1. Objectives

  • Introduction to serial communication protocols

  • Using the Serial Peripheral Interface (SPI) protocol

2. Parts List

  • LPC1768 mbed board

  • USB A-Type to Mini-B cable

  • Breadboard

  • Jumper wires

  • ADXL345 Accelerometer Module

    ADXL345 Accelerometer Module
    Figure 1. ADXL345 Accelerometer Module

3. Background

In this experiment, you will use one of the serial communication interfaces of the LPC1768 microncontroller, specifically the SPI interface (through the SSP controller), to interact with a digital accelerometer.

3.1. SPI Communication Using the LPC1768 Microcontroller

3.1.1. Serial vs. Parallel Communication

Serial communication is the process of sending data one bit at a time, sequentially. In contrast, parallel communication involves sending multiple bits at the same time, as illustrated in the Parallel vs. Serial Communication figure below.

Parallel vs. serial communication
Figure 2. Parallel vs. Serial Communication

Some of the main differences between serial and parallel communication are:

  • A parallel link requires more wires, occupying more space and resulting in higher cost.

  • To keep all wires in a parallel link synchronized, the link rate is limited. In contrast, serial links can sustain much higher clock rates.

  • Parallel links are more susceptible to crosstalk interference.

  • Parallel communication between ICs require more pins, increasing the IC cost.

  • Parallel communication is easier to implement because it does not require data serialization and deserialization.

Serial communication is becoming more common for transmitting data between a computer and a peripheral device or even another computer, as improved signal integrity and transmission speeds in newer serial technologies have begun to outweigh the parallel bus’s advantages.

3.1.2. Serial Communication Protocols

Example serial communication standards include USB, FireWire, Serial ATA (SATA), PCI Express (PCIe), and Ethernet. Serial protocols commonly used in embedded systems include UART, I2C, and SPI.

Serial communication protocols can be synchronous or asynchronous. An asynchronous protocol sends a start signal prior to each code word, and a stop signal after each code word. UART is an asynchronous serial protocol supported by UART interfaces.

A synchronous serial protocol sends a clock signal on a dedicated wire. Additional wire(s) are required for data. I2C and SPI are synchronous serial protocols.

3.1.3. LPC1768 Serial Interfaces

The LPC1768 microcontroller provides the following serial interfaces (LPC1768 Manual):

  • Two Synchronous Serial Port (SSP) controllers, SSP0 and SSP1, with multi-protocol capabilities. They can operate as SPI, 4-wire TI SSI, or Microwire bus controllers.

  • A Serial Peripheral Interface (SPI) controller. SSP0 is intended to be used as an alternative for the SPI interface. SPI is included as a legacy peripheral.

  • Three enhanced Inter-Integrated Circuit (I2C) bus interfaces, one supporting the full I2C specification, and two with standard port pins. I2C is pronounced I-squared-C.

  • Four UARTs.

  • A two-channel CAN controller.

  • Ethernet MAC with RMII interface and dedicated DMA controller.

  • USB 2.0 full-speed controller that can be configured for either device, host, or OTG operation with an on-chip PHY for device and host functions and a dedicated DMA controller.

In this experiment, we will use the SSP interface configured for the SPI protocol.

3.1.4. Serial Peripheral Interface (SPI)

SPI is a four-wire, full-duplex, master-slave bus that was created by Motorola. There can be only a single master. Multiple slaves are allowed with individual slave select (SS or SSEL) lines. The four wires are:

  1. SCLK: Serial Clock (output from master)

  2. MOSI: Master Output, Slave Input (output from master)

  3. MISO: Master Input, Slave Output (output from slave)

  4. SSEL: Slave Select (active low, output from master) — one per slave

The microcontroller is usually the master. It uses the MOSI pin to send data, and the MISO pin to read data. The SCLK pin dictates the transmission rate; a bit is sent/received every clock pulse. A simple timing diagram for writing data is shown below.

SPI Timing
Figure 3. Timing diagram for writing data on a SPI bus

The slave select (SSEL) signal is used to select the slave in a data transfer. SSEL is active low: it must be low before the transaction begins, and must stay low for the duration of the transaction.

To connect multiple slaves, you need a dedicated SSEL for each slave. All slaves can share the remaining wires.

Even though the SSEL signal is a part of the SPI protocol, it is not uncommon to leave its control to the software instead of the SPI/SSP controller. The LPC176x manual states that "This signal is not directly driven by the master. It could be driven by a simple general purpose I/O under software control." In the LPC1768 mbed Board, SSEL is connected to GPIO P0.16. It should be driven low (by software) prior to placing data in the Data Register (DR), and then switched back to high when the transmission is complete.

3.1.5. Using SSP/SPI in LPC1768

The section describes how to use the SSP interface of the LPC1768 microcontroller as an SPI interface by listing the involved registers and their functions.

Data Register (DR)

The data to be sent serially must be loaded into the SSP Data Register (LPC_SSP1→DR). The serial transfer rate is controlled by the SSP clock as described below.

The LPC_SSP1→DR register has both a transmitter FIFO and a receiver FIFO.

To transmit the value stored in x, you can use:

LPC_SSP1→DR = x;

Similarly, to receive a new value and store in x, you can use:

x = LPC_SSP1→DR;

Every time you send data by writing to the LPC_SSP1→DR register, some data are also received in that same register. Make sure you read that data to clear the receiver buffer.

