[FAQ] AM6422: SK and EVM EXT_REFCLK1 CLKOUT0 use case and configuration

Hello Shu  

Thank you for the query.

Please refer below.

wang shu said:

1. EXT_REFCLK1 is 3.3V. Why it connect to a 1.8V clock buffer. Will it damage the buffer?

Supply voltage: 3.3 V, 2.5 V, or 1.8 V – 3.3-V tolerant input at all supply voltages – Fail-safe inputs

wang shu said:

In AM6422 evaluation board SK-AM64B. It connect the EXT_REFCLK1 to CLKOUT0 signal. i have some question about it.

1. EXT_REFCLK1 is 3.3V. Why it connect to a 1.8V clock buffer. Will it damage the buffer?

2. If EXT_REFCLK1 output the signal to 1.8V buffer. How the clock generate from EXT_REFCLK1. looks like the buffer also support the main domain clock input. if the main domain doesn't has the input clock firstly, how the EXT_REFCLK1 generate clock signal. if I am wrong, how the circuit work?

This is used when the SOC had the crystal clock option configured.

I am not sure if this has been tested.

I need to check and comeback.

Regards,

Sreenivasa

Hello Shu  

Please refer below. The same pin can be configured as Clkout0. The EVM configuration is a valid configuration that can be used when the SOC is configured for Crystal.

6.3.22.3 System
6.3.22.3.1 MAIN Domain

CLKOUT0
O
RMII Clock Output (50 MHz). This pin is used for clock
source to the external PHY and must be routed back to
the RMII_REF_CLK pin for proper device operation. A19, U13

EXT_REFCLK1
I
External clock input to Main Domain, routed to Timer
clock muxes as one of the selectable input clock
sources for Timer/WDT modules, or as reference clock
to MAIN_PLL2 (PER1 PLL) A19

Regards,

Sreenivasa

Hi Sreenivasa,

That is my concern. looks like the EVM make the EXT_REFCLK1 to a clkout0 function, it is a output clock to a clock input buffer. the buffer is connected to SoC main clock input. So where is the first clock come from? if the SoC clock from a crystal, where the EXT_REFCLK1 clock go. In EVM, it go to same buffer input as the crystal. I don't think the buffer can use two inputs at same time.

Best Regards,

Shu

okay, that make sense. 

thank you very much