README

README

This sample displays the maximum measured IRQ latency, while different actions
will be executed in the program and by the user. The measurement uses a system
timer, which by default generates 25000 IRQs per second. On each IRQ the delay
between the moment, the IRQ has been triggered, and the time, when the IRQ
handler starts execution is calculated. Then the minimum, maximum and average
value of this delay will be determined. This is implemented in the class
CLatencyTester. The sample program does only work with a screen without
modification.

You can configure the following options, before building the program:

* System option REALTIME in include/circle/sysconfig.h:

  Modifies the spin lock handling in CScreenDevice::Write() and
  CSerialDevice::Write(), so that the IRQ latency should decrease radically.
  Disables the detection of low- and full-speed USB devices without
  USE_USB_SOF_INTR enabled, because they would increase the IRQ latency very much.

* System option USE_USB_SOF_INTR in include/circle/sysconfig.h:

  Should be enabled on Raspberry Pi 1-3 and Zero in any case for USB PnP
  operation, which is activated in the program. Without this option low- and
  full-speed USB devices cannot be used, when REALTIME is enabled too. Has no
  influence on Raspberry Pi 4.

* Option USE_BUFFERED_SCREEN in file kernel.h of the sample program:

  Messages won't be directly written to screen with this option. Instead they will
  be written to a ring buffer and displayed later, when
  CWriteBufferDevice::Update() is called at TASK_LEVEL. This allows displaying
  messages from IRQ_LEVEL, even when REALTIME is defined. Otherwise these messages
  are silently ignored.

While the sample is running, watch the displayed logger messages. The "Timer
elapsed" message is generated at IRQ_LEVEL every second and is only visible
without REALTIME or with both REALTIME and USE_BUFFERED_SCREEN enabled.

You can plugin or remove USB devices at any time to influence system operation.