SPI INTERFACE IN LM98722

Hello,

If you are enabling the external crystal oscillator mode, you should wait for some time for the oscillator to start up, and the internal PLLs to lock to that clock source before resuming SPI communications.

Please try this and let me know.

Regards,

Hooman

Dear Mr Hooman Hashemi

After Power on we are using this code for enabling the external crystal we are using 5 Mhz crystal as the input clock 

P4OUT&=~SEN;  ////Chip Select
__delay_cycles(100);/// 6 us delay
SPI_WRITE(0x1F);  ////// Addressing the first page 
SPI_WRITE(0x00);
__delay_cycles(100); /////6 us delay
P4OUT|=SEN;     //////Chip deselect
__delay_cycles(100);/// 6 us delay
P4OUT&=~SEN;   ////Chip Select
__delay_cycles(100);/// 6 us delay
SPI_WRITE(0x00); /// writing the first register to enable the external crystal
SPI_WRITE(0x28);
__delay_cycles(100); /////6 us delay
P4OUT|=SEN;   //////Chip deselect
__delay_cycles(1000000); 6.2 Milliseconds delay 

After this code there is no spi communication between the AFE and the Micro controller when we probe on the crystal there is no oscillation 

pls let us know that we are sending the correct sequence , is there any power up sequence that we need to go through before issuing the commands

with regards

Gokul

Hi Gokul,

Please make sure you are waiting at least 50ms or longer, if not already?

From page 86:

"After either starting or stopping INCLK, or after Software Reset (Register Page 0, Address 1, [4:3]), users should wait for 50 ms to allow the internal PLL and logic to stabilize before resuming Serial Interface communications."

If it never works after a long delay (1s or more) then it may be an issue with the oscillator circuit you may have.

Please post your schematics and layout information showing the crystal oscillator circuitry and LM98722. These circuits are very sensitive to loading on the two connections, especially the XTAL in (INCLK+ pin).

Regards,

Hooman

Dear Hooman

we have waited for more than 1 sec but the crystal is not loading here with i am attaching the circuit pls let us know if there is any issues in the circuit

The inclk from uC line has been isolated there is no connection between the LM98722 and the uC

Hi Gokul,
Can you please also supply your board layout image around the area of the XTAL?

Regards,
Hooman

Hi Hooman

C17 and C18 are Loading Capacitors , 18pF , and Y1 is 5 Mhz

Hi Gokul,
The layout / schematic look fine.
Any chance you can try a higher frequency XTAL or some sort of external differential clock source to debug the board otherwise?

How are you looking for the XTAL oscillation - Are you putting a scope probe on it? Could the scope probe capacitance be responsible for attenuating or stopping the XTAL from oscillation? Can the XTAL oscillation be "sniffed" with a high frequency probe close by without touching the leads?

Regards,
Hooman

Dear Hooman 

We have not tried the higher frequency XTAL oscillation we will try it and let you know in a short while

Gokul