Three classic questions
Q: What are HC-05 and HC-06?
A: They are modules that replace the UART byte stream with a classic Bluetooth wireless link. There is a section on both sides, and the middle section is invisible.
Q: Why should we replace CH340 with Bluetooth?
A: CH340 allows the car to connect to the computer, and Bluetooth allows the car to leave the desktop – the mobile phone in your hand is the host computer.
Q: Common usage scenarios of serial Bluetooth?
A: Mobile phone remote control, wireless parameter adjustment, on-site log viewing, and any occasion where “there should be no wires between the host and the car”.
When adjusting the serial port on the desktop, CH340 is very useful: plug a USB cable into the computer, turn on VOFA+, connect PA9 and PA10, send a string of hexadecimal, and the microcontroller will return the packet immediately.
After the car is placed on the ground, the USB cable becomes a towing rope. When the car moves, the computer, cables and batteries are dragged along, and your hands have to prevent the cables from getting entangled in the wheels. To make a car that can really run, this thread must be removed sooner or later.
The cost of getting rid of it is less than you think. The one last time AA 55 ... SUM8 0D There is no need to change a single byte of the frame. What is sent by the mobile phone is still the same string of hexadecimal – what is replaced is just “who sent the bytes to USART1”.
The serial port is not a thing, it is an agreement
You need to think this sentence through first, because there is almost no code to write later.
USART1 It’s not a path to computers. it’s just PA9 and PA10 A set of conventions on the two pins: how the high and low levels represent one bit, how many bits there are in a frame, and how many bits per second.Where it really “leads” depends on what is connected to these two pins.
- Connect to CH340: These two pins lead to the USB port of the computer;
- Connect to HC-05 or HC-06: these two pins are connected to the air, and the other end is the mobile phone.
So there are two ways to connect the same serial port, but the program sees the same thing:
| What’s in my hand | what is it actually |
|---|---|
| CH340 | USB ↔ serial port converter |
| HC-05 / HC-06 | Bluetooth (SPP) ↔ serial port converter |
In the Bluetooth profile,SPP The name is actually very straightforward: Serial Port Profile, which means “pretending to be a serial port cable.” The module receives the wireless data sent from the mobile phone, turns around and TXD delivered to PA10;STM32 from PA9 The bytes sent out are turned into wireless data and sent back to the mobile phone.In the middle part of the flight, the program is completely invisible.
So CH340 and HC-05 are the same thing in the eyes of the program: a UART byte stream. The protocol is still the same frame header, length, checksum and state machine as last time, and the business layer is still the same six commands, without changing a single line.
Wiring: four wires, and three questions that must be thought through first
蓝牙模块 VCC -> 5V
蓝牙模块 GND -> GND
蓝牙模块 TXD -> STM32 PA10（USART1_RX）
蓝牙模块 RXD -> STM32 PA9 （USART1_TX）
TX and RX should cross. The TX sent by the module should fall to the RX of the microcontroller; if both sides are connected to TX, it means that both people are talking and no one is listening.
The first problem is power supply. The electricity on the car is this chain: 12V battery → power switch → buck/power distribution module → 5V, 3.3V outputs, all of which share the same ground. Bluetooth module VCC Just take it from this line.
The bottom plate usually comes with its own voltage regulator.VCC Connecting to 5V is a common practice; there are also modules that only consume 3.3V.Follow the instructions on the board you havedon’t think that “it can be used if connected to 5V” means that all modules can be used.
What should be noted is the current: the Bluetooth module usually only has a few milliamperes, but it can reach tens of milliamperes when it is transmitting. When it shares a 5V channel with the motor, the voltage may be pulled down when the motor starts, and the module is reset first – this is exactly what the fourth experiment will focus on (current and voltage are sometimes very “metaphysical”, and this may not necessarily happen).
The second issue is level. STM32 is 3.3V logic, modular TXD The output is generally 3.3V, which is safe. in turn RXD Whether this end can accept a higher level depends on whether the backplane has voltage division or level conversion: common baseboards can be directly connected to 3.3V, some with voltage divider can be connected to a 5V serial port, and bare package ones RXD Read the manual before deciding whether to feed 5V directly.It’s also about checking the instructions, not guessing.(Although the modules I bought directly seem to have never encountered problems that cannot be connected)
The third issue is common ground. The high and low levels of the Bluetooth module must be the same reference as STM32; if only VCC, TX, and RX are connected without GND, the module’s light may remain on, but the communication will be like shouting in water.
