I am a professor at NHTI, Concord's Community College in Concord NH. I have just rewritten my Embedded Microprocessor course to incorporate Mazidi's "The 8051 Microcontroller, a system's approach" text book. On the recommendation of a professor at another school, I have purchased twenty-two Digilent MDE-8051 Trainers(USB) Rev b.
I have had to prohibit my students from using the red 8 bit slide switches on this board as the switches connect directly to 5 Volts. without any resistance. If a user has a switch connected to a IO Port and set to "ON" and then sends a zero to that port, the output transistor for the port is destroyed. It is clearly stated in Mazidi's text on Pages 599 and 600 under the heading "AVOID DAMAGING THE PORT" that with this IC, you must never connect an output port directly to 5 volts. Yet, this is what the MDE 8051 trainer does through the red switches.
In my opion this is a SERIOUS, unacceptable defect in the MDE-8051 trainer. We have always taught our students that to connect a switch to a microrocessor input port, you may switch it directly to ground, (when the switch is "ON" ) and when the switch is open, approx 20K pull-up resistors should pull the port to a logic 1. The only possible reason that I can think of for having the MDE 8051 trainer to switch directly to Vcc without any resistance is to get a "one" when the switch is 'ON". This is not necessary and every engineerng student that I have known realizes that you switch an input directly to ground and pull it up with resistance.
This is making me seriously think about abandoning the MDE-8051 as a trainer and either design our own boards with proper switches or find another vendor. Is anyone else concerned about blowing up DS89C450 chips at $25 per "POP"?
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William French
I am a professor at NHTI, Concord's Community College in Concord NH. I have just rewritten my Embedded Microprocessor course to incorporate Mazidi's "The 8051 Microcontroller, a system's approach" text book. On the recommendation of a professor at another school, I have purchased twenty-two Digilent MDE-8051 Trainers(USB) Rev b.
I have had to prohibit my students from using the red 8 bit slide switches on this board as the switches connect directly to 5 Volts. without any resistance. If a user has a switch connected to a IO Port and set to "ON" and then sends a zero to that port, the output transistor for the port is destroyed. It is clearly stated in Mazidi's text on Pages 599 and 600 under the heading "AVOID DAMAGING THE PORT" that with this IC, you must never connect an output port directly to 5 volts. Yet, this is what the MDE 8051 trainer does through the red switches.
In my opion this is a SERIOUS, unacceptable defect in the MDE-8051 trainer. We have always taught our students that to connect a switch to a microrocessor input port, you may switch it directly to ground, (when the switch is "ON" ) and when the switch is open, approx 20K pull-up resistors should pull the port to a logic 1. The only possible reason that I can think of for having the MDE 8051 trainer to switch directly to Vcc without any resistance is to get a "one" when the switch is 'ON". This is not necessary and every engineerng student that I have known realizes that you switch an input directly to ground and pull it up with resistance.
This is making me seriously think about abandoning the MDE-8051 as a trainer and either design our own boards with proper switches or find another vendor. Is anyone else concerned about blowing up DS89C450 chips at $25 per "POP"?
Professor William L. French
wfrench@ccsnh.edu
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