Answer

Correct option: (ii) Conversion of physical signals into digital form

Explanation

A microcontroller-based DAQ system acquires signals from sensors and converts them into digital values for processing.

The typical sequence is:

Sensor → Signal Conditioning → ADC → Microcontroller → Processing/Storage/Communication

For example, an analog temperature sensor may produce 0–5 V. The ADC inside or connected to the microcontroller converts this voltage into a digital code, which the microcontroller can use to calculate temperature.

Therefore, the correct answer is (ii).

Solution: Microcontroller-Based DAQ System

Definition

A microcontroller-based DAQ system uses a microcontroller to acquire, digitize, process, store and/or transmit sensor data.

Architecture

Sensor
  ↓
Signal Conditioning
  ↓
ADC ───────────────┐
  ↓                │
Microcontroller ← Timers / Interrupts
  ↓
Data Processing
  ├──→ Display
  ├──→ Memory / SD Card
  └──→ UART / SPI / I²C / USB → Computer/Network

Working

1. A sensor measures the physical quantity.
2. Signal conditioning amplifies and filters the sensor output.
3. The ADC converts the analog signal to digital data.
4. The microcontroller samples the data at a programmed rate.
5. Firmware performs scaling, calibration, filtering or feature extraction.
6. The processed data is displayed, stored or transmitted.
7. The controller can also generate control outputs based on measured data.

Important Components

• Sensors/transducers
• Signal-conditioning circuits
• ADC
• Microcontroller
• Timers and interrupts
• Memory/storage
• Communication interfaces
• Display or actuator interface

Advantages

• Compact and low cost.
• Programmable and flexible.
• Easy data logging.
• Real-time processing.
• Multiple communication options.
• Suitable for portable biomedical and industrial instruments.

Example: Temperature Logger

Temperature Sensor
       ↓
Amplifier + Low-pass Filter
       ↓
ADC
       ↓
Microcontroller
       ↓
Convert Code → Temperature
       ↓
SD Card / LCD / USB

The microcontroller periodically samples the temperature, converts the ADC code into temperature units, adds a timestamp and stores the result.

Conclusion

A microcontroller-based DAQ integrates measurement, conversion, processing, communication and storage into a compact embedded system.

Solution: Hardware Communication Interface Programming for Data Loggers

A data logger needs communication interfaces to transfer acquired data between the microcontroller, sensors, memory devices and computers.

1. SPI

SPI (Serial Peripheral Interface) is a synchronous, high-speed interface using signals such as SCLK, MOSI, MISO and CS.

MCU SCLK ───────── Device
MCU MOSI ───────── Device
MCU MISO ───────── Device
MCU CS   ───────── Device

It is commonly used with SD cards, external ADCs and DACs.

Programming sequence

1. Configure SPI clock and mode.
2. Configure GPIO pins.
3. Select the slave using CS.
4. Transmit command/address/data bytes.
5. Read returned data when required.
6. Release CS.

2. I²C

I²C uses two lines:

• SDA: data
• SCL: clock

It supports multiple addressed devices on the same bus.

Typical sequence:

START → Address + R/W → ACK → Data → ACK → STOP

It is useful for EEPROMs, sensors and low-speed peripherals.

3. UART/USART

UART is an asynchronous serial interface using TX and RX. A typical frame contains a start bit, data bits, optional parity and stop bit(s).

Typical configuration includes:

• Baud rate
• Data bits
• Parity
• Stop bits

UART is commonly used to transfer logged data to a PC or another controller.

4. USB

USB provides higher-speed communication between the embedded data logger and a host computer. Firmware must configure the appropriate USB device class and endpoints according to the application.

Basic Data Logger Flow

Sensor → ADC → MCU RAM/Buffer → Communication Interface → Storage/PC

The firmware normally uses a buffer so that data acquisition can continue while blocks of data are written to storage.

Interface Selection

Interface	Main advantage	Typical use
SPI	High speed, simple	SD card, ADC/DAC
I²C	Addressed multi-device bus	Sensors, EEPROM
UART	Simple point-to-point	PC/debug/modem
USB	High-speed host connection	PC data transfer

Conclusion

Communication interface programming involves configuring the peripheral, transferring data according to its protocol, handling errors/timeouts and buffering acquired samples so that continuous logging is reliable.

Solution: SPI 8-bit DAC

Given:

• DAC resolution = 8 bits
• Output range = 0–5 V
• SPI clock = 4 MHz

(i) SPI Timing and DAC Output

SPI is a synchronous serial protocol. The master provides the clock on SCLK, sends data on MOSI, and selects the DAC using chip select (CS).

Typical sequence:

CS ↓ → Send command/data bits on MOSI → Clock pulses → CS ↑ → DAC updates output

The exact sampling clock edge and timing depend on the DAC's SPI mode specified in its datasheet.

For an ideal 8-bit DAC, the input code ranges from:

$$0\text{ to }2^8-1=255$$

For a 0–5 V output using the common ideal endpoint convention:

$$V_{out}=\frac{D}{255}\times5$$

where $D$ is the decimal DAC code.

Thus:

• $D=0 \Rightarrow V_{out}=0$ V
• $D=255 \Rightarrow V_{out}=5$ V

The approximate voltage step is:

$$\frac{5}{256}=19.53\text{ mV}$$

The exact transfer function should follow the DAC datasheet if it uses a different reference/output convention.

(ii) Maximum Update Rate

An 8-bit DAC requires at least 8 clock cycles per update if the SPI frame contains only the 8 data bits.

SPI clock:

$$f_{SPI}=4\text{ MHz}$$

Therefore:

$$f_{update,max}=\frac{4\times10^6}{8}$$

$$\boxed{f_{update,max}=500,000\text{ updates/s}}$$

or:

$$\boxed{500\text{ kUpdates/s}}$$

Important practical note

If the DAC requires more than 8 bits per SPI frame—for example, command bits, address bits or control bits—the actual maximum update rate will be lower:

$$f_{update,max}=\frac{f_{SPI}}{N_{bits/frame}}$$

Final Answer

• DAC output: $V_{out}=\frac{D}{255}\times5$ V under the stated endpoint convention.
• Approximate LSB step: 19.53 mV.
• Maximum theoretical update rate for an 8-bit frame: 500 kUpdates/s.