The difference between HC-05 and HC-06
Remember one sentence first:HC-05 can usually be configured as a master or a slave, while HC-06 usually only acts as a slave, waiting to be connected.
| HC-05 | HC-06 | |
|---|---|---|
| Role | Can be set as master/slave | Generally only used as slave machine |
| Configuration method | AT command, complete functions | AT command, few items; different firmware writing methods vary greatly |
| Enter configuration | Generally rely on KEY/EN The button is pressed and held during power-up |
Generally no KEYuse AT directly when not connected |
| Who connects whom | Can be connected to both a mobile phone and another HC-05 | Wait for a mobile phone or a host to connect |
| Suitable for whom | Want to study master-slave and module-to-module | I just want my phone to connect to the car |
The firmware of baseboards from different manufacturers has changed the name, echo and parameter format, soAT instructions cannot just memorize an online form. Do three things first: write down the lighting rhythm when the module is powered on, check the default baud rate/name/password written on the product page, and send a sentence using the current serial port parameters. AT See if it comes back OK。
There is also a platform difference on the mobile phone that needs to be explained clearly first:iPhone cannot use HC-05/HC-06. iOS does not open the SPP of classic Bluetooth, only BLE; these two modules use SPP. Therefore, for this experiment, please use an Android phone with a “Bluetooth serial port” type App. For Apple devices, you need to find another way (MFi certification module, or change to BLE solution). There is no way around this step. Make sure the phone in your hand can be used before proceeding.
AT mode: let the module learn our parameters first
AT mode is not “stronger signal mode”. It means that the module does not connect to the mobile phone temporarily and reads the received UART bytes as configuration commands.
The entry method is different for each board, but the reproducible process is as follows:
断电
-> 按说明按住 KEY/板载按键（HC-05 常见做法）
-> 上电，观察指示灯节奏
-> 用 CH340 接模块的 RXD/TXD（仍然是交叉接）
-> 发 AT，确认回 OK
-> 一次只改一个参数
-> 退出 AT，重新上电
-> 再用手机搜索和连接
CH340 is still useful during configuration, but its identity at this time is not the link of the car, but the configuration tool of the module. After configuration, connect HC-05/HC-06 back to STM32.
Commonly used instructions are roughly as follows. The specific writing method is subject to the instructions of the firmware on hand:
| instruction | what do you want to do | Notice |
|---|---|---|
AT |
Make sure the module is listening | times OK Only then has it entered AT mode. |
AT+NAME=ZW-CAR |
Change Bluetooth name | Some firmware is AT+NAMEZW-CAR |
AT+PSWD=1234 |
Change pairing password | Default common 1234 or 0000 |
AT+UART=115200,0,0 |
Change communication baud rate and other parameters | Commonly seen on HC-06 are AT+BAUD8(8 corresponds to 115200) |
AT+ROLE=1 |
Set as host (HC-05) | After setting, power on again |
AT+RESET |
Let the module restart | Common ways to make parameters effective |
No reply to the order OK Don’t try changing ten commands in a row. Confirm three things separately:
- Is it really in AT mode?——The rhythm of the light is different from that of normal waiting for connection;
- What is the current baud rate?——The baud rate in AT mode is often different from the communication baud rate. Many firmwares use 38400 in AT mode and set another one during communication; the parameters on the CH340 end need to be changed accordingly;
- Do you want to enter and change the line?——Some firmware requires each command to be followed by
\r\nsome do not require it.
One more thing must be aligned:The communication baud rate of the module must be consistent with that of STM32. We have set this link at 115200 from the beginning, and the common communication baud rate when the module is shipped from the factory is 9600 – the serial port parameters of many Bluetooth routines on the Internet are written as 9600, which is the default value of the module. You have two options: change the module to 115200, or initialize the serial port Uart_Init() inside 115200 Change to 9600. It’s fine if both sides are consistent, but don’t let them work independently, otherwise the screen will be full of garbled characters.
