Theory of Operation
Main Board
Overview
The Main Board contains the Main microcontroller (U2) that controls all the logic and state machines on the MKP. In addition, a small microcontroller, called the Boot Controller, controls the boot-up sequence of the device. All power supply and external connectors are located on the Main Board, as well as the Ethernet physical interface, the serial interface and the USB-C connector.
Non-volatile Storage
The MKP uses non-volatile memory to store important parameters that are kept even if the power is turned off. Many similar devices use Electrically Erasable Programmable Read-Only Memory (EEPROM) as non-volatile memory. The EEPROM has limited numbers of write cycles. Even at 1,000,000 write cycles an EEPROM can reach its limit within a few years of operation. The MKP uses a different technology called Ferroelectric Random Access Memory (FRAM) yielding up to 1013 write cycles, enabling years of error free operation. The FRAM memory (U1) can hold up to 512 bytes.
Indicator LEDs
There are 7 LEDs on the Main Board. These are used during development of the firmware as well as being used for maintenance. The S1 switch enables and disables the LEDs. To prevent any possibility of lights from the LEDs to be visible through seams in the MKP housing, the LEDs are disabled during normal operations. The 7 LEDs are numbered 1-7, each indicating a different action performed by the Main Microcontroller. The following functions are indicated by the LEDs: 1) Device Power, 2) Heartbeat, 3) ARINC Out, 4) ARINC In, 5) Ethernet / RS232, 6) BT Module and 7) USB Thumb drive attached.
Boot Controller
The Boot Controller is a separate microcontroller on the Main Board. The firmware is embedded into the firmware loaded onto the Main Microcontroller. At initial start-up, or if the firmware has been updated, the Boot Controller is programmed by the Main Microcontroller. Once the Boot Controller is operational, it assists in updating the firmware or enabling the Bluetooth Module (located on the Keyboard). By reading certain keys directly on the Keyboard it can enable In System Programming (ISP) or enable firmware updates over BT. It can also connect the BT Module directly to the serial communication bus.
Serial-over-USB
The MKP can communicate using different technologies. The serial interface is an RS232 connection over USB. This USB interface is located on the DSUB connector, not the USB-C connector. The DSUB connector is not hot-pluggable when it is connected to a USB port on a PC. Since the DSUB connector does not have the ability to physically disconnect the data lines before the power lines, and because the DSUB connector can also deliver power to the device, the USB data lines may be loaded with excessive current which will cause damage to the USB interface. To eliminate this, please disconnect power before disconnecting the DSUB connector, when the USB interface is used. The serial connection is shared between the Main Microcontroller and the Bluetooth module. The Boot Controller will decide which of the two is connected to the serial interface by controlling relay (K1). The default state is to connect the Main Microcontroller to the serial interface. A series of commands can be sent over the serial interface to control and monitor the device. These commands are the same as those used over the Ethernet connection.
Ethernet Connection
The Ethernet connection is available whenever the device is powered. The Ethernet connection obtains its IP address via Dynamic Host Configuration Protocol (DHCP ) by default. The user can change the system to connect via static IP address by sending it a command over serial or using the Ethernet connection. The serial and Ethernet connection shares the same commands to monitor and control the MKP.
Power Supplies
Several internal power supplies produce internal power to drive the electronics on the Main Board. 28VDC is the power source for the device. For the device to survive power glitches and brown outs, a large capacitor (C25) is charged up as soon as power is applied. The capacitor can power the device for several hundreds of milliseconds, even under heavy load. Under light load the device is powered for several seconds. A resistor (R14) limits the charging current to under 600mA. This reduces the inrush current during start-up. U4 produces 3.3VDC power from the 28VDC source. 3.3VDC is used by the Main Microcontroller and most of the logic ICs on the Main Board. U3 produces 5.0VDC power from the same 28VDC supply. 5.0VDC is mainly used by the ARINC transceivers as well as the backlighting and panel backlighting of the buttons on the Keyboard. Besides the two main power supplies, the programming voltage (U5) for programming the Boot Controller and a negative 5.0VDC power for the ARINC Interface.
ARINC Connection
The MKP uses the same ARINC 429 protocol as the corresponding aircraft device. The ARINC 429 protocol is realized using and ARINC 429 transceiver (U12). The transceiver contains the both input and output hardware interface and the ARINC 429 protocol, all in one single integrated circuit (IC). The Main Microcontroller communicates to the IC using a serial peripheral interface (SPI). Additionally, a connection is used to indicate when an ARINC 429 message has been received.
USB Type C
The USB Type-C interface has been implemented to serve two main functions; 1) To provide a simple benchtop method to power the device using a 20VDC USB Type-C power supply and 2) To provide a method to update the firmware of the MKP using a thumb drive. Using a power supply capable of providing 20VDC over USB the MKP will power-up and enter “Demo Mode”. This allows the user to exercise the mic selectors and volume knobs, in addition to validating the operation of the panel backlight circuitry.
PWM Output
The MKP contains three PWM outputs for lighting remote panel backlighting panels. The rate of the PWM is controlled by incoming ARINC 429 commands. The three outputs have one power and one PWM controlled return circuit. The power output is a constant 5VDC which is turned on after the first PWM command is received over the ARINC 429 interface. This power output is limited to 200mA and will trip (turned off) if more than 400mA is drawn. The circuit will reset itself once the over current situation has been resolved.
Keyboard
Overview
The Keyboard contains all the keys of the MKP. It is designed to accurately resemble the same layout and the same feel as the corresponding aircraft unit. All keys are individually lit and are controlled separately. Each individual key is individually calibrated to the correct light level and stored in non-volatile storage. The Keyboard also provides the interface for the encoder and a Bluetooth module.
Lighted Keys
The 52 keys on the keyboard are connected to 3 IO expanders that will detect when a key is pushed. The IO expanders are controlled by the Main Microcontroller using an Inter-Integrated Circuit (I2C) protocol in addition to circuitry indicating if a button was pushed. Each key is illuminated by several light emitting diodes (LED). The LEDs lighting up a key are PWM controlled individually using 4 PWM LED drivers. The result is that each of the keys can be independently controlled and very accurate light level of each key is achieved. While the PWM LED drivers control the calibrated light level of each key, the overall light level is controlled by the Main Microcontroller. This is also a PWM signal. The circuitry effectively add these signals to produce the final light output.
Non-volatile Storage
During manufacturing, each of the keys are calibrated to a certain light level. These calibration values are stored on the Keyboard in non-volatile memory. The non-volatile memory is the same as is found on the Main Board. It is placed on the Keyboard so that the calibration values are stored on the same printed circuit board (PCB) as the keys to which they belong.
Encoder
The Keyboard provides a connector for the quadrature encoder. The signals are directly forwarded to the Main Microcontroller’s quadrature input.