however this can be adjusted using the nrk_set_next_wakeup() function. nrk_wakeup_signal is created by the kernel during nrk_init().  In order for nrk_event_wait() to receive the nrk_wakeup_signal from a task it must simply be registered and muxed in like any other signal.  Note, each task has its own instance of the nrk_wakeup_signal.  Unlike other signals, it is not globally broadcast to all tasks.  See below for an example of how to use it as an event  timeout.
131 19 Anthony Rowe
132 20 Anthony Rowe
'''int8_t nrk_set_next_wakeup(nrk_time_t timeout);'''
133 19 Anthony Rowe
134 19 Anthony Rowe
This function sets the task's next wakeup timer.  It returns NRK_OK upon success and NRK_ERROR on failure.  This function does not
135 18 Anthony Rowe
suspend the task, it only changes when the next wakeup will happen if the task suspends on an event.  Calling other suspend functions like nrk_wait_until_next_period() will replace whatever wakeup value you might have previously set. 
136 17 Anthony Rowe
137 17 Anthony Rowe
138 17 Anthony Rowe
139 8 Anthony Rowe
   nrk_time_t timeout;
140 17 Anthony Rowe
141 17 Anthony Rowe
142 1 Anthony Rowe
143 1 Anthony Rowe
144 1 Anthony Rowe
145 17 Anthony Rowe
146 17 Anthony Rowe
   my_sigs=nrk_event_wait( SIG(signal_one) | SIG(nrk_wakeup_signal) );
147 21 Anthony Rowe
148 18 Anthony Rowe
   // Lets check which signal we got...
149 18 Anthony Rowe
   if(my_sigs==0)                        nrk_kprintf( PSTR( "Error calling nrk_event_wait()\r\n" ));
150 18 Anthony Rowe
   if(my_sigs & SIG(signal_one))         nrk_kprintf( PSTR( "Task got signal_one\r\n") );
151 17 Anthony Rowe
   if(my_sigs & SIG(nrk_wakeup_signal))  nrk_kprintf( PSTR( "Task got timeout signal! \r\n") );
152 1 Anthony Rowe
153 1 Anthony Rowe
154 1 Anthony Rowe
| [wiki:nrk-api Contents] | [wiki:nrk-api-device-drivers Device Drivers] |