lab04

sensor reading tfmini lidar

contents

1. objective
2. tfmini lidar
3. tfmini sensor reader programming
4. implement tfmini data frame reader
5. control led based on distance
6. notes
7. how the tfmini sends data
8. tasks

objective

in this src, we will attach a small lidar sensor, which measures distance, to the hifive1 board via uart and develop software to read the sensor data.

tfmini lidar

• main product characteristics
• this product is based on tof (time of flight) principle and integrated with unique optical and electrical designs, so as to achieve stable, precise, high sensitivity and high-speed distance detection.
• it's operating range is from 0.3 m - 12 m
• the communication interface is via UART
• it's main applications is for drone altitude holding and terrain following, machine control and safety sensor, robot distance detection
• TOF is an abbreviation of time of flight technology and its operating principle is as follows: a modulated nearinfrared light is sent from the senor and reflect by an object; the distance to the object to be shot can be converted with the sensor by calculating the tie different or phase difference between the light sending and the light reflection, so as to produce the depth information.

the tfmini sensor

hifive1 rev b pinout

connections

tfmini	hifive1 board uart 0
tx (green)	rx pin 0 = gpio 0
rx (white)	tx pin 1 = gpio 17
5v (red)	5v line
gnd (black)	gnd line

• now let's look at the datasheet of the tfmini lidar in order to find out how to retrieve data from the sensor. the data exchange format of the sensor is described in section 6.1 standard data format serial port of teh data sheet

• tfmini datasheet

serial port data communication protocol and line sequence

• the following section describes the connections and communications between TF_mini and external devices, including the encoding format of sending data, the communication protocol between the module and the external devices, the hardware line connection indications, and related precaustions.

communication protocol	UART
baud rate	115200
data bits	8
stop bits	1
parity	none

• baud rate is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modulation rate in symbols per second or pulses per second

standard data format of serial port

• the module data is a hexadecimal output data
• each frame data is encoded with 9 bytes
• including 1 distance data (Dist)
• each distance data has corresponding signal strength information (strength)
• the frame end is the data parity bit

byte1-2	byte3	byte4	byte5	byte6	byte7	byte8	byte9
0x59 59	Dist_L	Dist_H	Strength_L	Strength_H	Resered	Raw.Qual	CheckSum_L

data type	data encoding intrepretation
Byte1	0x59 frame header, all frames are the same
Byte2	0x59 frame header, all frames are the same
Byte3	Dist_L distance value is low 8-bit, note the distance value is a hexadecimal value for example, Distance 1,000cm = 03 E8(hex)
Byte4	Dist_H distance value is a high 8-bit
Byte5	Strength_L is a low 8-bit
Byte6	Strength_H is a high 8-bit
Byte7	Reserved is a reserved byte
Byte8	Raw.Qual is the original signal quality degree
Byte9	CheckSum_L parity bit is a low 8-bit, checksum = Byte1 + Byte2+ ... + Byte8, checksum is the sum of the first 8 bytes of actual data here is only a low 8-bit

• in this sensor, each measured data is encoded in a 9 byte data frame.
• the first two bytes Byte1 & Byte2 are frame headers 0x59 = Y is ASCII.
• the next two bytes Byte3 & Byte4 encode the actual distance data (in centimeter).
• Byte3 (Dist_L) is the low 8 bits
• Byte4 (Dist_H) is the high 8 bits of the measured distance data 16 bit
• the rest of the bytes of the data frame are not used in this lab
• note that the sensor operates at 100 Hz generating one data frame (distance measurement) at every 10 ms

tfmini sensor reader programming

you will re-use the ser_read() function you implemented in Lab 2 or use the provided ser_read() function in the souce code you downloaded. you only need to modify eecs388_tfmini.c

implement tfmini data frame reader

the first task is to interact with the tfmini sensor via UART0 to parse the sensor's distance data. note that the distance data is stored in Byte3 and Byte4 of the sensor's data frame. so you need to do a bit of data manipulation to ontain the distance value. note that the variable dist should contain the obtained distance value. you will then print the value to the console (i.e. serial monitor), by using either printf, sprintf, and ser_printline combination.

reference eecs388_tfmini.c reference bitwise operations

#include <stdio.h>
#include <stdint.h>

#include "eecs388_lib.h"

int main()

    gpio_mode(RED_LED, OUTPUT);
    gpio_mode(GREEN_LED, OUTPUT);
    ser_setup();

    ser_printline("setup complete\n");

    uint16_t dist = 0;

    while (1) {

        if ('Y' == ser_read() && 'Y' == ser_read()) {

            uint8_t dist_1 = ser_read();
            uint8_t dist_2 = ser_read();

            dist = (dist_2 << 8) | dist_1;

            if (dist < 50) {
                gpio_write(RED_LED, ON);
                printf("distance: %d cm\n", dist);
            } else {
                gpio_write(GREEN_LED, ON);
                printf("distance: %d cm\n", dist);
            }

        }

    }

control led based on distance

notes

• light detection and ranging lidar[1]

• it is an instrument to measure proximity or distance

• tfmini is a lidar sensor module

• connects and communicates using UART

• sends you an angle value from 0 to 180, 30 degrees at a time

• all it needs to send is the on and off time

how the tfmini sends data

• the actual distance measure is 16 bit number
• this 16 bit number is sent in two 8 bit chucks
• byte 3 is lower 8 bits, byte 4 is higher 8 bits

tasks

• connect tfmini to hifive board
• careful with the devices
• implement tfmini data frame reader
• contol led based on distance, prints the distance
• submit the eecs388_tfmini.c file