As discussed in the previous blog, the LEDs in a DMD panel are organized as small groups in a matrix form. With this the number of pins and power required are significantly reduced. But it is preferable to be able to control with 4-5 pins so that even an 8 bit MCUs can manage such display. This calls for a bit complex electronics in the panel and software in the MCU, but should be manageable in terms of cost and usability.

This blog covers the internal circuitry inside DMD in detail. The block diagram of the overall set up is depicted below.

DMD - System Block Diagram

Dot Matrix Display – Overall Setup Block Diagram

As it can be seen, there are shift registers and demulitplexers used to simplify the effort in MCU. The dual P-channel MOSFET feeds the positive supply voltage to the LED anodes which denotes the row control and the shift registers provides the return path for the LED’s which denotes the column control. The MCU controls the demultiplexer with GPIO interface and interfaces with the shift registers through serial interface, most commonly SPI. More details of them are covered below. For further explanation, we will consider a DMD panel of 512 LEDs, each row is a collection of anode of 32 LEDs and each column is a collection of cathode of 4 groups of 4 LEDs (one column of 16 LEDs divided into 4 groups each of 4 LEDs for reducing IO pins required). The following figure depicts the arrangement of 16X32 LED panel

Row and Column arrangement in 16 X 32 DMD panel

16 X 32 DMD panel arrangement

Role of Serial Shift Register (74HC595)

The purpose of the shift register is to reduce the number of GPIOs required to drive the column of the LED matrix. 74HC595 is an 8-bit serial shift register with output latches and storage register.  The block diagram of the shift register commonly used inside DMD is given below (courtesy of 74HC595 datasheet)

74HC595 shift register block diagram

Serial Shift Register – Block Diagram

With one shift register we get 8 GPIOs possible. Hence for driving 32 columns of 16 LEDs, (total 128 GPIOs) we require 16 serial shift register. SPI clock, SPI MOSI and a latch signal acts a shift register inputs. The main advantage of this serial shift register is that it outputs the serial input fed to it in its serial output pin only when the latch signal is provided. The serial output of the first shift register is connected to the serial input second register. Likewise 16 shift registers are cascaded in series resulting in 128 GPIO pins.

Each of the 128 pin will in turn control 4 LEDs in a column resulting 512 LEDs on the whole. The data to be displayed can be fed as a 128 bit data with 128 clock pulse to the serial shift register. The data will not appear on the output unless the data is transferred to the storage register. Only upon the positive transition of the latch signal, the data will be transferred to the storage register. The data will automatically appear on the output since the output enable pin is permanently grounded.

Since the shift register corresponds to the control of 4 groups of LEDs in 32 columns, the Demultiplexer IC is required to drive the LEDs in 16 rows corresponding to the required data bits.

The following figure depicts the serial shift register circuitry in 16X32 DMD panel, with 128 output lines. Each shift register outputs has 8 outputs, hence 16 shift registers are serially connected for 128 output lines for column control. One output line is connected to 1 group of 4 LEDs in a column. Hence 128 output lines will be connected to 128 groups of 4 LEDs across 32 columns (i.e. one column has 4 groups of 4 LEDs).

Serial shift register with SPI interface

Serial Shift Register Circuit in 16 X 32 DMD panel

Role of Demultiplexer

Demultiplexer is dedicated for row control. 74HC138 is a 3 to 8 line demultiplexer with eight mutually exclusive inverting outputs. Out of three available address inputs only two inputs are selected for 4 individual inverting outputs. The four outputs from the demux will control the gates of four dual P-channel MOSFET where we get 4 pairs of drive outputs which in turn will drive the necessary current to the LEDs in the 16 rows. Finally there are 4 individual sets of multiplexed rows within the DMD. The block diagram of demultiplexer is depicted below (courtesy of 74HC138 datasheet)

3 to 8 line demultiplexer block diagram

Demultiplexer Block Diagram

With this arrangement only four rows will be illuminated at a time while the other is not illuminated. Hence the values of 4 outputs from the demux should be toggled periodically to illuminate all the sets of multiplexed rows. With persistence of human eye, if the LEDs are refreshed once around 20 ms, it is sufficient to show a flicker free display.

Following figure shows the four individual sets of the multiplexed rows inside DMD. The Color coding differentiates the 4 multiplexed rows and the four demux outputs are connected in the following manner

Y0 – connected to the rows R16, R12, R8, R4

Y1 – connected to the rows R15, R11, R7, R3

Y2 – connected to the rows R14, R10, R6, R2

Y3 – connected to the rows R13, R9, R5, R1

Row multiplex inside Dot Matrix Display

Multiplexed rows inside DMD

The demultiplexer input/output combination and the DMD row illumination sequence is illustrated in the following table

DMD-TB

Example

  1. Consider a 32X16 DMD panel
  2. The following figures illustrates the sequence of bit shifting

A single data bit is shifted in to the DMD and it is effectively present in the [R16, C8] location.

