Fast NCO switching with AD6688 / AD6902

Question

Hi, 
 I'm trying to operate the Fast NCO switching feature of AD6688 / AD6902 on 5 different phases with no success. 
 
 I'll explain what I did so far : 
 
 Step 1 : 
 Calculate 5 FTWs and set each in the appropriate NCO channel : 
 1. Chose NCO channel to update by setting Bits[3:0] in the NCO control registers 0x0374. 
 2. Calculate the FTW using this formula : (page 49 in the Data Sheet) and setting the 48bits word into registers : 0x0376 to 0x037B. 
 3. Repeat steps 1 through 2 for all other 4 channels. 
 
 Step 2 : 
 Chose the GPIO option for transferring between channels - GPIO_B1 in Edge Control Mode : 
 1. Wrote 0x0 to Bits[7:4] in Register 0x0042 . 
 2. Configured the NCO channel selector in GPIO edge control mode by setting Bits[7:4] in the NCO control registers 0x0374 to 0xB . 
 3. Configured the wrap point for the NCO channel selection by setting Bits[3:0] in the NCO control registers 0x0374 to 4. 
 
 Step 3 : 
 Toggling GPIO_B1 0>1 
 
 Questions : 
 1. What am I doing wrong ? 
 2. Does setting the selector - red circled FTW/POW write index (in the below Figure 75.) is by writing to register 0x0374 bits [3:0] ?

Thank you

ADIApproved · Accepted Answer

Hello KobiP 
 I did not see you mentioned steps "a" and "b" on the list below. 
 1. Configure one or more GPIO pins as NCO channel selection inputs. GPIO pins not configured as NCO channel selection are internally tied low. 
 a. To use GPIO_A0, write Bits[2:0] in Register 0x0040 to 0x6 and Bits[3:0] in Register 0x0041 to 0x0. 
 b. To use GPIO_B0, write Bits[5:3] in Register 0x0040 to 0x6 and Bits[7:4] in Register 0x0041 to 0x0. 
 c. To use GPIO_A1, write Bits[3:0] in Register 0x0042 to 0x0. 
 d. To use GPIO_B1, write Bits[7:4] in Register 0x0042 to 0x0 
 
 Please let me know if you follow the steps on AD6688 spec sheet page 50. 
 Thanks 
 Sergio Valenzuela)

Electronic Design Services - PartnerZone

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Fast NCO switching with AD6688 / AD6902