Read level detector via I2C and convert data

Hello,

I'm picking up on this old topic ADAU1701 Low Instruction Level Detection Readback

I want to read back the level meter data and process it in my controller but I'm stuck with the final conversion.

First I generated this very simple project to get myself some test values.

This results in the following register Values from the Trap.Dat file

Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1X trap value 40 register
Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1LinX trap value 45 register
Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1LinX_4 trap value 46 register
Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1LinX_lo trap value 48 register
Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1X_4 trap value 49 register
Instance 3 Signal SingleBandLevelDetwOutDisplay2ndGen1X_lo trap value 50 register

Next I used a scope to record the I2C bus for different levels. For example here a recording at -11 dB shown at the level meter

I2C, Time, Address, R/W, Data
1,-31.1484ms,0x68,W,0x08 1A 00 A2
2,-30.3461ms,0x68,W,0x08 1A
3,-15.5413ms,0x69,R,0x00 A0 B9

4,-14.4372ms,0x68,W,0x08 1A 00 C6
5,-13.4924ms,0x68,W,0x08 1A
6,-6.93889as,0x69,R,0x03 80 00

7,965.060us,0x68,W,0x08 1A 00 CA
8,1.79494ms,0x68,W,0x08 1A
9,15.8698ms,0x69,R,0x05 48 CE

10,16.9898ms,0x68,W,0x08 1A 00 B6
11,17.8147ms,0x68,W,0x08 1A
12,31.4520ms,0x69,R,0xFE 8C 76

13,32.4421ms,0x68,W,0x08 1A 00 BA
14,33.3045ms,0x68,W,0x08 1A
15,47.2218ms,0x69,R,0x00 0C 66

16,48.2506ms,0x68,W,0x08 1A 00 C2
17,49.1155ms,0x68,W,0x08 1A
18,63.0903ms,0x69,R,0x07 00 00~A

Then I figured out with read matches the Trap.Dat register values. According to the datasheet on page 38 the Data capture registers need the register value as well a the register type specified in the I2C write command. Therefore, if we for example take a look at the first write command.

00 A2 equals to 0000 0000 1010 0010

the last two bit 10 are indicating to read out the MAC_Out trap data, which is the same for all 6 I2C write commands. The rest of the command will translate to

0000 0000 0010 1000 or 0x28 which is 40 decimal.

Here we have our first register "Gen1X". If we repeat this for all I2C write commands we can see that read order is

40 - Gen1x

49 - Gen1_x4

50 - Gen1x_lo

45 - LinX

46 - LinX_4

48 LinX_lo

fabulous. But now I encounter my problems. First of all, which set of registers is correct?
According to this topic Reading meters with a controller on the ADAU1452
The LinX registers are the unchanged input values. While in this conversation ADAU1701 Low Instruction Level Detection Readback the LinX registers seem to be used. I think that the values, which already include some processing like decay and averaging are benefitial. So my guess would be to read the Gen1 registers. Can you confirm?

Next is the unanswered question where in the _x4 registers the additional 4 bits for the 5.23 format are located exactly. From my test data I suspect that I need the lower nibble of the first byte for the calculation. In my example this would be the 3 left to the 0x80 in row 6.

And if I know the location. The final conversion needs to happen how? The 56 bits I get from all those reads are what? A double precision version of the 5.23 format? Does this mean I need to handle the data like 10.46?

For the other cases I think I figured out the conversion.

If I use only the data from the first register. For example Gen1x 00 A0 B9

Convert A0 B9 in decimal = 41145
Divide by 2^19 for the number of bits we have ~ 0,0784778595

And finally 10*log(0,0784778595) = -11,05 dB

But this only works down to values of roughly -40 dB since this first register will then always be set to all 0x00. So for more precision I can use the 4 bits from the Gen1_x4 register.

Convert A0 B9 3 in decimal = 658323
Divide by 2^23 now since we added the nibble of the x4 register ~ 0,0784782171

And convert to dB like to get the known -11 dB.

This works down to roughly - 60 db before the nibble will also be set to 0. Hence if I need even lower dB values I need to work with the whole 54 bit register. But for this case I can't figure out the final conversion. If I use the demonstrated way

A0B930548CE = 11044827515086

I get the closest result by dividing with 2^47. But why 2^47 and not 2^46 or 2^51 which would make more sense to me?

For other cases this becomes even more challenging to me. For example -71 dB reads in my registers to

Gen1X 00 00 00

Gen1_x4 00

Gen1x_lo 05 3A 29

If I only convert 53A29 in decimal and follow the same way I get -86 dB if I use 2^47 as divider. In this case 2^42 would give me the nearest result.

Yours faithfully

Chris

Edit Notes
Added more examples

[edited by: Chris49 at 9:00 AM (GMT -5) on 24 Dec 2025]

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