IMX6ULL GPT input capture mode

[sergei_p]

Hello,

We need to measure input frequency of about 30 MHz. The idea is to use IMX6ULL GPT2, clocked internally with the 24MHz IPG clock, and configure the input frequency pin JTAG_MOD (ie to be measured) as External Clock in GPT2_CR, and JTAG_TDO as GPT2_CAPTURE2.

The Reference Manual has a statement:

To ensure proper operations of GPT, the external clock input frequency should be less than 1/4 of frequency of the peripheral clock (ipg_clk).

Now the question: if we use the divider "4" set in GPT2_PR, to divide the input 30MHz to the safe value of (30/(4+1)) 6MHz, will that satisfy the "1/4" restriction above?

Thanks!

Regards,

Sergei

[igorpadykov]

Hi Sergei

yes that condition will satisfy the "1/4" restriction.

Best regards
igor
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[sergei_p]

Thanks, Igor!

[sergei_p]

Igor,

I'm sorry but my tests don't confirm your statement that the EXT_CLK input frequency could be divided by prescaler in GPT_PR for proper measurements. What we see, is that irrelevant to the prescaler,  measurements of the high frequencies  are not correct . Here is my test, with 39.6MHz supplied to JTAG_MOD, with IPG_CLK=66 MHz:

# Configure JTAG_MOD as GPT2_CLK

U-Boot > mw.l 0x20E0044 1

# Reset GPT2
U-Boot > mw 0x20e8000 0x8000

# Configure and start GPT2: EXT_CLK, WAITEN, ENMOD, EN

U-Boot > mw 0x20e8000 0x000cb

# Set prescaler to (19 + 1)
U-Boot > mw 0x20e8004 0x13

# MEasure the counter
U-Boot > md 0x20e8024 1;sleep 10;md 0x20e8024 1
020e8024: 123b89c3 ..;.
020e8024: 13050c6b k...

U-Boot >

Calculations shows that calculated value doesn't match 39.6MHz:

(gdb) p (0x13050c6b-0x123b89c3)*20/10 $1 = 26412368 (gdb) 

If I set JTAG_MOD (GPT2_CLK) to a lower frequency (eg 16 MHz), measurements are correct.

Could you please comment on the above data?

[igorpadykov]

Hi Sergei

seems you are right, from sect.30.2.1 External Clock Input i.MX6ULL Reference Manual :

"The external clock input (GPT_CLK) is treated
as asynchronous to the peripheral clock (ipg_clk). To ensure proper operations of GPT,
the external clock input frequency should be less than 1/4 of frequency of the peripheral
clock (ipg_clk)."

http://www.nxp.com/docs/en/reference-manual/IMX6ULLRM.pdf 

Best regards
igor