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ADS8558 busy stays high

Marco Schindler
Prodigy 50 points
Other Parts Discussed in Thread: ADS8558, SN74AHC138

I am trying to evaluate an ADS8558 on the ADS8558EVM eval board.

In short my problem is that when I start a sample cycle by asserting the "convst" pin,
the busy signal goes high, but never back to low.
In this case it does not seem to be an improper reset as I have read in similar post for a different chip.

I'm also not reading any data other than '0's when I ignore the busy signal and start an SPI access after the conversion time.

Does anyone have any idea what I might be operating wrong or have set up incorrectly?




In detail
=========

I have connected +/-12V, 5V and 3.3V and can measure them both at the test points, as well as directly on the ADS8558 pins.

(side note, the power LEDs D1 and D2 don't light up, because they are assembled with the wrong polarity,
they do light up when I test them with a multimeter)


DIP-switch settings:
--------------------

Byte - Word:     0  (don't care)
Range 2x - 4x:   0
SFT - HDW:       0
SER - PAR:       1
REFBUF ENA:      0
Daisy Ch ENA:    0
SERA enable:     1
SERB enable:     1
SERc enable:     1
REF en:          1



Jumpers:
--------

I tried out both settings for JP1, JP2, JP5, JP6, JP7 and JP8.

JP3  is set to 7-8
JP4  is set to 2-3  (I am connected to an FPGA with 3.3V I/O voltage)
JP9  is set to 1-2  for all 3 pins
JP10 is set to 2-3
JP11 is set to 2-3
JP12 is open




Analog Inputs:
--------------

For a first test I have connected 3.3 V to A0.

After reading the following thread I also connected the other channels to ground, so they are not floating.
e2e.ti.com/.../628420




Digital Interface:
------------------

I have connected the following pins to an FPGA

J4-19: busy
J4-17: convst
J1-13: SDO_A
J1-11: SDI     (driving to ground since I am in HW mode)
J1-7:  FS_n
J1-3:  SCLK
J1-1:  ADC_rst

TP13:  SDO_C
TP12:  SDO_B




Test Cases / Observations
-------------------------

1)
When I drive a '1' on convst the ADC's busy pin goes to '1' immediately and stays there.
Figure 2. of the ADC's manual implies that busy should return to '0' after the conversion time.

I also tried ignoring the busy signal and started an SPI access after the conversion time.
I'm not reading anything other than '0' though.


2)
When I drive a reset pulse on ADC_rst I expected a pulse on busy but I never saw that.
Figure 38. of the ADC's manual seems to imply that there should be one.

The reset itself does seem to work though, because it causes the busy signal, that is stuck at '1' from 1) (see above) to return to '0'.


  • 0
    TI__Guru**** 172646 points
    Hi Marco,

    Can you verify that the ADS8558 is not in standby mode? Check the logic level of U3 pin 7, in the default setup, that pin should be low since the A/B/C inputs are pulled high. Adding a shunt jumper between the two pins on J4 DC_A0, DC_A1 or DC_A2 (pins 7, 9, 11 and 8, 10, 12) should change the the address input to the SN74AHC138 and set pin 7 to logic high.
  • 0 in reply to Tom Hendrick
    Prodigy 50 points

    Tom,

    standby mode indeed was active. It is not really spelled out in the eval board documentation that this is the default state and I have to do anything about it.

    Anyway it works now.

    The other question was, if I should expect a pulse on busy after a reset, see Figure 38. of the ADC manual.  I still don't see this one.

    Should I see one or is that for SW mode only or am I misinterpreting that diagram entirely?

    Thanks so far.

    Marco

  • 0 in reply to Marco Schindler
    TI__Guru**** 172646 points

    Hi Marco,

    It's' an oversight on my part about the STBY mode - I designed the EVM to operate the way it does and by default it was set up for parallel mode operation, I did not completely think through the serial mode operation (obviously) and failed to highlight that little feature in the EVM Users Guide. 

    As for Figure 38 in the datasheet, what is not included there is the active CONVST input.  The assumption here is that the part receives a reset after power up (or just the normal POR) followed by a regular stream of CONVST inputs (not shown).  Issuing a RESET won't make BUSY pulse - it will bring it back low in the event where the ADC is in standby mode as you've already seen.  Please do let me know if you run into any additional trouble with the ADS8558, I'm happy to help out where I can!

  • 0 in reply to Tom Hendrick
    Prodigy 50 points
    Thank you, Tom.
    Sampling and transferring data works now.