5. Accepted Technical Design
5.1.1 General Overview
The I/O Master hardware consists of three main subsystems and three voltage regulators. Figure 7 shows a level 1 block diagram of the system and the various connections between the
subsystems. The hardware can be broken down into two separate signal domains. So-called “low voltage” signals are 3.3V-level signals and include the computer interface and microcontroller subsystem. So-called “high voltage” signals are at the voltage level of the target device, which depending on the signal may be anywhere from -15V to +15V and include the signals that pass through the circuit protection subsystem to the target device. As the I/O Master consists of multiple I/O pins, the level shifter and circuit protection subsystems are repeated for each I/O pin, giving each individual pin full configurability and circuit protection independent from other pins. There is one level shifter subsystem for each pair of I/O pins, and there is one circuit protection subsystem for each I/O pin.
The microcontroller subsystem has two tasks: configuring the other hardware subsystems and transferring data between the digital interface and the computer. Depending on the protocol selected, the level shifter subsystem must be configured for the electrical requirements of the protocol, including logic voltage levels, pull-up/pull-down resistors, and single-ended/differential communication modes. The adjustable regulator must also be configured for the correct voltage to power the target device. The microcontroller takes configuration commands from the
computer and outputs the correct signals, whether a digital signal or analog voltage, to configure the hardware. Once configured, the microcontroller is responsible for taking digital data from the computer and generating the proper digital signal for the protocol selected as input to the level shifter. Conversely, the microcontroller must read received signals from the level shifter and send the received digital data to the computer.
The level shifter subsystem separates these two signal domains and shifts signals from the low-voltage domain to the high-voltage domain and vice versa. Based on a configuration signal from the microcontroller, the level shifter generates two logic voltage rails, V H and V L , used for
high-voltage signalling. Since a low-voltage signal state can be either 0V or 3.3V, the
corresponding high-voltage signal state is either V L or V H , respectively. The logic voltage rails are
also used to generate a threshold voltage for interpreting the state of received signals. As all microcontroller signals are single-ended, the level shifter subsystem is also responsible for converting single-ended signals to differential signals and vice versa. For this reason, there is one level shifter subsystem for each pair of I/O pins. In differential mode, I/O pins are paired together and two pairs of logic voltage rails, V H1 , V L1 and V H2 , V L2 , are used to generate differential voltages.
The circuit protection subsystem is designed to protect the target device from over-current and over-voltage related failures. To achieve this, the voltage is limited using zener diodes to clamp the voltage while the over-current protection is made of op-amps, an NMOS transistor and a voltage divider to control the gate voltage.
To power the various subsystems, there are three separate voltage regulators. The 3.3V regulator generates a 3.3V voltage rail for powering the microcontroller subsystem and low-voltage side of the level shifter subsystem. The ±15V regulator generates two opposite voltage rails, +15V and -15V, used for powering the high-voltage side of the level shifter subsystem and circuit
protection subsystem. Finally, the adjustable regulator is used solely for powering the target device, as long as the target device’s power requirements are small. A control signal from the microcontroller can set the output voltage anywhere between 3.3V and 15V.
Tables 5, 6, 7, 8, 9, and 10 list the functional requirements for the subsystems shown in Figure 7.
Table 5: Microcontroller Subsystem Functional Requirements
Module Microcontroller Subsystem
Designer Ian Glen & Nik Untch
Inputs - 3.3V power input
- USB interface
- Low-voltage signals from level shifter subsystem
Outputs - USB interface
- Low-voltage signals to level shifter subsystem - Control signals to level shifter subsystem - Control signal to adjustable regulator
Description The microcontroller is responsible for configuring the level shifter hardware for the protocol that the user has selected and transferring data between the USB interface and low-voltage digital I/O.
Table 6: Level Shifter Subsystem Functional Requirements
Module Level Shifter Subsystem
Designer Ian Glen
Inputs - 3.3V power input
- ±15V power input
- Low-voltage signals from microcontroller subsystem - Control signals from microcontroller subsystem - High-voltage signals from circuit protection subsystem
Outputs - Low-voltage signals to microcontroller subsystem - High-voltage signals to circuit protection subsystem
Description The I/O level shifter is responsible for translating digital signals between the low voltages required by the microcontroller and the high voltages required by the target device. For protocols that require differential signalling, this module also converts the high voltage signals to
single-ended signalling as required by the microcontroller. This module accepts control signals from the microcontroller so that its hardware can be configured in specific ways depending on the protocol selected.
Table 7: Circuit Protection Subsystem Functional Requirements
Module Circuit Protection Subsystem
Designer Nik Untch
Inputs - ±15V power input
- High-voltage signals from level shifter subsystem - High-voltage signals from target device
Outputs - High-voltage signals to level shifter subsystem - High-voltage signals to target device
Description The I/O circuit protection module protects the Target Device against issues such as overcurrent and overvoltage, and protects the I/O Master from reverse polarity
Table 8: 3.3V Regulator Functional Requirements
Module 3.3V Regulator
Designer Nik Untch
Inputs - 5V to 12V power input
Outputs - Regulated 3.3V power rail
Description The 3.3V regulator is designed to provide a 3.3V power rail for the microcontroller subsystem and low-voltage side of the level shifter subsystem.
Table 9: ±15V Regulator Functional Requirements
Module ±15V Regulator
Designer Ian Glen
Inputs - 5V to 12V power input
Outputs - Regulated +15V power rail
- Regulated -15V power rail
Description The ±15V is designed to provide both +15V and -15V power rails for the high-voltage side of the level shifter subsystem and the circuit protection subsystem.
Table 10: Adjustable Regulator Functional Requirements
Module Adjustable Regulator
Designer Nik Untch
Inputs - 5V to 12V power input
- Control signal from microcontroller subsystem
Outputs - Regulated power rail, adjustable between 3.3V to 15V
Description The adjustable regulator is designed to provide a regulated power rail for the purpose of powering the target device. Depending on the control signal, the output voltage is adjustable between 3.3V and 15V.