About why I open sourced the schematic diagram of this course design

My original plan was to open source this project when the course design progress is almost complete (around mid-April 2021), so why should I release it in advance? Because something happened that made me very angry.

The first version of the design has been completed on 2021/3/9, and the schematic was sent to the teacher for checking the circuit on 2021/3/18, which is the screenshot below.

but! ! !

2021/3/21 Someone from class one suddenly sent me this message

I don't know where he got the schematic diagram from. Since it has been leaked out, it is a problematic circuit anyway. It is a plan that I have abandoned. After thinking about it, I might as well forget it. Is this the end? No, think too much!

2021/4/1 My roommate told me that the people in the next class got not photos, but the entire PDF document! ! ! (Why do my roommates know it’s mine, because the Lichuang EDA schematic document has my signature underneath it) Moreover, people in the first class said that almost everyone has a copy! ! !

It took me two full days from checking the information to drawing the schematic diagram! The power supply part is okay. I participated in training camps organized by Tuya and Lichuang during the winter vacation and learned a lot from the big guys. Moreover, the data sheet of the power supply chip is marked, which is relatively simple. However, this is the first time I have used this core FPGA/CPLD chip, but there is little information on the Internet, and it is difficult to find the minimum system. I also made it by referring to the development board of Punctual Atom (which uses different chips). Good guy, you plagiarized directly without my permission, I'm really angry.

So I am now open-sourcing this schematic in advance!

The first version of the project has been covered by the second version , leaving only a PDF, which I will upload to the attachment. This open source project is the second version, but the one we use has been modified again. This second version does not draw the PCB or make the board for verification! ! ! After the project is accepted, I will open source the modified version (schematic diagram and PCB) currently used by our group.

This version generally has no problems. Let’s explain the circuit below.

1. Power supply

power input

Use the DC interface for power supply, and then connect it to a 6V/1.1A self-restoring fuse to protect the charging circuit and 5V power supply circuit behind it.

Lithium battery charging and discharging circuit

1. The charging circuit battery power management chip uses Fuman’s TC4056A, which has low cost and simple peripheral circuits. The charging current is set through R1, here it is set at 400mA. I misplaced the device in the schematic and used TP4056X, but the schematics and packages of the two chips are the same, so I didn't change it. Don't buy TP4056X based on the BOOM list, use TC4056A!!! 2. Lithium battery The charge and discharge protection also uses Fuman's chip - DW06D. Compared with another chip of his, DW01A-G, there is no need to use additional NMOS tubes, which reduces the difficulty of circuit design.

3. Lithium battery boost

This part was not included in the initial design. After sending it to the teacher to check the circuit, the teacher gave his opinion: a boost circuit for the lithium battery needs to be added. Check the data sheet of the lithium battery and the DC-DC chip used for voltage reduction later - TLV62568DBVR. The output voltage of the lithium battery is 3.6V~4.1V, and the input voltage to the DC-DC chip is indeed very reluctant, so a boost part was added. I chose SX1308 from Shuoxin Technology, or you can use MT3608 from Xi'an Aerospace Minxin. The packaging and peripheral circuits of the two chips are the same (the price of MT3608 is slightly lower). Even the data manuals have different brands and models. The content and layout are almost the same... The feedback resistors are R4=22K and R5=2.7K. The voltage of the power supply will be different between no-load and loaded. The voltage will drop after loading, and there are problems with the resistance accuracy and the power supply on the PCB. Regarding the voltage drop when changing layers, since I have never used this chip and do not know how big the actual voltage drop is, I increased the output voltage to a range that the chip used in the back-end buck circuit can withstand.

Power selection

This part is based on the design of the Tuya training camp boss ( mimiww's [Tuya Smart] IoT temperature and humidity sensor ). I have to say that this part is really cleverly designed, with a pull-down resistor, a PMOS, and a Schottky The diode can realize the power switching function, which can realize: ① When only DC power supply is provided, the PMOS is turned off, and DC_VCC supplies power to the subsequent circuit through the Schottky diode. Since the voltage drop of the Schottky diode is low, the actual measured voltage is about 0.3V, so the voltage obtained by VCC is about 4.7V. ② When only using lithium battery for power supply, R3 (either 10k or 100k) pulls the gate voltage down to 0V, PMOS is turned on, and +5V supplies power to subsequent circuits through the MOS tube. The actual measured voltage drop is very small. ③When DC power supply and lithium battery are connected at the same time, DC_VCC flows through D2, VCC≈4.7V, and the gate voltage is DC_VCC = 5V, PMOS is cut off.

step down

Use the TLV62568DBVR provided by the teacher. The specific circuit data sheet is also written in it, and it is easy to use. Reduce the input 5V voltage to 3.3V and 2.5V. There is a circuit loss problem, which will pull the voltage slightly higher. The 2.5V output is 2.52V, and the 3.3V output is 3.327V.

There is not much to say about the rest. It just meets the requirements of the course design and adds some innovations. The most important thing is that this is the first time I have come into contact with this EPM240T100C5N chip. I am not familiar with this chip myself, so I better wait for the third version to be tested successfully. Let’s talk about it later.

Again, I remind you that I did not draw the PCB or board for verification of this version of the design! ! ! After the project is verified and accepted, I will open source the modified (third version) schematic diagram and PCB actually used.

Attached is a PDF of the first version of the schematic