Code: No need to change a single line
It’s the easiest here. The current main loop looks like this (last time, in order to hand over the work of moving bytes to DMA, the ring buffer was replaced by two buffers;Key_Tick() Still in the 1ms interrupt,Mode_Loop() Still in the main loop):
static uint8_t ready_index;
static uint16_t ready_length;
static uint16_t i;
while (1)
{
Mode_Loop();
if (rx_ready != 0U)
{
ready_index = rx_ready_index;
ready_length = rx_block_length;
rx_ready = 0U;
for (i = 0U; i < ready_length; i++)
{
Protocol_Service(rx_block[ready_index][i]);
}
}
}
Regardless of whether these bytes are DMA from USART1->DR Those who moved here were moved in manually one by one from the interruption.Protocol_Service() What is received is always “one byte”. It searches according to the state machine set last time AA 55read length, count SUM8neither knows nor needs to know whether these bytes just sat on USB or radio.
The same goes for the business layer. The first two branches of the distribution function last time (speed_percent It is the default speed of the file scope last time.static int16_t speed_percent = 30;）：
static uint8_t CarCommand_Execute(uint8_t command, const uint8_t *payload, uint8_t length)
{
switch (command)
{
case CAR_CMD_STOP:
Motor_Stop();
LED_WriteOne(0, 0U);
return 0U;
case CAR_CMD_FORWARD:
Motor_SetTargetPercent(speed_percent, speed_percent);
LED_WriteOne(0, 1U);
return 0U;
...
Motor_SetTargetPercent() What is given is the percentage of the left and right wheels, and the negative number is the reversal;Motor_Stop() Stop both roads;LED_WriteOne(0, 1U) It is to write the 0th external lamp as bright. None of them knew where the order came from. They only know “Someone told me to stop” or “Someone told me to go forward.”
Changing the link does not change the protocol, changing the protocol does not change the service——The previous articles were separated layer by layer, so that I don’t have to worry about it this time.
What to send in the mobile app
The Bluetooth serial port App on your mobile phone must have one skill:Send hex. Apps that can only send text cannot do this – AA 55 ... When text is copied, characters are sent out 'A'、'A'spaces and numbers, the bytes received by the microcontroller are another matter entirely.
If you can send hexadecimal, just use the frame on hand. These are the most commonly used:
停止 AA 55 01 01 01 00 01 04 0D
前进 AA 55 01 01 02 00 01 05 0D
设速度 100 AA 55 01 01 06 01 01 64 6E 0D
After sending it, the board will reply with a response, for example, the one corresponding to the speed:
AA 55 01 81 06 01 01 00 8A 0D
The checksum algorithm remains unchanged: the device ID is added up to the last content byte, leaving only the lower 8 bits. If you want to change the speed, change the content byte to the percentage you want, and then recalculate the checksum.
Let the phone find it first
The device cannot be found in the Bluetooth settings of the mobile phone, which is the easiest way to lead people into a dead end. Let’s break the question into several paragraphs:
灯不亮 -> 先查 VCC/GND、底板稳压和供电电流
灯快闪但搜不到 -> 模块没在可发现状态，或已经被别的设备连着
能搜到但配对失败 -> 密码不对、手机缓存了旧配对记录、模块角色不对
刚连上马上断开 -> 供电掉压、模块复位、手机 App 主动关掉了连接
连上但数据乱码 -> 两边波特率、校验位、停止位不一致
能连上、发帧没反应 -> App 发的不是十六进制，或者模块没接到 PA9/PA10
“Automatic disconnection” is not necessarily due to the Bluetooth protocol. The current change when the motor starts may cause the 5V line to drop instantly. The Bluetooth module restarts first, and what the phone sees is disconnection. Observe the motor power supply and Bluetooth logic power supply separately, or remove the motor first and only do desktop transceiver. You can quickly separate the power supply problem from the protocol problem.
Five field experiments
Experiment 1: CH340 and Bluetooth alternate access
The same program, just change the link:
- CH340 pick up
PA9/PA10the computer sends a speed frame; - Power off and unplug CH340;
- HC-05/HC-06 press TX/RX to cross-connect and the mobile phone sends the same frame.
The number of correct frames, the number of incorrect frames, and the last command on the screen should change in the same way. The only thing that changes is “who sends the bytes to USART1”.
Experiment 2: Bluetooth pairing on the computer, VOFA+ still looks at the curve
This one bypasses the limitations of the mobile app: let the computer be the host.
