Altium Designer promotes hierarchical design, which seems to make sense. It can help manage large designs and encourage design reuse. Those concepts have long been prevalent in software design. In a true hierarchical design, the local nets should stay local, but Altium seems to have problems with sheets with same named local nets. We are forced to define unique names to different sheets.
The ports are considered differently from the wires; so the port names do not become signal names even when the wires are not named. This may be a little unexpected. If we think of ports are module interface declaration, inside the module, the port name should be used as signal names. More confusion is created when the nets are flatten: what net names are ultimately used if nets are assigned names in the hierarchy. Altium would even complain about same net names in different hierarchy. There does not seem to have a well defined way of resolving net names.
Another thing that could cause confusion is the netlist option "Allow Ports to Name Nets". If we try to connect nets by name, we find Altium actually consider the net with net label and the net connect to the port with the same name to be different in PCB while showing exactly the same name. The rule for flattening the netlist seems applied inconsistently. For some nets, the net labels are used and for other the port names are used in the final PCB netlist even though both net labels and port names are assigned.
Altium cannot use a multi-part component in the multiple instances. A hex inverter IC has six inverters. If one is used in a sheet and the sheet is used in multiple instances, Altium will end up with multiple ICs rather than distribute subparts to different instances. But there is not an easy way to resolve this.
In conclusion, the hierarchical design in Altium has to applied with greater caution. I do find some utility for it when a design block are used more than once, but otherwise it is better to avoid at this moment
Sunday, January 29, 2012
Monday, January 23, 2012
EEssential Weekly Notes #3
Backdrilling
Backdrilling is a PCB design technique to remove stub from a through-hole. It is to reduce reflection from the stub for the high-speed signals (>5Gb/s). Blind vias and buried vias can achieve the same effect for vias, but at a increased manufacturing cost. But backdrilling is especially useful for backplane connectors, but seems only applicable for press-fit connectors.
Backdrilling is a PCB design technique to remove stub from a through-hole. It is to reduce reflection from the stub for the high-speed signals (>5Gb/s). Blind vias and buried vias can achieve the same effect for vias, but at a increased manufacturing cost. But backdrilling is especially useful for backplane connectors, but seems only applicable for press-fit connectors.
Monday, January 16, 2012
EEssential Weekly Notes #2
Magnets flux density vs temperature
Because thermal motion randomizes magnetic domain, the flux density of magnets decreases with increasing temperature. NdFeB magnets has a temp co of dB/B/dT = -1.1e-3 /C.
Magnetostriction
The inductor humming is caused by magnetostriction, magnetic material changing shape in response to a changing magnetic field.
RS232 receiver
It is customary to use a RS232 driver IC to convert a TTL signal to RS232 signal level. The RS232 driver IC includes a charge pump to generate the required negative voltage. But if we look at the RS232 receiver spec, we'll notice that the transition actually happens at positive voltage; for example, MAX232A RS232 input threshold low is typically 1.3V and threshold high is 1.8V. So it is possible to send digital output (0 to 3.3V or 5V) to a RS232 receiver IC and be decode correctly (even when it is outside of the RS232 spec).
Rise/Fall time measurement
The rise time of the first order low-pass step response is 0.35/BW. The measured rise time is the square root of the sum of the square of the signal rise time and the square of the scope rise time. This is only strictly true for Gaussian step and Gaussian impulse response, but it is a good approximation with the signal rise time is greater than the scope rise time. The scope frequency response can be well approximate by a second order low pass with a damping factor of 1/sqrt(2).
Because thermal motion randomizes magnetic domain, the flux density of magnets decreases with increasing temperature. NdFeB magnets has a temp co of dB/B/dT = -1.1e-3 /C.
Magnetostriction
The inductor humming is caused by magnetostriction, magnetic material changing shape in response to a changing magnetic field.
RS232 receiver
It is customary to use a RS232 driver IC to convert a TTL signal to RS232 signal level. The RS232 driver IC includes a charge pump to generate the required negative voltage. But if we look at the RS232 receiver spec, we'll notice that the transition actually happens at positive voltage; for example, MAX232A RS232 input threshold low is typically 1.3V and threshold high is 1.8V. So it is possible to send digital output (0 to 3.3V or 5V) to a RS232 receiver IC and be decode correctly (even when it is outside of the RS232 spec).
Rise/Fall time measurement
The rise time of the first order low-pass step response is 0.35/BW. The measured rise time is the square root of the sum of the square of the signal rise time and the square of the scope rise time. This is only strictly true for Gaussian step and Gaussian impulse response, but it is a good approximation with the signal rise time is greater than the scope rise time. The scope frequency response can be well approximate by a second order low pass with a damping factor of 1/sqrt(2).
Wednesday, January 11, 2012
EEssential Weekly Notes #1
Tin Whisker Investigation
NASA Goddard has a presentation on Tin Whisker on Toyota accelerator pedal position sensor failure. It shows ribbon connector leads (copper alloy with 2um pure tin) shorted by tin whisker ~2mm long,1um diameter, 240 Ohms. Standard Ohm-meters may not be able to detect the whiskers because the voltage is too low to break down insulating films.
Analog Devices iCoupler
The analog devices iCoupler technology is described in this article. The transform is built with CMOS metal layer, a high breakdown polyimide layer and a gold layer. High speed CMOS circuits with 1ns edges are connected to the transformer with wafer scale packaging.
Free PCB Layout Software
Some free PCB layout software worth noting are the open-source KiCAD and gEDA, Sunstone PCB123, DesignSpark PCB. These tools seem to be able to handle moderately complex designs.
Make You Own PCB
A relatively simple and low-cost procedure of making your own PCB is shown here. It starts with pre-sensitized circuit board, then developer and etching solution. A board can be made for less than $50.
NASA Goddard has a presentation on Tin Whisker on Toyota accelerator pedal position sensor failure. It shows ribbon connector leads (copper alloy with 2um pure tin) shorted by tin whisker ~2mm long,1um diameter, 240 Ohms. Standard Ohm-meters may not be able to detect the whiskers because the voltage is too low to break down insulating films.
Analog Devices iCoupler
The analog devices iCoupler technology is described in this article. The transform is built with CMOS metal layer, a high breakdown polyimide layer and a gold layer. High speed CMOS circuits with 1ns edges are connected to the transformer with wafer scale packaging.
Free PCB Layout Software
Some free PCB layout software worth noting are the open-source KiCAD and gEDA, Sunstone PCB123, DesignSpark PCB. These tools seem to be able to handle moderately complex designs.
Make You Own PCB
A relatively simple and low-cost procedure of making your own PCB is shown here. It starts with pre-sensitized circuit board, then developer and etching solution. A board can be made for less than $50.
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