TMDS171: adaptive equalization not working

stefano vaghi

Part Number: TMDS171

in our system we use the TMDS171.

In some cases we are facing an issue with the adaptive equalization that seems to not behave as expected.

Looking with the LinkEye Tool at the equalization gains imposed by the adaptive equalizer we have found that on 1 or 2 lane we have the gains at the maximum levels even if we are using a very short cable(1m). If we manually set the equalization gain to a lower level we have a clean video signal.

In our application we have to support both short cables and longer cables(max 20m), so the adaptive equalizer is a feature we would like to have enabled and working.

What can cause the adaptive algorithm to work in a wrong way and set the gain at the maximum?

Thank you

Best Regards

over 6 years ago

0 David (ASIC) Liu over 6 years ago

TI__Guru**** 176136 points

Stefano

When you are switching between a short cable and a long cable, does the HPD_SNK get toggled?

Can you please send me a Page 0 and a Page 1 register dump between the short and the long cable?

Thanks

David

0 stefano vaghi over 6 years ago in reply to David (ASIC) Liu

Prodigy 75 points

here I have the register dump in the case of a short cable(1m).

As i said the adaptive equalizer set the gain at an high levels and the equalizer gain is changing continuously

Fullscreen RegisterMap.txt Download

CSR PAGE 0
00	54
01	4D
02	44
03	53
04	31
05	37
06	31
07	20
08	01
09	02
0A	B3
0B	00
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0D	00
0E	00
0F	0F
10	00
11	00
12	00
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15	80
16	00
17	F0
18	FF
19	0F
1A	FF
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1D	0F
1E	00
1F	00
20	0A
21	00
22	00
23	00
24	00
25	00
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27	00
28	00
29	00
2A	00
2B	00
2C	00
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30	00
31	00
32	00
33	00
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36	00
37	00
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39	00
3A	00
3B	00
3C	00
3D	00
3E	00
3F	00
40	00
41	00
42	00
43	00
44	00
45	00
46	00
47	00
48	00
49	00
4A	00
4B	00
4C	00
4D	00
4E	00
4F	00
50	00
51	00
52	00
53	00
54	00
55	00
56	00
57	00
58	00
59	00
5A	00
5B	00
5C	00
5D	00
5E	00
5F	00
60	00
61	00
62	00
63	00
64	00
65	00
66	00
67	00
68	00
69	00
6A	00
6B	00
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6F	00
70	00
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76	00
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79	00
7A	00
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80	00
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88	00
89	00
8A	00
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8D	00
8E	00
8F	00
90	00
91	00
92	00
93	00
94	00
95	00
96	00
97	00
98	00
99	00
9A	00
9B	00
9C	00
9D	00
9E	00
9F	00
A0	00
A1	00
A2	00
A3	00
A4	00
A5	00
A6	00
A7	00
A8	00
A9	00
AA	00
AB	00
AC	00
AD	00
AE	00
AF	00
B0	00
B1	00
B2	00
B3	00
B4	00
B5	00
B6	00
B7	00
B8	00
B9	00
BA	00
BB	00
BC	00
BD	00
BE	00
BF	00
C0	00
C1	00
C2	00
C3	00
C4	00
C5	00
C6	00
C7	00
C8	00
C9	00
CA	00
CB	00
CC	00
CD	00
CE	00
CF	00
D0	00
D1	00
D2	00
D3	00
D4	00
D5	00
D6	00
D7	00
D8	00
D9	00
DA	00
DB	00
DC	00
DD	00
DE	00
DF	00
E0	00
E1	00
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E4	00
E5	00
E6	00
E7	00
E8	00
E9	00
EA	00
EB	00
EC	00
ED	00
EE	00
EF	00
F0	00
F1	00
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F3	00
F4	00
F5	00
F6	00
F7	00
F8	00
F9	00
FA	00
FB	00
FC	00
FD	00
FE	00

CSR PAGE 1
00	C3
01	01
02	3F
03	00
04	A0
05	00
06	00
07	00
08	01
09	00
0A	00
0B	33
0C	00
0D	00
0E	11
0F	00
10	0F
11	80
12	00
13	F7
14	00
15	00
16	00
17	00
18	00
19	00
1A	00
1B	00
1C	00
1D	00
1E	00
1F	00
20	00
21	00
22	00
23	00
24	00
25	00
26	00
27	00
28	00
29	00
2A	00
2B	00
2C	00
2D	00
2E	00
2F	00
30	07
31	B0
32	08
33	00
34	00
35	00
36	00
37	00
38	00
39	00
3A	00
3B	08
3C	04
3D	06
3E	00
3F	00
40	80
41	80
42	80
43	80
44	CB
45	00
46	00
47	00
48	01
49	01
4A	7F
4B	FF
4C	03
4D	30
4E	7B
4F	74
50	00
51	00
52	00
53	00
54	00
55	00
56	00
57	00
58	00
59	D7
5A	00
5B	FF
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5D	40
5E	40
5F	40
60	00
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65	00
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B0	3E
B1	90
B2	00