Bit 1 shifted into R16, C8

First Bit shift into DMD

On further data in, the Old data moves one bit to [R16, C7] and new data bit is loaded at [R16, C8]

Bit 2 loaded at R16, C8

After 2nd bit input

After the input of 9th bit, the first bit is moved to the twelfth row at [R12, C8] and the bit 9 is loaded at [R16, C8]

Bit 9 loaded at R16, C8

After 9th Bit input

Similarly with 32nd bit input, the bit one is moved to the forth row at [R4, C1] and the bit 32 is loaded at [R16, C8]

Bit 32 is loaded at R16, C8

After 32nd bit input

Upon the input of 33rd bit, the bit one is again moved to the sixteenth row but this time at [R16, C16] and the new bit 33 is loaded at [R16, C8]

New bit 33 is loaded at R16, C8

After 33rd bit input

Likewise on input of 128 bits, the bit 1 is moved to the consecutive rows and columns till [R4, C25] and the bit 128 is loaded at [R16, C8]

Bit 128 is loaded at R16, C8

After 128th bit input

Thus the entire pattern to be displayed can be loaded bit by bit. Running even at a low clock frequency of 8KHz, the 128 bits can be easily shifted in 16 ms, more than needed for human eye to detect the change.

DMD – Daisy Chain

It is possible to connect the multiple DMD panels in series using ribbon cables. This is called daisy-chaining. The number of the DMD in series is limited to the RAM size and the SPI clock frequency.
Even though the DMD can also come with multiple LEDs of varying color such as RED, GREEN, BLUE, etc, the underlying connection mechanism is same and each of the colored LED’s are controlled separately.

Now that we can control each LED of the DMD display independently, we can create any pattern to be displayed. In the upcoming blogs, we will discuss in detail about the software based control mechanism and creating rolling displays.

 

Understanding the growth of new technologies and keeping in mind of giving an added value to its existing product eStorm-L1, Embien launched an Android app named “DAQ Suite”. With experience in developing Android app for many segments such as IoT, healthcare, automotive etc we have developed “DAQ Suite” for enhancing user experience in multiple industrial applications such as remote control, remote data acquisition/logging, etc.

This blog is the sequel of the blog “eStorm-L1 as GSM based remote switch” demonstrating the relay control and sensor monitor via an Android app. The following video shows the remote monitoring and control application typically applicable in industrial domain and many other verticals. The demo is setup using the GSM based remote switch and android application controlling the device through SMS command.

For the demo purpose we have connected two LED’s and two switches for demonstrating the relay control and sensor monitor functionality respectively. In the real time application, the LED could actually be a relay with suitable external DC supply for coil excitation and switch could be a sensor/transducer output preferably a digital high/low signal or even a pulse type output. The device stores all the configuration settings corresponding to the user, IO, etc in the EEPROM and hence preserved even on power cycling. Also the device has an option to erase the memory on site and start the user settings from the initial factory default.

The DAQ Suite android app includes the following menu options

  1. Device type selection setting – To select the type of the device need to be monitored or controlled. Available device will be listed in the Device model list box.
  2. User settings – To set the user details such as name, phone number, password, device description, and phone number to be taken for call actions etc.
  3. Relay settings – To set the on duration, ON/OFF delay etc. For multiple relays, the name of the device attached to the relay can be set.
  4. Update settings – To set the update interval for how much time interval the user need to receive the SMS about the current status of the relay and digital input. Many options can be chosen from the list view such as on change, periodic, both and none.
  5. Call actions settings – To set the action to be done on the relay upon calling the device. Either the relay can be switched ON/OFF.

The main screen of the DAQ suite includes the status of the SMS command being sent and time information of the last received/sent SMS. The control switch to toggle the relay and the status LED of the digital inputs is also included in the main screen.

Upon every operation, a SMS will be sent to the device with the predefined command set and the respective status of the digital inputs and relay will be displayed on the main screen based on the acknowledge SMS from the device.

DaqSuite, the user friendly Android App for GSM based remote switch and sensor monitor applications is available in Google Play store. Feel free to try it or contact us for more information.

About Embien: Primary focus of Embien technologies lies in the Industrial Automation segment. We have a rich experience in working on industrial automation and control systems with customers across geographies to enable their factories and assembly lines run efficiently. Our team have developed protocol stacks for various industrial protocols and enabled them in customer devices. We have created various Human Machine Interfaces (HMI) systems to make easier the interface with the machine. Our Machine to Machine (M2M) service offering includes developing system capable of remote monitoring and controlling of machines, PLC’s, etc.