Keil MCB1700 Serial/USB Hub

My name is Vincent, I am a second year student at the University of Waterloo in Electrical Engineering. In undergraduate labs at the university, we use Keil MCB1700 Evaluation boards to develop skills in ARM Assembly Language. The MCB1700 requires 2 USB ports and 2 serial RS-232 connectors in order to use the debugging interface and download flash. A robust improvement to this board would be to design a shield that attaches to the serial ports on the MCB1700, which takes the 2 USB connections and the 2 serial connections as inputs and has one USB output to the PC. This would enable the Keil boards to be functional even when used on new compact computers such as the Intel NUC with limited USB ports and no serial ports. 

For this project, there are two main components. The first being handling the upstream and downstream USB ports of the hub, and second being converting the dual serial ports into a single USB. 

The TI TUSB4041I is a 4 port USB 2.0 Hub which supports high-speed and full-speed connections on the upstream port. The IC requires 1.1 V and 3.3V supply voltages. A USB 2.0 hub was chosen since it supports all Keil board functionality. Due to current requirements, a USB 3.0 type B receptacle will be used to support up to 900mA of current though the 5V line of the upstream USB port. The TUSB4041I will have 3 additional USB 2.0 type A receptacles, one of which will support power for the Keil Board. This board also uses a TPS2553 precision adjustable current limited power switch to control power to individual USB ports. The IC will be customized using I²C EEPROM.

The FTDI FT2232H is a dual UART/FIFO to USB 2.0 high-speed converter IC. It will use an EEPROM and two SP3243 RS-232 transceivers to convert the RS-232 UART Keil interface into a single USB 2.0 I/O going into the TUSB4041I. 

The ICs above, combined with LDO regulators will draw a total current of 760mA. Since USB 3.0 can only supply up to 900mA, it would not be feasible to incorporate an additional powered USB port for user charging, nor practical to use a USB 3.0 hub IC as its power and speed potential would be limited by the maximum allowed current draw.

As recommended by TI, the final board will be 6 layers, and comply with all requirements to support maximum signal integrity of the USB differential pairs. These differential pairs require a differential impedance of 90 Ohms, and the trace widths and separation have been calculated based on the stack-up and thickness of the final board. For this reason, the distance between the top copper signal layer and the adjacent ground plane must be approximately 0.28mm, using 1 oz. copper for internal and external layers. 

To develop this project, I have used a free software called Diptrace to develop footprints, schematics and a board layout.

This board design has taught me a lot about PCB design, serial protocol, signal integrity, and the process of such a project. If successful, this design could be considered by the University as a feasible solution to using the Keil MCB1700 with modern day computers and extend the usage life of the board. The education sponsorship program at PCB Way is greatly appreciated, and this project can not come to completion without their support.

3D Image from Diptrace of the design, Keil MCB1700, Intel NUC