DAC39RFS10 datasheet(49/175 Pages) TI2

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Table 7-3. Requirements for SYSREF period

Requirements on SYSREF Period

Reason

SYSREF period must be a multiple of 16 CLK cycles.

DAC Encoder/DEM always operates with a FDAC/16 clock that is

aligned to SYSREF.

SYSREF period must be a multiple of LT CLK cycles This constraint

does not apply to DDS mode (DDS_EN=1).

Makes sure the SYSREF period is a multiple of the input sample

period.

SYSREF period must be a multiple of 4*LT*S/F CLK cycles. This

constraint does not apply to DDS mode (DDS_EN=1).

Makes sure the SYSREF period is a multiple of the effective link

layer clock period.

SYSREF period must be a multiple of LT*S*K CLK cycles. This

constraint does not apply to Subclass 0 mode (SUBCLASS=0) or

DDS mode (DDS_EN=1)

Makes sure that SYSREF period is a multiple of the LMFC/LEMC

period. Note that K=256*E/F in 64b/66b mode.

7.3.5.2 SYSREF Position Detector and Sampling Position Selection (SYSREF Windowing)

The SYSREF Windowing block is used to first detect the position of SYSREF relative to the input clock

CLK± rising edge. Based on the window information, an optimum SYSREF sampling time is selected to

maximize setup and hold timing margins relative to the input clock. In many cases, a single SYSREF sampling

position SYSREF_SEL is sufficient to meet timing for all systems (device-to-device variation) and conditions

(temperature and voltage variations). However, this feature can also be used by the system to expand the timing

window by tracking the movement of SYSREF as operating conditions change or to remove system-to-system

variation at production test by finding a unique optimal value at nominal conditions for each system.

This section describes proper usage of the SYSREF Windowing block (SYSREF_RECV_SLEEP must be

programmed to 0). First, apply the device clock and SYSREF to the device. The location of SYSREF relative

to the device clock cycle is determined and stored in the SYSREF_POS field. Each bit of SYSREF_POS

represents a potential SYSREF sampling position. If a bit in SYSREF_POS is set to 1, then the corresponding

SYSREF sampling position has a potential setup or hold violation. Upon determining the valid SYSREF sampling

positions (the positions of SYSREF_POS that are set to 0) the desired sampling position can be chosen

by setting SYSREF_SEL to the value corresponding to that SYSREF_POS position. In general, the middle

sampling position between two setup and hold instances is chosen. The determination of SYSREF_SEL is

performed at the nominal operating conditions of the system (temperature and supply voltage) to provide

maximum margin for operating condition variations. This process can be performed at final test and the optimal

SYSREF_SEL setting can be stored for use at every system power up. Further, SYSREF_POS can be used to

characterize the skew between CLK± and SYSREF± over operating conditions for a system by sweeping the

system temperature and supply voltages. For systems that have large variations in CLK± to SYSREF± skew,

this characterization can be used to track the optimal SYSREF sampling position as system operating conditions

change. In general, a single value can be found that meets timing over all conditions for well-matched systems,

such as those where CLK± and SYSREF± come from a single clocking device.

The step size between each SYSREF_POS sampling position can be adjusted using SYSREF_ZOOM. When

SYSREF_ZOOM is set to 0, the delay steps are coarser. When SYSREF_ZOOM is set to 1, the delay steps

are finer. See the electrical specifications table for delay step sizes when SYSREF_ZOOM is enabled and

disabled. In general, SYSREF_ZOOM is recommended to always be used (SYSREF_ZOOM = 1) unless a

transition region (defined by 1's in SYSREF_POS) is not observed, which can be the case for low clock rates.

Bits 0 and 19 of SYSREF_POS are always 1 because there is insufficient information to determine if these

settings are close to a timing violation, although the actual valid window can extend beyond these sampling

positions. The value programmed into SYSREF_SEL is the decimal number representing the desired bit

location in SYSREF_POS. Table 7-4 lists some example SYSREF_POS readings and the optimal SYSREF_SEL

settings. Although 20 sampling positions are provided by the SYSREF_POS status register, SYSREF_SEL only

allows selection of the first 16 sampling positions, corresponding to SYSREF_POSbits 0 to 15. The additional

SYSREF_POS status bits are intended only to provide additional knowledge of the SYSREF valid window. In

general, lower values of SYSREF_SEL are selected because of delay variation over supply voltage, however in

the fourth example a value of 14 provides additional margin and can be selected instead.

If SYSREF_PS_EN is set to 0, only the last SYSREF edge is used for the SYSREF_POS values.

Setting SYSREF_PS_EN to 1 enables an "infinite persistence" mode, where if any SYSREF edge since

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