LS1020A JTAG does not work

[dainiuszilys]

I'm trying to run custom PCB with LS1020A. I use J-Link as JTAG adapter.

But the LS1020A does not respond properly. Here is a log of access:

Tue Jun 06, 2017 16:07:39: Could not measure total IR len. TDO is constant high.
Tue Jun 06, 2017 16:07:39: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFF814C7B00009
Tue Jun 06, 2017 16:07:39: Could not measure total IR len. TDO is constant high.
Tue Jun 06, 2017 16:07:39: TotalIRLen = ?, IRPrint = 0x..FFFFFFFFFFFFFE0C80000009
Tue Jun 06, 2017 16:07:39: Could not measure total IR len. TDO is constant high.
Tue Jun 06, 2017 16:07:39: TotalIRLen = ?, IRPrint = 0x..000000000000000000000000

There are no any other devices in the JTAG chain. No visual noise on jtag lines, pulses are nice.

RCW is hard-coded. RESET_REQ_B is released (logic 1).

Tryed with uSD card RCW + software. Situation is the same - the response on TDO is almost random, software starts running (output on UART exists). So CPU is alive, but I can't access it via JTAG to debug.

Tryed various JTAG clocks - 32khz - 1MHz, - no influence.

Hardware connection seems to be fine: SCAN_MODE_B, TEST_SEL_B - pull-ups to 1.8V, TRST_B connected to PORESET_B. TCK has pull-up of 10k, as recommended. System clock is 100MHz.

Where could be a problem?

J-Link works on LS1021A Tower board, so adapter should be compatable with my PCB as well.

[dainiuszilys]

I made the comparison of JTAG signals. No answer so far. Wrong PCB returns random response, while the Tower board always returns the same response to the same request.

Here are diagrams of wrong response of LS1020A:

Here is the response of Tower board, which is always the same, and understandable by JTAG adapter:

TCK frequency in both cases is 1MHz.

TRST_B was reset just before capturing diagrams in all 3 cases.

So still no idea, what's wrong with the PCB...

Maybe a kind of security bits are on by default?

RTC has no 32kHz clock in my PCB. Maybe that's the reason?

Anything else can disable debug port?

[dainiuszilys]

I connected 32kHz RTC clock. Didn't help.

[alexander_yakov]

LS1021A and LS1020A are based on the same silicon, so if your J-link works on LS1021A, than it should also work on LS1020A.

Please perform the following:

1. Please verify all power supplies are stable at their desired values, and power supply noise is as expected.

2. Please check TRST state. If TRST is connected to PORESET, please make sure TRST is asserted at least once at power-on, and than released.

3. Please look JTAG pins by scope for extra noise caused by signal reflections on long traces.

4. Please compare scope screenshots of JTAG signals with similar screenshots from "good" board

Have a great day,
Alexander
TIC

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