LMK04821
,
LMK04826
,
LMK04828
SNAS605AR –MARCH 2013–REVISED DECEMBER 2015
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3. Now that dividers are synchronized, disable SYNC from resetting these dividers. It is not desired for
SYSREF to reset it's own divider or the dividers of the output clocks.
(a) Prevent SYNC (SYSREF) from affecting dividers: SYNC_DIS0 = 1, SYNC_DIS2 = 1, SYNC_DIS4 = 1,
SYNC_DISSYSREF = 1.
4. Release reset of local SYSREF digital delay.
(a) SYSREF_CLR = 0. Note this bit needs to be set for only 15 VCO clocks after SYSREF_PD = 0.
5. Set SYSREF operation.
(a) Allow pin SYNC event to start pulser: SYNC_MODE = 2.
(b) Select pulser as SYSREF signal: SYSREF_MUX = 2.
6. Complete! Now asserting the SYNC pin, or toggling SYNC_POL will result in a series of 2 SYSREF pulses.
9.3.2.1.2 SYSREF_CLR
The local digital delay of the SDCLKout is implemented as a shift buffer. To ensure no un-wanted pulses occur at
this SYSREF output at startup, when using SYSREF, requires clearing the buffers by setting SYSREF_CLR = 1
for 15 VCO clock cycles. After a reset, this bit is set, so it must be cleared before SYSREF output is used.
9.3.2.2 SYSREF Modes
9.3.2.2.1 SYSREF Pulser
This mode allows for the output of 1, 2, 4, or 8 SYSREF pulses for every SYNC pin event or SPI programming.
This implements the gapped periodic functionality of the JEDEC JESD204B specification.
When in SYSREF Pulser mode, programming the field SYSREF_PULSE_CNT in register 0x13E will result in the
pulser sending the programmed number of pulses.
9.3.2.2.2 Continuous SYSREF
This mode allows for continuous output of the SYSREF clock.
Continuous operation of SYSREF is not recommended due to crosstalk from the SYSREF clock to device clock.
JESD204B is designed to operate with a single burst of pulses to initialize the system at startup, after which it is
theoretically not required to send another SYSREF since the system will continue to operate with deterministic
phases.
If continuous operation of SYSREF is required, consider using a SYSREF output from a non-adjacent output or
SYSREF from the OSCout pin to minimize crosstalk.
9.3.2.2.3 SYSREF Request
This mode allows an external source to synchronously turn on or off a continuous stream of SYSREF pulses
using the SYNC/SYSREF_REQ pin.
Setup the mode by programming SYSREF_REQ_EN = 1 and SYSREF_MUX = 2 (Pulser). The pulser does not
need to be powered for this mode of operation.
When the SYSREF_REQ pin is asserted, the SYSREF_MUX will synchronously be set to continuous mode
providing continuous pulses at the SYSREF frequency until the SYSREF_REQ pin is un-asserted and the final
SYSREF pulse will complete sending synchronously.
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