IMX 8 clock/xtal inputs

[enoch_arthur]

Hi, I am designing the IMX 8 onto a very limited board space so I am trying to restrict any unnecessary components I don't need. we believe the majority of our clocks can be run from the 25Mhz input and the 27Mhz input is just a back up. Is that true? Apart from it being used as the reference clock for HDMI PHY?  We are not using HDMI so does it mean we can get away with not putting this crystal on the board? do we require any termination or the inputs can be left floating?

What about the clk1 and clk2 differential inputs, what are they really for? If not used, can they be left floating?

Any help will be very useful. Many thanks,

Enoch

[mrudangshelat1]

Hi Enoch,

I believe you are referring the i.MX8M D/Q. You have multiple queries, please see my comments as below.

• we believe the majority of our clocks can be run from the 25Mhz input and the 27Mhz input is just a backup. Is that true?

Ans: Which reference design you are referring to? Most of the design has 24Mhz CLKin for CPU's XTALOSC.

• Apart from it being used as the reference clock for HDMI PHY?  We are not using HDMI so does it mean we can get away with not putting this crystal on the board?

Ans: It depends on the HDMI Phy chip. Most of the Phy IC require external crystal/oscillator.

• Do we require any termination or the inputs can be left floating?

Ans: Which CLK you are asking for? 24Mhz CLKin for CPU's XTALOSC or HDMI CLK between CPU and HDMI through MIPI?

• What about the clk1 and clk2 differential inputs, what are they really for? If not used, can they be left floating?

Ans: We can terminate on TP. Please see below table.

Regards,

Mrudang

[enoch_arthur]

Hi Shelat,

Yes, I am  referring the i.MX8M D/Q. Please see added comments:

• we believe the majority of our clocks can be run from the 25Mhz input and the 27Mhz input is just a backup. Is that true?

Ans: Which reference design you are referring to? Most of the design has 24Mhz CLKin for CPU's XTALOSC.

comments: no, the datasheet, reference manual and reference design(MCIMX8M-EVKB) all use 25Mhz for xtal1 and 27mhz for xtal2. My question is can I just use xtal1 (25mhz crystal) and leave crystal 2 unconnected as I don't need HDMI?

• Apart from it being used as the reference clock for HDMI PHY?  We are not using HDMI so does it mean we can get away with not putting this crystal on the board?

Ans: It depends on the HDMI Phy chip. Most of the Phy IC require external crystal/oscillator.

comment: I don't need HDMI, can I  leave crystal 2 unconnected? or xtal 2 is required for something else I am not aware of?

• Do we require any termination or the inputs can be left floating?

Ans: Which CLK you are asking for? 24Mhz CLKin for CPU's XTALOSC or HDMI CLK between CPU and HDMI through MIPI?

Comment: I mean if I don't put a crystal at xtal 2, can I leave the pins uncconnected or I have to terminate it somehow?

• What about the clk1 and clk2 differential inputs, what are they really for? If not used, can they be left floating?

Ans: We can terminate on TP

comments: thanks :smileyhappy:

[mrudangshelat1]

HI Enoch,

As per datasheet, "The 27 MHz oscillator is used as the reference clock for HDMI PHY. And It can also be used as the alternative source for the fractional PLLs." I request you go through section 5.1-Clock Control Module (CCM) of the Reference Manual for 27MHz CLK requirement. I suggest you keep the provisional for 27Mhz (Do Not Populate on board) for the safe side to avoid the redesign cost and do testing based on your application requirements. The later stage you can remove if your system is running without Xtal2.

Mrudang