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LMX2491: RAMP_CMP0 not working

Brian Smith
Prodigy 90 points
Part Number: LMX2491

LMX2491 ramping 1500 to 1515.625MHz. Operation is zero-increment RAMP0 followed by RAMP1 as in datasheet page 32 example. Using 40MHz PLL reference, Multiplier x1, R Divider x1, We initially load all the registers and then use writes to R2 power-up/power-down to minimise power. Interestingly we do not seem to need any signal to kick off the ramp; it starts itself on power-up. On R2 = 0x01 (POWERUP), locks fine after about 70us. RAMP_CMP0 has been preset to 1505MHz (10485760 0xA00000, R60 = 0x00, R61 = 0x00, R62 = 0xA0 as per TICS-Pro) with CMP0RAMP brought out through TRIG1 (R36 = 0xB2, R64 = 0x02). TRIG1 goes high just 2us after lock initially goes low at the beginning of the ramp and stays high throughout ramp. Any ideas - cannot get ramp comparison to work, it just goes high at the very beginning of the ramp and stays high until the end and power-down?

  • 0
    TI__Guru** 108480 points

    Hi Brian,

    If you make RAMP_EN = 0 before power it down, will it start ramping automatically after it is power up?

    Regarding RAMP_CMPx, the flag signal will turn HIGH once the threshold is hit. The flag signal will stay HIGH until the next ramp start. The example in datasheet section 8.2.8 has the details.

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Making RAMP_EN = 0 prior to power-down stops the ramp starting automatically on power up - Thank You. It will now start in the normal way with R58 set to 1.

    However, CMP0RAMP also goes high at the the beginning of the ramp, not when the threshold is hit. Being pedantic, the example in 8.2.8 does not use the 'FLAGS' as in Flag0 and Flag1. I had expected the comparator to set 'Output CMP0RAMP' via R36[7:3] = 22 and this is what I am monitoring via the TRIG1 output. Does the comparator need any sort of reset or to be set up after powerup?

  • 0 in reply to Brian Smith
    TI__Guru** 108480 points

    Hi Brian,

    Could you provide your configuration in TICS Pro format?

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Noel,

    Attached below please find TICS Pro file, plus a screen shot. Reading top to bottom - Power, CPout, Lock, CMP0RAMP.

    When we power up we set R58 to 0x01. Thank you for your help.

    AI.tcs

  • 0 in reply to Brian Smith
    Prodigy 90 points

    Apologies, Noel - file in previous submission was not .txt and had a spurious character in it.  Here is the file again as an IMPORT file for TICS Pro:

    Fullscreen AI.txt Download 
    R141	0x008D00
R140	0x008C07
R139	0x008BD0
R138	0x008A00
R137	0x008900
R136	0x008800
R135	0x008700
R134	0x008600
R133	0x008507
R132	0x0084D0
R131	0x008300
R130	0x008200
R129	0x008100
R128	0x008000
R127	0x007F00
R126	0x007E07
R125	0x007DD0
R124	0x007C00
R123	0x007B00
R122	0x007A00
R121	0x007900
R120	0x007800
R119	0x007707
R118	0x0076D0
R117	0x007500
R116	0x007400
R115	0x007300
R114	0x007200
R113	0x007100
R112	0x007007
R111	0x006FD0
R110	0x006E00
R109	0x006D00
R108	0x006C00
R107	0x006B00
R106	0x006A00
R105	0x006907
R104	0x0068D0
R103	0x006700
R102	0x006600
R101	0x006500
R100	0x006400
R99	0x006300
R98	0x006279
R97	0x0061E0
R96	0x006000
R95	0x005F00
R94	0x005E02
R93	0x005DEE
R92	0x005C34
R91	0x005B79
R90	0x005AE0
R89	0x005900
R88	0x005800
R87	0x005700
R86	0x005600
R85	0x005500
R84	0x005400
R83	0x005300
R82	0x005203
R81	0x005100
R80	0x005000
R79	0x004F00
R78	0x004E00
R77	0x004DA0
R76	0x004C00
R75	0x004B00
R74	0x004A00
R73	0x004900
R72	0x004800
R71	0x004700
R70	0x004608
R69	0x004500
R68	0x004400
R67	0x004300
R66	0x004200
R65	0x004100
R64	0x004002
R63	0x003F00
R62	0x003EA0
R61	0x003D00
R60	0x003C00
R59	0x003B01
R58	0x003A00
R57	0x003900
R45	0x002D00
R44	0x002C00
R43	0x002B00
R42	0x002A00
R41	0x002900
R40	0x002800
R39	0x00270F
R38	0x00265A
R37	0x002517
R36	0x0024B2
R35	0x002341
R34	0x002204
R33	0x002120
R32	0x002000
R31	0x001F32
R30	0x001E0A
R29	0x001D00
R28	0x001C18
R27	0x001B08
R26	0x001A00
R25	0x001901
R24	0x001800
R23	0x001703
R22	0x0016E8
R21	0x001580
R20	0x001400
R19	0x001300
R18	0x00122C
R17	0x001100
R16	0x001025
R15	0x000F00
R14	0x000E00
R13	0x000D00
R12	0x000C00
R11	0x000B00
R10	0x000A00
R9	0x000900
R8	0x000800
R7	0x000700
R6	0x000600
R5	0x000500
R4	0x000400
R3	0x000300
R2	0x000201
R1	0x000100
R0	0x000018

    File loads ok with Import Register Map.

    Brian

  • 0 in reply to Brian Smith
    TI__Guru** 108480 points

    Hi Brian,

    Regrading the yellow trace, is it Vcc power supply or CE pin signal?

    The purple trace is toggling for about 50µs, I assume the PLL is acquiring lock during this period of time. If this is true, then you should enable ramp only after the PLL is locked. 

    Your .txt file does not have correct RAMP information. 

  • 0 in reply to Noel Fung
    Prodigy 90 points

    Hi Noel,

    The yellow trace is actually the power rail to the VCO, and I'm just using it as a 'scope trigger here; apologies for any mislead.  I keep chipEnable low and just enter POWERDOWN/POWERUP via R2[0] with physical power always applied to the PLL.

    TICS Pro does not seem to import the reference (40MHz) and it needs to be set manually  after an IMPORT of register values. I think this may be why the ramp values appeared incorrect. Also, using an external trigger signal for the ramp initiation, one can leave the ramp enabled between power-ups and the ramp only starts on the trigger.

    However, I have eventually got to the bottom of the issue. I wanted to output the CMP0 signal at a particular ramp count as a reference for data acquisition after lock.  Taking guidance from Figure 27 (datasheet) where they refer to RAMP_CMP0, I selected CMP0RAMP as output (22). Actually I needed  to select 'Output CMP0 (8)'. Lack of description of the mnemonics in Table 15 and what was (at least to me) a misleading name in Figure 27!