There are 3 main usage for VDD pin 1) Some programmers actually use the target +5V via VDD to generate the +12/+13 vpp as the serial port voltage & power is not reliable. These programmers have no other PSU. I have decided these programmers are a bad idea. If you have one, convert it to be self powered by cut up a USB cable.
2) Some programmers can power the target, if the target has no PSU connected. These programmers do not rely on RS232 for power but use USB or a separate PSU. 3) Professional programmers will deliberately vary VDD to test properly the reliability of the programming. Some programmers are RS232 powered only, this is to be avoided. Some programmers are self powered (by USB, RS232 or external dedicated PSU) but don't connect to VDD. Some ICSP target boards solve this by jumper between cpu vdd and rest of circuit, to allow (2) & (3), the "proper programmers" or leave jumper in for cheap (1) type programmer. Other target boards use diode isolation (recommended design). In this case a programmer that varies VDD can't damage rest of circuit, but the target board can supply a cheap programmer. In all cases VDD of CPU connects direct to VDD of ICSP cable. So you EITHER have a jumper between CPU vdd and rest of board, or preferably a good low voltage drop Schottky feed from rest of circuit or target PSU to cpu VDD pins. In all cases VDD of ICSP and cpu are directly connected with no diodes or jumpers. -- You received this message because you are subscribed to the Google Groups "jallib" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/jallib?hl=en.
