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--- technical_information [2018/08/29 16:06]
giometti created
+++ technical_information [2025/02/24 10:02] (current)
giometti [Parallel port generator]
@@ Line 1: / Line 1: @@
-====== RFC considerations ======
+====== Technical Information ======
+===== RFC considerations =====
 While implementing a PPS API as RFC 2783 defines and using an embedded
@@ Line 26: / Line 28: @@
 into files ''/dev/pps0'', ''/dev/pps1'', etc.
-====== PPS with USB to serial devices ======
+===== PPS with USB to serial devices =====
 It is possible to grab the PPS from an USB to serial device. However,
@@ Line 42: / Line 44: @@
 ch341 and pl2303 examples).
-====== Coding example ======
+===== Coding example =====
 To register a PPS source into the kernel you should define a ''struct
@@ Line 87: / Line 89: @@
 Please see the file ''drivers/pps/clients/pps-ktimer.c'' for example code.
-====== SYSFS support ======
+===== SYSFS support =====
 If the SYSFS filesystem is enabled in the kernel it provides a new class:
@@ Line 117: / Line 119: @@
    PPS source is connected to (if it exists).
-====== Testing the PPS support ======
+===== Testing the PPS support =====
 In order to test the PPS support even without specific hardware you can use
@@ Line 142: / Line 144: @@
 This is available in the **pps-tools** repository mentioned above.
-====== Generators ======
+===== Generators =====
 Sometimes one needs to be able not only to catch PPS signals but to produce
 them also. For example, running a distributed simulation, which requires
-computers' clock to be synchronized very tightly. One way to do this is to
+computers' clock to be synchronized very tightly.
-invent some complicated hardware solutions but it may be neither necessary
-nor affordable. The cheap way is to load a PPS generator on one of the
-computers (master) and PPS clients on others (slaves), and use very simple
-cables to deliver signals using parallel ports, for example.
-  Parallel port cable pinout:
+To do so the class pps-gen has been added. PPS generators can be
-  pin     name    master      slave
+registered in the kernel by defining a ''struct pps_gen_source_info'' as
-       STROBE    *------     *
+follows:
-       D0        *     |     *
-       D1        *     |     *
+    static struct pps_gen_source_info pps_gen_dummy_info = {
-       D2        *     |     *
+            .name                   = "dummy",
-       D3        *     |     *
+            .use_system_clock       = true,
-       D4        *     |     *
+            .get_time               = pps_gen_dummy_get_time,
-       D5        *     |     *
+            .enable                 = pps_gen_dummy_enable,
-       D6        *     |     *
+    };
-       D7        *     |     *
-      ACK       *     ------*
+Where the ''use_system_clock'' states if the generator uses the system
-      BUSY      *           *
+clock to generate its pulses, or they are from a peripheral device
-      PE        *           *
+clock. Method ''get_time()'' is used to query the time stored into the
-      SEL       *           *
+generator clock, while the method ''enable()'' is used to enable or
-      AUTOFD    *           *
+disable the PPS pulse generation.
-      ERROR     *           *
-      INIT      *           *
+Then calling the function ''pps_gen_register_source()'' in your
-      SELIN     *           *
+initialization routine as follows creates a new generator in the
--25   GND       *-----------*
+system:
+    pps_gen = pps_gen_register_source(&pps_gen_dummy_info);
+===== Generators SYSFS support =====
+If the SYSFS filesystem is enabled in the kernel it provides a new class:
+    $ ls /sys/class/pps-gen/
+    pps-gen0/  pps-gen1/  pps-gen2/
+Every directory is the ID of a PPS generator defined in the system and
+inside of it you find several files:
+    $ ls -F /sys/class/pps-gen/pps-gen0/
+    dev  enable  name  power/  subsystem@  system  time  uevent
+To enable the PPS signal generation you can use the command below:
+    $ echo 1 > /sys/class/pps-gen/pps-gen0/enable
+===== Parallel port generator =====
+One way to do this is to invent some complicated hardware solutions but it
+may be neither necessary nor affordable. The cheap way is to load a PPS
+generator on one of the computers (master) and PPS clients on others
+(slaves), and use very simple cables to deliver signals using parallel
+ports, for example.
+Parallel port cable pinout:
+        pin     name    master      slave
+       STROBE    *------     *
+       D0        *     |     *
+       D1        *     |     *
+       D2        *     |     *
+       D3        *     |     *
+       D4        *     |     *
+       D5        *     |     *
+       D6        *     |     *
+       D7        *     |     *
+      ACK       *     ------*
+      BUSY      *           *
+      PE        *           *
+      SEL       *           *
+      AUTOFD    *           *
+      ERROR     *           *
+      INIT      *           *
+      SELIN     *           *
+-25   GND       *-----------*
 Please note that parallel port interrupt occurs only on high->low transition,
@@ Line 177: / Line 224: @@
 using polling in the interrupt handler which actually can be done way more
 precisely because interrupt handling delays can be quite big and random. So
-current parport PPS generator implementation (''pps_gen_parport'' module) is
+current parport PPS generator implementation (pps_gen_parport module) is
 geared towards using the clear edge for time synchronization.
@@ Line 184: / Line 231: @@
 latencies. But if it is too small slave won't be able to capture clear edge
 transition. The default of 30us should be good enough in most situations.
-The delay can be selected using ''delay'' ''pps_gen_parport'' module parameter.
+The delay can be selected using ''delay'' pps_gen_parport module parameter.

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