Design of ISP download line for AT89S series microcontroller

1 Introduction

With the increasing application of single-chip microcomputers, in order to reduce the cost and difficulty of single-chip microcomputer system development, many single-chip microcomputer manufacturers have launched single-chip microcomputers that support ISP technology, including the AT89S series single-chip microcomputers produced by the mainstream manufacturer ATMEL. ISP (InSySTem Programming) is online system programming. This technology allows users to erase and write the final program code to the single-chip microcomputer on the circuit board, eliminating the need for emulators, programmers and other development tools required in the traditional development process, and enables users to upgrade the software of the single-chip microcomputer online.

ISP technology needs to be implemented through ISP download cable and corresponding program download software. At present, most of the download cables are parallel port download cables, but the parallel port has been gradually eliminated by the market. Many notebook computers and desktop computers now have cancelled the parallel port configuration. Therefore, the ISP download cable with USB interface has become the mainstream. This paper proposes a design scheme of ISP download cable based on USB interface, which has low cost, simple circuit, easy to make and convenient to use.

2 Design Solution Description

The design scheme is based on ATMEGA8 microcontroller, and the specific circuit schematic is shown in Figure 1.

Figure 1 ISP download line circuit schematic

The ATMEGA8 microcontroller is a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISC structure. The pinout of the PDIP package is shown in Figure 2.

Figure 2 ATMEGA8 pin diagram

In Figure 1, the ATMEGA8 microcontroller performs software simulation of the USB serial port protocol. Combined with the computer driver and target program download software, it can connect to the computer's USB interface and erase and write code to the target device in ISP mode. The definition of the ISP port in the circuit diagram is shown in Figure 3.

3 Implementation of the design

3.1 Hardware welding

Purchase components as indicated in Figure 1 and complete the soldering of the ISP download line hardware circuit board. The component list is shown in Table 1.

Table 1 Components List

3.2 Writing firmware

After the download line circuit board is soldered, the USB serial port simulation protocol and the underlying program of the ISP download mode need to be written to the ATMEGA8 on the circuit board (i.e., write the firmware). Before writing the firmware, three preparations must be made: compile the underlying program into a target file (HEX format file), prepare a parallel port ISP download line, and install the MCU program download software (such as ISPLAY 1.5) on the computer. After the preparations are completed, you can write the firmware. The specific steps are as follows:

(1) Make hardware connections as shown in Figure 4.

Figure 4 Schematic diagram of hardware connection for writing firmware

It is worth noting that J1, J2, and J3 are all shorted at this time. J1 is a speed reduction jumper, which is slow when shorted, and it is required to be in slow speed when writing firmware; J2 is shorted to supply power to the original parallel port download line; J3 is the ATMEGA8 reset jumper, which is required to be in reset state when writing firmware.

(2) Run the MCU program download software on the computer to write the firmware.

The software interface is shown in Figure 5. It is worth noting that after writing the firmware, the fuses of ATMEGA8 should be configured, that is, the SUT0 and CKSEL0 options in Figure 5 should be configured.

After the firmware writing operation is completed, disconnect J1, J2, and J3. When using it for the first time, install the USB device driver on the computer according to the prompts and it can be used normally. If you need the relevant driver software package, please contact the author.

Figure 5 Schematic diagram of writing firmware software operation

4 Conclusion

The USB interface ISP download cable designed according to the above scheme can effectively program the ATS series microcontrollers. If the firmware is upgraded, it can also program the AVR series microcontrollers. In addition, due to the use of the USB interface, it is easy to use, low cost, stable performance, easy to implement, greatly reducing the hardware cost of the microcontroller system development, such as mass production, has a certain market value.