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+What: /sys/class/timecard/
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This directory contains files and directories
+ providing a standardized interface to the ancillary
+ features of the OpenCompute timecard.
+
+What: /sys/class/timecard/ocpN/
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This directory contains the attributes of the Nth timecard
+ registered.
+
+What: /sys/class/timecard/ocpN/available_clock_sources
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) The list of available time sources that the PHC
+ uses for clock adjustments.
+
+ ==== =================================================
+ NONE no adjustments
+ PPS adjustments come from the PPS1 selector (default)
+ TOD adjustments from the GNSS/TOD module
+ IRIG adjustments from external IRIG-B signal
+ DCF adjustments from external DCF signal
+ ==== =================================================
+
+What: /sys/class/timecard/ocpN/available_sma_inputs
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Set of available destinations (sinks) for a SMA
+ input signal.
+
+ ===== ================================================
+ 10Mhz signal is used as the 10Mhz reference clock
+ PPS1 signal is sent to the PPS1 selector
+ PPS2 signal is sent to the PPS2 selector
+ TS1 signal is sent to timestamper 1
+ TS2 signal is sent to timestamper 2
+ TS3 signal is sent to timestamper 3
+ TS4 signal is sent to timestamper 4
+ IRIG signal is sent to the IRIG-B module
+ DCF signal is sent to the DCF module
+ FREQ1 signal is sent to frequency counter 1
+ FREQ2 signal is sent to frequency counter 2
+ FREQ3 signal is sent to frequency counter 3
+ FREQ4 signal is sent to frequency counter 4
+ None signal input is disabled
+ ===== ================================================
+
+What: /sys/class/timecard/ocpN/available_sma_outputs
+Date: May 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Set of available sources for a SMA output signal.
+
+ ===== ================================================
+ 10Mhz output is from the 10Mhz reference clock
+ PHC output PPS is from the PHC clock
+ MAC output PPS is from the Miniature Atomic Clock
+ GNSS1 output PPS is from the first GNSS module
+ GNSS2 output PPS is from the second GNSS module
+ IRIG output is from the PHC, in IRIG-B format
+ DCF output is from the PHC, in DCF format
+ GEN1 output is from frequency generator 1
+ GEN2 output is from frequency generator 2
+ GEN3 output is from frequency generator 3
+ GEN4 output is from frequency generator 4
+ GND output is GND
+ VCC output is VCC
+ ===== ================================================
+
+What: /sys/class/timecard/ocpN/clock_source
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) Contains the current synchronization source used by
+ the PHC. May be changed by writing one of the listed
+ values from the available_clock_sources attribute set.
+
+What: /sys/class/timecard/ocpN/clock_status_drift
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Contains the current drift value used by the firmware
+ for internal disciplining of the atomic clock.
+
+What: /sys/class/timecard/ocpN/clock_status_offset
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Contains the current offset value used by the firmware
+ for internal disciplining of the atomic clock.
+
+What: /sys/class/timecard/ocpN/freqX
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Optional directory containing the sysfs nodes for
+ frequency counter <X>.
+
+What: /sys/class/timecard/ocpN/freqX/frequency
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Contains the measured frequency over the specified
+ measurement period.
+
+What: /sys/class/timecard/ocpN/freqX/seconds
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) Specifies the number of seconds from 0-255 that the
+ frequency should be measured over. Write 0 to disable.
+
+What: /sys/class/timecard/ocpN/genX
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Optional directory containing the sysfs nodes for
+ frequency generator <X>.
+
+What: /sys/class/timecard/ocpN/genX/duty
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Specifies the signal duty cycle as a percentage from 1-99.
+
+What: /sys/class/timecard/ocpN/genX/period
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Specifies the signal period in nanoseconds.
+
+What: /sys/class/timecard/ocpN/genX/phase
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Specifies the signal phase offset in nanoseconds.
+
+What: /sys/class/timecard/ocpN/genX/polarity
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Specifies the signal polarity, either 1 or 0.
+
+What: /sys/class/timecard/ocpN/genX/running
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Either 0 or 1, showing if the signal generator is running.
+
+What: /sys/class/timecard/ocpN/genX/start
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Shows the time in <sec>.<nsec> that the signal generator
+ started running.
+
+What: /sys/class/timecard/ocpN/genX/signal
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) Used to start the signal generator, and summarize
+ the current status.
