ADS7924: Reading Multiple Registers

Brian Haught

Part Number: ADS7924

We are using a ADS7924 in a new design. On power up, we are writing Awake Mode followed by Manual-Scan Mode. Then, every half second, we are reading six bytes of data starting at address 02h (INC bit set) and repeating Manual-Scan Mode. The issue we are having is that the first two bytes that are received are always zero (confirmed with scope). The remaining four byte are correct.

If we change the starting address to 00h, the third and fourth bytes are correct for channel 0. So, it appears it is not an issue with the channel 0. The first two bytes are still zero.

The I2C communication routines are the same as we have successfully used in the past with other devices.

Any ideas?

over 6 years ago

0 Evan Sawyer over 6 years ago

TI__Expert 3175 points

Hi Brian,

I am looking into this and will get back to you.

0 Evan Sawyer over 6 years ago in reply to Evan Sawyer

TI__Expert 3175 points

Hi Brian,

Thank you for your patience.

When reading the registers (regardless of which is first), are you following the timing diagram from Figure 33 of the datasheet for reading multiple registers? Specifically I want to make sure that the repeated start (or a stop then start) is being provided.

Can you provide scope shots of your read command and the data bytes being sent (at least the first two bytes that contain the expected data)?

0 Brian Haught over 6 years ago in reply to Evan Sawyer

Prodigy 80 points

Looks like we missed the third repeated start. Is it required?

Note: Overlap from frame to frame

0 Evan Sawyer over 6 years ago in reply to Brian Haught

TI__Expert 3175 points

Hi Brian,

These all look correct to me. Instead of reading multiple registers, can you please try reading the same register (02h) multiple times using the single register read and then reading a previously addressed register (see figures 31 and 32)? Be sure that your input signal will result in values in this register.

0 Evan Sawyer over 6 years ago in reply to Evan Sawyer

TI__Expert 3175 points

Hi Brian,

A couple of questions for you:

1) How long are you waiting between each of the following: power up and awake, awake and manual scan, manual scan and read?
2) Are you setting the ACQCONFIG or PWRCONFIG registers?

0 Brian Haught over 6 years ago in reply to Evan Sawyer

Prodigy 80 points

1) Power up to awake is 500 ms then manual scan right after. Then, every half second, we are reading and repeating manual scan.
2) Not setting ACQCONFIG or PWRCONFIG registers.

0 Brian Haught over 6 years ago in reply to Evan Sawyer

Prodigy 80 points

Result is the same.

0 Evan Sawyer over 6 years ago in reply to Brian Haught

TI__Expert 3175 points

Hi Brian,

A few more questions:

1) What are you programming for the ModeCNTRL register? (if you can send your startup waveforms that would be helpful)
2) Can you try adding a delay between your read and scan cycles (a couple hundred milliseconds or more)?
3) After power up, please try a reset before configuring the device.

0 Brian Haught over 6 years ago in reply to Evan Sawyer

Prodigy 80 points

1) Programming Awake mode:

2) Result is the same.

3) Result is the same.

0 Brian Haught over 6 years ago in reply to Brian Haught

Prodigy 80 points

As it turns out, in addition to the ADS7924, we have three DAC7578s (TSSOP-16 package) on the bus. Their addresses are 90h, 94h and 98h. The address conflict is likely the issue.

0 Evan Sawyer over 6 years ago in reply to Brian Haught

TI__Expert 3175 points

Hi Brian,

Great to hear that you've identified the issue. If you have any further questions, please let us know!

Data converters

Data converters forum

ADS7924: Reading Multiple Registers