| ??? 08/31/06 20:33 Modified: 08/31/06 20:35 Read: times |
#123469 - Only because it looks simple does not mean... Responding to: ???'s previous message |
Jan said:
One form of it which comes back from time to time is that by Mohammad Asim Khan, which appears to be quite popular also with the members of this forum. It comes in two forms: as v3.0 and as v3.7; And many newbies have reported here that it doesn't work! Ok, let's have a look at the schematic: 
1. Line D3 is used to turn the micro on and off. If D3 goes low after being high, 74HCT367 outputs are immediately activated. But due to the fact, that the BC557 needs some microseconds to turn-on, the 74HCT367 drives the inputs of micro, allthough the micro isn't turned-on yet. That's not a good idea. 2. If line D3 goes high again, outputs of 74HCT367 are immediately turned-off, while the micro still gets power. Due to the internal pull-down resistor at reset pin of micro, the potential at reset pin becomes low. And this during power down, where the reset pin should be high to prevent damage from the flash memory content! The urgently needed pull-up at reset pin is omitted. 3. Not even one series termination resistor can be seen for the signals entering the cables! 4. No decoupling cap for the micro! 5. In the line D4 a strange 1k5 resistor is inserted, looking like a "solved" timing problem... Do you really think that this is a circuit suited for newbies?? Kai |