'DATA FILE FORMAT: <HEX ADDRESS><TAB><HEX DATA>. EVERYTHING AFTER <DATA> IS IGNORED.
'USE <'> FOR IN-LINE COMMENTING.
'DATA FILE PARSING STOPS AT FIRST BLANK LINE.

lane 0 min=11.5dB max=14.62dB

lane1 min=8.8dB max=14.62dB

lane 2 min=3.8dB max=8.8dB

Why is it doing so? The best eye with eyescan is at an equalizer gain of 0dB. Why it is not finding the best gain?

0 David (ASIC) Liu over 6 years ago in reply to stefano vaghi

TI__Guru**** 176136 points

Stefano

This looks to be a sink design. When you are switching between the two cables, does HPD_SNK get toggled? You need HPD_SNK to be toggled so the TMDS171 RX can re-lock and re-train its equalizer.

Thanks

David

0 stefano vaghi over 6 years ago in reply to David (ASIC) Liu

Prodigy 75 points

if I change the cable from short to long and viceversa the adaptive equalizer change the equalizer gain even if in our board we have the HPD_SNK fixed with a pull-up.

In case of long cable the adaptive equalizer find a equalizer gain set that is near to the optimal.

In case of short cable the adaptive equalizer find a equalizer gain set that is far from the optimal as you can see in the following image:

What can cause the adaptive equalizer to work in this wrong way?

If I toggle the HPD_SNK manually nothing change.

If I scan the eye with all the different fixed gain value it is confirmed that the best setting is the one with fixed gain set to 0 and the adaptive equalizer set the gain to 11, that seems to me a wrong behavior, as you can see in the following image:

Can you please help us in understanding why this happen?

Best Regards

Stefano

0 David (ASIC) Liu over 6 years ago in reply to stefano vaghi

TI__Guru**** 176136 points

Stefano

1. If I change the cable from short to long and vice versa the adaptive equalizer change the equalizer gain even if in our board we have the HPD_SNK fixed with a pull-up.

2. In case of long cable the adaptive equalizer find a equalizer gain set that is near to the optimal.

3. In case of short cable the adaptive equalizer find a equalizer gain set that is far from the optimal.

Are you saying that for short cable, even though the equalizer is changing, the equalizer is changing to a less optimized position?

Can we try the following experiment?

1. Remove cable

2. If you are using the TMDS171 HPD_SNK and HPD_SRC, please make sure HPD_SRC is low when HPD_SNK is low, so the source knows the TMDS171 is disconnected

3. Plugging in the short cable

4. Drive HPD_SNK high, HPD_SRC should be high, and check the EQ gain

For the 2nd experiment

1. Remove cable

2. Plugging in the short cable

3. Write a 1 to the APPLY_RXTX_CHANGES bit and check the EQ gain

For the 3rd experiment

1. Remove cable

2. Plugging in the short cable

3. Toggle the PD_EN bit and check the EQ gain

Thanks

David

0 David (ASIC) Liu over 5 years ago in reply to David (ASIC) Liu

TI__Guru**** 176136 points

Stefano

Do you have an update to this thread?

Thanks

David

0 stefano vaghi over 5 years ago in reply to David (ASIC) Liu

Prodigy 75 points

Hi David,

I've made the experiments you have suggested.

None of those has brought to a change in the adaptive equalizer gain settings.

It returns back to the non-optimal point.

The problem arise only on short cable distance(1m).

We are using a TX output swing of 700mV that should be compliant with the TMD171 receiver.

Do you have other suggestion?

Thanks

Regards

Stefano

0 David (ASIC) Liu over 5 years ago in reply to stefano vaghi

TI__Guru**** 176136 points

Stefano

Please give me few days to set up and duplicate the issue on the TMDS171 EVM, and I will reply to you what I find.

Thanks

David

0 David (ASIC) Liu over 5 years ago in reply to David (ASIC) Liu

TI__Guru**** 176136 points

Stefano

Here are the results from my testing on the TMDS171 EVM. I am seeing the equalizer adapts correctly to both the long and short cable.

Have you seen this issue on multiple boards and with multiple cables?

No Cable Connected

Long Cable Connected

Short Cable Connected

Thanks

David

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TMDS171: adaptive equalization not working