AM1808: some pin output low pulse during power up

Tony Tang

Part Number: AM1808

Hello,

#1. on a customer's old product found pulse on G3 and C17 during power up lead to LCD flick. according to Errata, the 4.7Kohm pulled up resistor <4.81Kohm already.

#2. verified 3 boards, all have the same issue.

Seems the pin's behavior can't be explained by AM1808 errata Advisory 2.3.12. Your advice?

over 5 years ago

0 Yordan Kovachev over 5 years ago

TI__Guru**** 161600 points

Hi Tony,

According to AM1808 Datasheet, pin G3 is PRU0_R30[22] / PRU1_R30[8] / UPP_CHB_WAIT / GP8[12] /PRU1_R31[24]. How is this connected in their board? How does it affect the device bootup, so errata Advisory 2.3.12 is referred?

Could you share the relevant part of the scheamtics?

Best Regards,
Yordan

0 Tony Tang over 5 years ago in reply to Yordan Kovachev

TI__Mastermind 29202 points

Yordan,

as in the first snapshot, Pin G3 was used to control LCD back light, the low pulse result in the LCD a short flicker during power up, it doesn't impact device boot up. but it bring end customer bad user experience.

the low pulse is about 20us.

0 Yordan Kovachev over 5 years ago in reply to Tony Tang

TI__Guru**** 161600 points

Okay, sorry, I got confused because you involved the errata Advisory 2.3.12.
I don't have an AM1808 board at my disposal to probe the pin and see what is the case on the reference EVM, so can you share the relevant part of the schematics? It is possible that his a fault in their design.

Best Regards,
Yordan

0 Tony Tang over 5 years ago in reply to Yordan Kovachev

TI__Mastermind 29202 points

Yordan,

In order to isolate the issue, customer disconnected the external LCD, only remain the 4.7Kohm pull up resistor on the pin to capture the waveform.

0 Yordan Kovachev over 5 years ago in reply to Tony Tang

TI__Guru**** 161600 points

Hi,

In this case the only thing that comes to my mind is to check weather the power supplies (levels should match the recommended operating conditions, there should be enough decoupling and bulk capacitors (they can check the reference boards for this), etc..).

Best Regards,
Yordan

0 Tony Tang over 5 years ago in reply to Yordan Kovachev

TI__Mastermind 29202 points

I think I find the cause, I modified LCDK to power up 3V3 and 1V8 at the same time, but the real wave is 1V8 a little after 3V3. there is also low pulse on G3.

BTW, with default LCDK power sequence, there is not such low pulse on G3 during power up. see below wave, yellow: reset, green: G3.

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AM1808: some pin output low pulse during power up