+
+ The signal generator may be started by writing the signal
+ period, followed by the optional signal values. If the
+ optional values are not provided, they default to the current
+ settings, which may be obtained from the other sysfs nodes.
+
+ period [duty [phase [polarity]]]
+
+ echo 500000000 > signal # 1/2 second period
+ echo 1000000 40 100 > signal
+ echo 0 > signal # turn off generator
+
+ Period and phase are specified in nanoseconds. Duty cycle is
+ a percentage from 1-99. Polarity is 1 or 0.
+
+ Reading this node will return:
+
+ period duty phase polarity start_time
+
+What: /sys/class/timecard/ocpN/gnss_sync
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Indicates whether a valid GNSS signal is received,
+ or when the signal was lost.
+
+What: /sys/class/timecard/ocpN/i2c
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This optional attribute links to the associated i2c device.
+
+What: /sys/class/timecard/ocpN/irig_b_mode
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) An integer from 0-7 indicating the timecode format
+ of the IRIG-B output signal: B00<n>
+
+What: /sys/class/timecard/ocpN/pps
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This optional attribute links to the associated PPS device.
+
+What: /sys/class/timecard/ocpN/ptp
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This attribute links to the associated PTP device.
+
+What: /sys/class/timecard/ocpN/serialnum
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RO) Provides the serial number of the timecard.
+
+What: /sys/class/timecard/ocpN/sma1
+What: /sys/class/timecard/ocpN/sma2
+What: /sys/class/timecard/ocpN/sma3
+What: /sys/class/timecard/ocpN/sma4
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) These attributes specify the direction of the signal
+ on the associated SMA connectors, and also the signal sink
+ or source.
+
+ The display format of the attribute is a space separated
+ list of signals, prefixed by the input/output direction.
+
+ The signal direction may be changed (if supported) by
+ prefixing the signal list with either "in:" or "out:".
+ If neither prefix is present, then the direction is unchanged.
+
+ The output signal may be changed by writing one of the listed
+ values from the available_sma_outputs attribute set.
+
+ The input destinations may be changed by writing multiple
+ values from the available_sma_inputs attribute set,
+ separated by spaces. If there are duplicated input
+ destinations between connectors, the lowest numbered SMA
+ connector is given priority.
+
+ Note that not all input combinations may make sense.
+
+ The 10Mhz reference clock input is currently only valid
+ on SMA1 and may not be combined with other destination sinks.
+
+What: /sys/class/timecard/ocpN/tod_correction
+Date: March 2022
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) The incoming GNSS signal is in UTC time, and the NMEA
+ format messages do not provide a TAI offset. This sets the
+ correction value for the incoming time.
+
+ If UBX_LS is enabled, this should be 0, and the offset is
+ taken from the UBX-NAV-TIMELS message.
+
+What: /sys/class/timecard/ocpN/ts_window_adjust
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) When retrieving the PHC with the PTP SYS_OFFSET_EXTENDED
+ ioctl, a system timestamp is made before and after the PHC
+ time is retrieved. The midpoint between the two system
+ timestamps is usually taken to be the SYS time associated
+ with the PHC time. This estimate may be wrong, as it depends
+ on PCI latencies, and when the PHC time was latched
+
+ The attribute value reduces the end timestamp by the given
+ number of nanoseconds, so the computed midpoint matches the
+ retrieved PHC time.
+
+ The initial value is set based on measured PCI latency and
+ the estimated point where the FPGA latches the PHC time. This
+ value may be changed by writing an unsigned integer.
+
+What: /sys/class/timecard/ocpN/ttyGNSS
+What: /sys/class/timecard/ocpN/ttyGNSS2
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: These optional attributes link to the TTY serial ports
+ associated with the GNSS devices.
+
+What: /sys/class/timecard/ocpN/ttyMAC
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This optional attribute links to the TTY serial port
+ associated with the Miniature Atomic Clock.
+
+What: /sys/class/timecard/ocpN/ttyNMEA
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: This optional attribute links to the TTY serial port
+ which outputs the PHC time in NMEA ZDA format.
+
+What: /sys/class/timecard/ocpN/utc_tai_offset
+Date: September 2021
+Contact: Jonathan Lemon <jonathan.lemon@gmail.com>
+Description: (RW) The DCF and IRIG output signals are in UTC, while the
+ TimeCard operates on TAI. This attribute allows setting the
+ offset in seconds, which is added to the TAI timebase for
+ these formats.
+
+ The offset may be changed by writing an unsigned integer.