Pair with HC-05 in Windows (the password is the pairing password of the module). After pairing is complete, in the device managerThere are usually two “standard serial over Bluetooth link” COM ports.——One is for connection, one is not, choose the one that can be connected. Then select this COM port in VOFA+ and align the parameters with the baud rate of the module.
The curve will still come out. What this means is:VOFA+ only recognizes the COM port, and it doesn’t care whether the COM port is behind a USB cable or Bluetooth. The functions that sent the data onto the curve last time have not changed a word here.
Experiment 3: Deliberately change the STM32 baud rate
Set the module to 115200 and temporarily change the project to 9600. The phone still displays “Connected”, but the received code will be garbled, and the binary frame will not pass the verification. Change it back to 115200, re-burn, and send another stop frame.
Bluetooth connection status and UART data status are two different things. If the mobile phone shows that the connection is successful, it only means that the wireless link is established, but it does not mean that the serial port parameters on both sides are consistent.
Experiment 4: Disconnection caused by motor starting
First connect only the Bluetooth module and STM32, and send stop, speed, and stop messages continuously to see if the connection is stable. Then connect the motor driver, let the motor start in the overhead state, and continue to send the same frame.
If “the phone cuts off as soon as the motor starts”, use a multimeter to measure the Bluetooth power supply. First check the motor power supply, buck module capacity, ground wire and power supply capacitance. Don’t modify the protocol state machine first – it has no chance of receiving the next byte.
Experiment 5: Distance, occlusion and walls
Place the car on the table, slowly move the phone away, and at the same time send a stop frame at regular intervals to see when the number of error frames on the screen starts to increase. Try using a wall or putting the phone on the other side of your body.
The nominal distance of classic Bluetooth is a number under “ideal conditions”, which actually depends on antenna placement, obstruction, congestion of the 2.4G frequency band, and the transmit power of the module itself. This experiment doesn’t give you an exact distance, it gives you one thing:How far away can you be sure the car will stop? This range is the real number to remember when remote controlling a car.
Command sequence when landing
I keep the stop frames handy when it comes to actually getting the car running. First, put it in the air and send forward frames to see if the wheel direction is correct; then put it on the ground and start the speed from a very small value. Set the stop frame as an obvious shortcut button in the mobile app. Don’t let the stop command be hidden in a dozen lines of history.
The corresponding relationship between the commands and the two wheels is still the six from the previous episode: stop, move forward, move backward, turn left, turn right, and set speed. The steering depends on “one wheel is slow and the other is fast”. The actual duty cycle must be retried based on the drive board, motor, tires and ground – below a certain duty cycle, the motor cannot overcome static friction on the ground, and the same command will not give the same result when it is overhead and on the ground.
Finally the orange cat said
If you look at these layers separately, changing modules is not scary:
配对、名称、密码、AT 配置 -> 模块自己管
波特率、校验位、停止位 -> UART 这一层管
帧头、长度、SUM8、重同步 -> 协议这一层管
前进、停止、差速、PID -> 小车业务这一层管
HC-05/HC-06 do not make the serial port disappear: they only replace the physical link “CH340 to computer”. So put SUM8 There is no need to relearn AT commands when upgrading to CRC16, and there is no need to rewrite the motor code when changing to a Bluetooth module.
There are other wireless solutions on the same link, but they are not connected to the same layer: some are point-to-point modules of the SPI interface, which have their own packet formats and do not go through the serial port; some are Wi-Fi or LoRa, each of which solves different combinations of distance, bandwidth and power consumption. Bluetooth’s place here is clear –It is responsible for shortening a section of the serial port line into air, and the rest is not under its control.
Finally, there are two of the most common questions of this kind. Let’s answer them here first:
- “I have two modules. Can I let them connect by themselves without using a mobile phone?” Yes, but you need to pair characters: one of the two HC-05s must be set as the host (
AT+ROLE=1) and bind the other party’s address; an HC-06 can only be used as a slave, waiting for a host to connect to it. The specific way of writing the binding address is different for each firmware, please follow the instructions. - “It’s connected, but the microcontroller doesn’t seem to have received anything?” Check three things in order: Is the app sending hexadecimal and module information?
TXDDid you really receive it?PA10Are the baud rates on both sides consistent? These three things account for the majority of this type of problem.