Also, to be able to receive something from a slave, you need to trigger the two way communication by putting dummy data in the DR.
SSP Control Registers

There are two control registers for the SSP1 interface (see LPC17xx.h):

  1. SSP1CR0: can be accessed as LPC_SSP1→CR0

  2. SSP1CR1: can be accessed as LPC_SSP1→CR1

The CR0 register has 5 fields:

  1. Data size (bits 0-3): the number of bits transferred in each frame.

  2. Frame Format (bits 4-5): the serial protocol to be used.

    00

    SPI
    01

    TI
    10

    Microwire
    11

    Not supported

  3. Clock Out Polarity (bit 6): should be 1 in our application.

  4. Clock Out Phase (bit 7): should be 1 in our application.

  5. Serial Clock Rate (SCR) (bits 8-15): used with the Clock Prescale Register (CPSR) to control the SSP clock. This is crucial when the SSP peripheral requires a specific value or range of frequencies.

The CR1 register has 4 fields, the most crucial of which is bit 1: SSP enable.

In addition to CR0 and CR1, there is the SSP Clock Prescale Register (CPSR). The CSPR register contains a single field, CPSDVSR, in bits 0-7. Its remaining bits are reserved (unused).

The SSP clock frequency is calculated using the formula:

\[\text{SSP frequency} = \frac{\tt PCLK}{\tt CPSDVSR \; (SCR + 1)}\]

The SSP’s CPSR register must be properly initialized. Otherwise, the SSP controller will not be able to transmit data correctly.

For details, see Tables 371, 372, and 375 in the LPC176x manual.
Exercise

What values of CPSDVSR and SCR will result in the highest SSP frequency?

Exercise

If the frequency of PCLK is 25 MHz, what would be the shortest possible amount of time to generate eight SCLK pulses?

3.2. Using the ADXL345 Accelerometer

The ADXL345 chip is a system-in-package featuring a 3D digital linear acceleration sensor. It includes both I2C and SPI interfaces. It also can be configured to generate an interrupt signal for activity and inactivity, sensing detect the presence or lack of motion by comparing the acceleration on any axis with user-set thresholds. The accelerometer part can be enabled or put into power-down mode.

To be able to conveniently use the ADXL345 chip, we will be using the ADXL345 carrier module/board.

3.2.1. Accelerometers

An accelerometer is an electromechanical device that will measure acceleration forces. These forces may be static, like the constant force of gravity pulling at your feet, or they could be dynamic, caused by moving or vibrating the accelerometer.

An accelerometer can help your project understand its surroundings better. Is it driving uphill? Is it going to fall over when it takes another step? Is it flying horizontally? A good programmer can write code to answer all of these questions using the data provided by an accelerometer. An accelerometer can even help analyze problems in a car engine using vibration testing.

In the computing world, IBM and Apple have been using accelerometers in their laptops to protect hard drives from damage. If you accidentally drop the laptop, the accelerometer detects the sudden freefall, and switches the hard drive off so the heads don’t crash on the platters. In a similar fashion, high-g accelerometers are the industry standard way of detecting car crashes and deploying airbags at just the right time. [accelerometers]

3.2.2. The ADXL345 SPI Interface

The ADXL345 chip provides an SPI interface with the device acting as a slave on the SPI bus. It allows writing and reading the registers of the device. The serial interface interacts with the outside world through 4 wires: CS, SCL, SDA and SDO.

Check the ADXL345 datasheet. Read the `SPI bus Interfaces` section to find out how to read from and write to the registers of ADXL345.

3.2.3. Using the ADXL345 Accelerometer

The accelerometer measures acceleration along the three dimensions, and makes them available in the following registers:

DATAX0 (32h) LSB, DATAX1 (33h) MSB

X-axis acceleration data. The value is expressed in 10 bits as 2’s complement (right justified).

DATAY0 (34h) LSB, DATAY1 (35h) MSB

Y-axis acceleration data. The value is expressed in 10 bits as 2’s complement (right justified).

DATAZ0 (36h) LSB, DATAZ1 (37h) MSB

Z-axis acceleration data. The value is expressed in 10 bits as 2’s complement (right justified).

The Directions of the Three Accelerometer Readings figure shows the directions corresponding to positive values along each of the three axes, relative to the chip.

Directions of the Three Accelerometer Readings Relative to the Chip
Figure 4. Directions of the Three Accelerometer Readings

You must configure the POWER_CTL register in order to read the accelerometer data.

Reading data from the accelerometer device is completed in 16 clock pulses. Thus, in order to read the data correctly from the registers, you have 2 options: send multiple 8-bit data, or send 16-bit data. The description is as follows:

  1. Send the first 8 bits, which include the read/write bit and the address bits of the register that you want to read. As a result of generating the clock pulses required to send this byte, you will receive dummy data. Then, send another 8 bits of dummy data just to generate the required clock pulses to receive the requested 8-bit data.

  2. Send 16-bit data, where the first 8 bits include the read/write bit and the register address, and the next 8 bits contains the data to write, in case of a write command, or dummy data if you are reading.

4. Tasks

  1. Use the LPC1768’s SSP/SPI interface to read the accelerometer data from the ADXL345 device.

  2. Write a simple application to indicate different stationary positions. For example, indicate whether the device is tilted to the right or to the left, tilted forward or backward, and whether it’s facing upward or downward. Use some output device to reflect this data in real-time. The following table summarizes the readings corresponding to each of the six stationary positions.

    Stationary Position Ax Ay Az

    Z down

    0

    0

    -

    Z up

    0

    0

    +

    Y down

    0

    -

    0

    Y up

    0

    +

    0

    X down

    -

    0

    0

    X up

    +

    0

    0

5. Grading Sheet

Task Points

Operate a seven-segment display using the SSP/SPI interface

7

Discussion

3

Resources