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Author Topic: BT816 signal level on RGB interface  (Read 9190 times)

Spyy

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BT816 signal level on RGB interface
« on: January 05, 2021, 12:16:32 PM »

Hi,

i am using the BT816 chip in my design and i am struggeling a bit. I supply the BT816 chip with 3.3V for VCCA, VCCIO1, VCCIO2 and VCCIO3. VOUT1V2 is on 1.2V as forseen.
Now my issue:
On the RGB lines, where i can see active pulsing signals (BACKLIGHT, DE, VSYNC, HSYNC, DISP, PCLK), i only measure approx 0.8V-1V. I connected those lines via 33Ohms in serial to a ST7701S chip. It seems that this signal level is too low for this chip which is also on 3.3V.
So my question is: Is this signal level normal ? I tried to force the signal strenth via register to higher amps without success.
What can i do here?

Thank you a lot

Torsten
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Re: BT816 signal level on RGB interface
« Reply #1 on: January 05, 2021, 06:15:25 PM »

Hello,

Thank you for your question.
Would it be possible for you to provide your schematic?

VCCIO2 is the supply concerned with the RGB and touch I/O pins, when supplied with 3V3 you should see 3v3 signals on the applicable pins.

Could you confirm that the RGB pins are driven at 3v3 when the ST7701S is disconnected?

Best Regards,
BRT Community
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Spyy

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Re: BT816 signal level on RGB interface (solved)
« Reply #2 on: January 07, 2021, 10:27:39 PM »

Hi,

thank you for your quick response....i disconnected the ST7701 from the BT81x and level is still low....so i assume the device is damaged...will try with a new one...

Thank you

Torsten
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Re: BT816 signal level on RGB interface
« Reply #3 on: January 08, 2021, 04:20:37 PM »

Hello,

Please keep us updated with how you get on replacing the device.

Best Regards,
BRT Community
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Spyy

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Re: BT816 signal level on RGB interface
« Reply #4 on: January 24, 2021, 04:47:31 PM »

Hi,

after changing the chip, signal level is now OK :).

What i am struggling is, is with the external crystal. It seems not to work (no oszillation at all). When i am trying to switch to external clock the EVE stays in the "reading the  REG_ID" loop. When i switch to internal it works and the chip starts to work.

So...is the crystal necessary or is the internal crystal sufficient ? What is the difference ?

Thank you

Torsten
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Re: BT816 signal level on RGB interface
« Reply #5 on: January 25, 2021, 11:52:19 AM »

Hello,

Glad to hear the new IC is working for you.
As for the selection for the clock source, external/internal, would it be possible to see your initialization code?

Best Regards,
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Spyy

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Re: BT816 signal level on RGB interface
« Reply #6 on: January 25, 2021, 07:53:05 PM »

Hi,

sure I can:

Code: [Select]
Serial.println("Waking up EVE chip");
// Wake-up FT81x
ft81xPowerDown(); // 1) lower PD#
delay(300); // 2) hold for 20ms
ft81xPowerUp(); // 3) raise PD#
delay(300); // 4) wait for another 20ms before sending any commands
ft81xCmdWrite(FT81x_RST_PULSE);
delay(300);

ft81xCmdWrite(FT81x_CLKINT); //send command "CLKEXT" to use the external clock source
//ft81xCmdParamWrite(FT81x_CLKSEL, 0x00); //select the system clock frequency => default 60 Mhz
ft81xCmdWrite(FT81x_ACTIVE); //send host command "ACTIVE" to wake up
SPI_TO_EVE_FAST;
delay(500); // Give some time to process

// --------------- Check that FT8xx ready and SPI comms OK -----------------
unsigned char regId = ft81xMemRead8(REG_ID);
while (regId != 0x7C)                                        // Read REG_ID register (0x302000) until reads 0x7C
{
Serial.println("Reading EVE REG_ID...failed");
delay(500);
}
Serial.print("EVE REG_ID = ");
Serial.println(regId, HEX);
delay(100);

This seems to be the standard init i found somewhere...
First reset is a hardware reset via /PD, the line below it is an additional SW reset just for trying it but seems to make no difference.
When i put in FT81x_CLKINT the chip starts working (pulsing, answering register reads and so on) with CLKEXT it hangs in the "// Read REG_ID register (0x302000) until reads 0x7C" loop forever...
I also tried with lower SPI speeds without success.

So in general, is there an external crystal needed ? Are there limitations if there is no crytal and i rely only on the internal clocking ?
Is there a crystal type you recommend ? Perhaps i have the wrong one...it is 12 Mhz, are 18 pF the right Cs ?, for the crystal it says it has 20pF 70Ohms...connections on the PCB to the BT818 are short (approx 2...4 mm) i am not the hardware expert here...for oscillating circuits...

Thank you

Torsten
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Re: BT816 signal level on RGB interface
« Reply #7 on: January 26, 2021, 02:05:27 PM »

Hello,

Thank you for providing your initialization code.
Could you try adding a 20ms delay after you have sent the host command to select the clock source?

In general, no it is perfectly acceptable to use the internal clock on the BT81x, and as far as I am aware there are no limitations when using the internal clock vs an external crystal. But I will double check this with the hardware team for you.

We have previously used the following Abracon Crystal on our BT81x development boards:
MFP#: ABM8G-12.000MHZ-18-D2Y-T
DigiKey Part # : 535-10261-1-ND

For your given crystal I would go with what the datasheet notes for the load capacitors, could you link its datasheet?
Also would you be able to attach the crystal section of your schematic?

Best Regards,
BRT Community
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Spyy

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Re: BT816 signal level on RGB interface
« Reply #8 on: January 27, 2021, 10:04:24 AM »

Hi,

thank you very much for your help, very appreciated...will add 20 ms after activating the ext clock...and try

From my side it is OK not using a crystal when there are no restrictions...perhaps my Cs are not well chosen...will then erase the external crystal..

Torsten
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Re: BT816 signal level on RGB interface
« Reply #9 on: January 27, 2021, 11:34:21 AM »

Hello,

Please let me know how you get on.

Best Regards,
BRT Community
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