Newhaven Display Forum

Newhaven Products => TFTs => Topic started by: georgek on December 11, 2018, 12:36:07 PM

Title: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: georgek on December 11, 2018, 12:36:07 PM
We need to control the backlight of the NHD-5.0-HDMI-N-RTXL-CTU using the digital dimming method of the T.I. TPS61165 LED driver via the CN4 header. We are following the instructions outlined in the 9.2.1.2.1 LED Brightness Dimming Mode Selection of the tps61165.pdf data sheet.

We can successfully control the dimming of the NHD-5.0-HDMI-N-RTXL-CTU when the HDMI cable is disconnected.
Unfortunately when the HDMI cable is connected the digital dimming doesn't work.

Can you help?

Thank you,
George
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: Alee_S on December 20, 2018, 09:09:07 AM
Hi Georgek,

I just powered on the NHD-5.0-HDMI-N-RTXL-CTU using the digital dimming method you were using, and it seems to work fine for me.

The HDMI cable being connected shouldn’t have any impact on the brightness, as it is responsible for the Video aspect of the display.
The DC jack is responsible for supplying power to the whole display module, including the LED driver circuit, and the PWM pin is responsible for adjusting the brightness to the users desired level.

To try and replicate your problem, I applied DC power and sent a pulse signal to the PWM pin, and I noticed a significant dimming in brightness level. Then I connected the HDMI cable, which gave the display an image to show, but had no impact in altering the brightness of the display. I was able to manipulate the duty cycle of the signal applied to PWM pin from 0% to 100% afterward, and the brightness correspondingly adjusted as expected.

Are you disconnecting the PWM signal upon connecting the HDMI cable? Please note that when the PWM pin is left unconnected, it is considered to be in a “floating state”, meaning the logic voltage will be set to HIGH. So when the DC jack and the HDMI cable are connected, but no signal is sent to the PWM pin, the display will be configured for 100% brightness. 

Can you also provide more information on how you are applying a signal to the PWM pin of the CN4 header?
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital (1-wire) Dimming Control
Post by: georgek on December 20, 2018, 06:26:24 PM
Hello Alee_s,

Thank you for spending the time and effort to help me.
I haven't explained it well. By digital dimming I meant 1-wire method of dimming, as explained in the tps61165.pdf guide - http://www.ti.com/lit/ds/symlink/tps61165.pdf.

We use an Arduino Micro controller to implement EasyScale One-Wire Digital Dimming.

Here is the problem:

When we sent digital dimming commands while the HDMI cable is connected, the back-light brightness remains unchanged.

By accident we discovered that if we disconnect the HDMI cable, send a One-Wire Digital Dimming command and after that connect again the HDMI cable, the back-light brightness changes properly to the required level.

Below are brief extracts from the relevant sections from ps61165.pdf:

9.2.1.2 Detailed Design Procedure

9.2.1.2.1 LED Brightness Dimming Mode Selection
The TPS61165 features two dimming modes:
To enter 1 wire mode, the following digital pattern on the CTRL pin must be recognized by the
device every time the device starts from the shutdown mode.
1. Pull CTRL pin high to enable the TPS61165, and to start the 1 wire detection window.
2. After the EasyScale detection delay (tes_delay, 100 μs) expires, drive CTRL low for more than the EasyScale
detection time (tes_detect, 260 μs).
3. The CTRL pin has to be low for more than EasyScale detection time before the EasyScale detection window
(tes_win, 1 msec) expires. EasyScale detection window starts from the first CTRL pin low to high transition.
The device immediately enters the one-wire mode once the preceding three conditions are met. The EasyScale
communication can start before the detection window expires. Once the dimming mode is programmed, it can
not be changed without another start up. This means the device needs to be shutdown by pulling the CTRL low
for 2.5 ms and restarts. See Figure 9 for a graphical explanation.

9.2.1.2.3 Digital One-Wire Brightness Dimming
The CTRL pin features a simple digital interface to allow digital brightness control. The digital dimming can save
the processor power and battery life as it does not require a PWM signal all the time, and the processor can
enter idle mode if available.
The TPS61165 adopts the EasyScale protocol for the digital dimming, which can program the FB voltage to any
of the 32 steps with single command. The step increment increases with the voltage to produce pseudo
logarithmic curve for the brightness step. See Table 3 for the FB pin voltage steps. The default step is full scale
when the device is first enabled (VFB = 200 mV). The programmed reference voltage is stored in an internal
register and is not changed by pulling CTRL low for 2.5ms and then re-enabling the device by taking CTRL high.
A power reset clears the register value and reset it to default.

9.2.1.2.4 EasyScale: One-Wire Digital Dimming
EasyScale is a simple but flexible one-pin interface to configure the FB voltage. The interface is based on a
master-slave structure, where the master is typically a microcontroller or application processor. Figure 11 and
Table 4 give an overview of the protocol. The protocol consists of a device specific address byte and a data byte.
The device specific address byte is fixed to 72 hex. The data byte consists of five bits for information, two
address bits, and the RFA bit. The RFA bit set to high indicates the Request for Acknowledge condition. The
Acknowledge condition is only applied if the protocol was received correctly. The advantage of EasyScale
compared with other on-pin interfaces is that its bit detection is in a large extent independent from the bit
transmission rate. It can automatically detect bit rates between 1.7 kBit/sec and up to 160 kBit/sec.
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: georgek on December 29, 2018, 10:40:03 AM
I've been experimenting with the NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control with the one-wire interface and I can provide additional information.
I removed D8, this eliminates the ACTIVE signal. With D8 removed, the one-wire dimming control works fine.
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: Alee_S on December 31, 2018, 01:27:06 PM
Hi georgek,

I'm glad you got it running! Thanks for posting your solution here as well!  :)
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: georgek on January 01, 2019, 04:52:03 PM
It is not a solution, it is just a test that proves that our one-wire commands are correct. Removing D8 from the Display board eliminates the ACTIVE signal. This affects negatively the behavior of the display when it is powered on.
Title: Re: NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control
Post by: georgek on January 25, 2019, 09:08:12 AM
Any chance for finding a solution for the NHD-5.0-HDMI-N-RTXL-CTU Digital Dimming Control with one-wire interface?