SNLS643C March 2019 – April 2024 DS90UB953A-Q1
PRODUCTION DATA
When using the DS90UB953A-Q1 in either synchronous or non-synchronous external clock modes, CLK_OUT is intended as a reference clock for the image sensor. CLK_OUT functionality is disabled when operating in non-synchronous internal clocking mode. The frequency of the external CLK_OUT is set by (see Equation 1 and Equation 2).
where
The PLL that generates CLK_OUT is a digital PLL, and as such, has very low jitter if the ratio N/M is an integer. If N/M is not an integer, then the jitter on the signal is approximately equal to HS_CLK_DIV/FC—so if not possible to have an integer ratio of N/M, then select a smaller value for HS_CLK_DIV.
If a particular CLK_OUT frequency, such as 37.125MHz, is required for a system, the designer can select the values M=9, N=0xF2, and HS_CLK_DIV=4 to achieve an output frequency of 37.190MHz and a frequency error of 0.175% with an associate jitter of approximately 1ns. Alternately, the designer can use M=1, N=0x1B, HS_CLK_DIV=4 for CLK_OUT = 37.037MHz, and a frequency error of 0.24% for less jitter. A third alternative is to use M=1, N=0x1B, and HS_CLK_DIV=4, but rather than using a 25.000MHz reference clock frequency (REFCLK) for the deserializer in synchronous mode, use a frequency of 25.059MHz. The 2x reference then fed to the DS90UB953A-Q1 from the deserializer back channel allows generating CLK_OUT = 37.124MHz with both low jitter and a low frequency error.