LMX2531: LQ1778E

Madhusri Ghosh

Part Number: LMX2531

To program the LMX2531LQ1778E in an Evaluation Board (ADC12D1x00RFRB), I used the following values in corresponding 24 bit registers as per the specifications of

foscin=60 MHz

fFout= 1800 MHz

R=8

N=240

(vco divider is disabled,delta-sigma modulator order=3,loop filter=2nd order)

But still I am not getting the output frequency at the output pin of LMX2531(pin no:21)

REG0 = 24'hF000_00
REG1 = 24'h2000_01
REG2 = 24'h4004_82
REG3 = 24'h7C80_03
REG4 = 24'h0000_04,
REG5 = 24'h8400_05, (INIT 1)
REG5 = 24'h8007_F5, (INIT 2)
REG6 = 24'h4866_56,
REG7 = 24'h05CB_07,
REG8 = 24'h0280_18,
REG9 = 24'h000B_A9,
REG12 = 24'h0104_8C

What is possible solutions?? please help..

over 9 years ago

0 Julian Di Matteo over 9 years ago

TI__Expert 4085 points

Hi Madhusri,

Please check this blog that my colleague wrote:

https://e2e.ti.com/blogs_/b/analogwire/archive/2016/07/12/what-to-do-when-your-pll-does-not-lock

It will help you debug.

How are you programming LMX2531? did you use TICS Pro or Code Loader GUI to extract register settings?

Thanks,

Julian

0 Madhusri Ghosh over 9 years ago in reply to Julian Di Matteo

Prodigy 50 points

Sir,

Thank you so much ..

I am using FPGA to program the LMX2531..Actually this pll is in the ADC12D1x00RFRB evaluation board..

From the Code Loader I am getting the registers value as per my specifications then I am programming the FPGA

to transfer the registers value into LMX2531.

0 Noel Fung over 9 years ago in reply to Madhusri Ghosh

TI__Guru** 122030 points

Hi Madhusri,

did you follow the programming sequence as described in the datasheet? That is, R5[INIT1] ->R5[INIT2]->R5->R12->........->R0.
Furthermore, can you also try put register XTLSEL to 2?

0 Madhusri Ghosh over 9 years ago in reply to Noel Fung

Prodigy 50 points

Hello Sir,

Yes I followed the exact sequence as described in datasheet and also made reg XTLSEL = 2 as my OSCin is 60 MHz.

0 Noel Fung over 9 years ago in reply to Madhusri Ghosh

TI__Guru** 122030 points

Hi Madhusri,

I think the problem is you did not release the RESET bit in your programming sequence.
R5[INIT1] = 0x840005 (correct)
R5[INIT2] = 0x800005 (your setting is not right, your setting is actually R5 not R5[INIT2])

You need three R5 in the programming sequence.
R5[INIT1] = 0x840005
R5[INIT2] = 0x800005
R5 = 0x8007F5

0 Madhusri Ghosh over 9 years ago in reply to Noel Fung

Prodigy 50 points

Hello Sir,

The problem is solved...Actually the problem was in the R6 register values regarding the internal loop filter and corresponding value of R3 and R4 resistors.

Now I have disabled the Internal loop filter and there is a external loop filter on board..

So now the PLL is locked and I am getting my desired frequency at output..Now It's working fine.

Thank you sir for your concern...

Madhusri

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LMX2531: LQ1778E