Even the smallest system board of a DSP includes power supply, clock, Io, and JTAG circuits. In order to carry out more convenient modular design, we adopt a top-down design idea. First, plan the overall minimum system, which includes six sub-modules:
- DSP itself: On the A4 size drawing, 176 pins and QFP packages 335 are used. The Protel dxp software does not directly have a 28335 principle image library, but it is not difficult. I extracted a 176-pin square schematic package from the schematic library of another model DSP, then, the definitions and network labels of each pin are modified accordingly.
- Decoupling Network: This part involves many capacitors, mainly for decoupling of various power supply pins.
- Start Mode Selection: one is the bootmode selection, which can be achieved by using the dial switch with the pull-up and drop-down circuits; the other is the selection of clock signals, here we mainly choose the source crystal oscillator or the passive crystal oscillator. We can also add some test circuits with LEDs, such as red LED lights when the power-on is successful. When the LED was selected, the small led was too bright, causing glare after each power-on.
- Io expansion: Because the smallest system board is used here, I/O is not processed too much. I/O is cited as a double-row plug-in as much as possible to facilitate subsequent expansion.
- JTAG circuit: This is mainly used to select the typical capacitance and resistance values. The detailed requirements in the device manual will be discussed later. The design of the reset circuit is also described below. After comparing the foreign demo with the previous Board, we found that there is a significant difference here, that is, foreigners use ESD Protection (TVS diode) in the reset circuit ); no wonder the board we made before is so fragile that it is slightly inappropriate to break down the device.
- Power Circuit Design: This is a key component and is directly related to whether the entire system can work normally. In the past, DSP, for example, also involved the issue of power-on sequence, which took some effort to deal. Fortunately, Ti provides us with a ready-made power supply solution, which can be applied directly.
The specific design of each part and some issues that I think need to be paid attention to will be continued next time.
