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diff --git a/host/docs/general.rst b/host/docs/general.rst new file mode 100644 index 000000000..7952abb8b --- /dev/null +++ b/host/docs/general.rst @@ -0,0 +1,170 @@ +======================================================================== +UHD - General Application Notes +======================================================================== + +.. contents:: Table of Contents + +------------------------------------------------------------------------ +Tuning notes +------------------------------------------------------------------------ + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Two-stage tuning process +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +A USRP device has two stages of tuning: + +* RF front-end: translates bewteen RF and IF +* DSP: translates between IF and baseband + +In a typical use-case, the user specifies an overall center frequency for the signal chain. +The RF front-end will be tuned as close as possible to the center frequency, +and the DSP will account for the error in tuning between target frequency and actual frequency. +The user may also explicitly control both stages of tuning through the tune_request_t object. + +Pseudo-code for tuning the receive chain: +:: + + //tuning to a desired center frequency + usrp->set_rx_freq(my_frequency_in_hz); + + --OR-- + + //advanced tuning with tune_request_t + uhd::tune_request_t tune_req; + tune_req.target_freq = my_frequency_in_hz; + //fill in tune request fields... + usrp->set_rx_freq(tune_req); + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +RF front-end settling time +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +After tuning, the RF front-end will need time to settle into a usable state. +Typically, this means that the local oscillators must be given time to lock before streaming begins. +Lock time is not consistent; it varies depending upon the device and requested settings. +After tuning and before streaming, the user should +wait for the "lo_locked" sensor to become true, +or sleep for a conservative amount of time (perhaps a second). + +Pseudo-code for dealing with settling time after tuning on receive: +:: + + usrp->set_rx_freq(...); + sleep(1); + usrp->issue_stream_command(...); + + --OR-- + + usrp->set_rx_freq(...); + while (not usrp->get_rx_sensor("lo_locked").to_bool()){ + //sleep for a short time in milliseconds + } + usrp->issue_stream_command(...); + +------------------------------------------------------------------------ +Overflow/Underflow notes +------------------------------------------------------------------------ +**Note:** The following overflow/underflow notes do not apply to USRP1, +which does not support the advanced features available in newer products. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Overflow notes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +When receiving, the device produces samples at a constant rate. +Overflows occurs when the host does not consume data fast enough. +When UHD detects the overflow, it prints an "O" to stdout, +and pushes an inline message packet into the receive stream. + +**Network-based devices**: +The host does not back-pressure the receive stream. +When the kernel's socket buffer becomes full, it will drop subsequent packets. +UHD detects the overflow as a discontinuity in the packet's sequence numbers, +and muxes an inline message packet into the receive stream. + +**Other devices**: +The host back-pressures the receive stream. +Therefore, overflows always occur in the device itself. +When the device's internal buffers become full, streaming is shutoff, +and an inline message packet is sent to the host. +If the device was in continuous streaming mode, +the UHD will automatically restart streaming. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Underflow notes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +When transmitting, the device consumes samples at a constant rate. +Underflow occurs when the host does not produce data fast enough. +When the UHD detects underflow, it prints an "U" to stdout, +and pushes a message packet into the async message stream. + +------------------------------------------------------------------------ +Threading notes +------------------------------------------------------------------------ + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Thread safety notes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +For the most part, UHD is thread-safe. +Please observe the following limitations: + +**Fast-path thread requirements:** +There are three fast-path methods for a device: send(), recv(), and recv_async_msg(). +All three methods are thread-safe and can be called from different thread contexts. +For performance, the user should call each method from a separate thread context. +These methods can also be used in a non-blocking fashion by using a timeout of zero. + +**Slow-path thread requirements:** +It is safe to change multiple settings simultaneously. However, +this could leave the settings for a device in an uncertain state. +The is because changing one setting could have an impact on how a call affects other settings. +Example: setting the channel mapping affects how the antennas are set. +It is recommended to use at most one thread context for manipulating device settings. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Thread priority scheduling +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +When the UHD spawns a new thread it may try to boost the thread's scheduling priority. +When setting the priority fails, the UHD prints out an error. +This error is harmless, it simply means that the thread will have a normal scheduling priority. + +**Linux Notes:** + +Non-privileged users need special permission to change the scheduling priority. +Add the following line to */etc/security/limits.conf*: +:: + + @<my_group> - rtprio 99 + +Replace <my_group> with a group to which your user belongs. +Settings will not take effect until the user has logged in and out. + +------------------------------------------------------------------------ +Misc notes +------------------------------------------------------------------------ + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Support for dynamically loadable modules +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +For a module to be loaded at runtime, it must be: + +* found in the UHD_MODULE_PATH environment variable, +* installed into the <install-path>/share/uhd/modules directory, +* or installed into /usr/share/uhd/modules directory (unix only). + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Disabling or redirecting prints to stdout +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +The user can disable the UHD library from printing directly to stdout by registering a custom message handler. +The handler will intercept all messages, which can be dropped or redirected. +Only one handler can be registered at a time. +Make "register_handler" your first call into UHD: + +:: + + #include <uhd/utils/msg.hpp> + + void my_handler(uhd::msg::type_t type, const std::string &msg){ + //handle the message... + } + + uhd::msg::register_handler(&my_handler); |