The programmer has computer ground as global ground. It is possible to let the programmer stay in a circuit connected to some of the pc's ports (e.g. LPT or COM) while programming, and it is not a problem with main-ground connections on both programmer and PC.
5V power | supply (or 4.5V battery.) | - R| |100 ohm (down to 56 ohm). | |0.5W - | 14| ------- | VDD |PIC16C84 | | | | | MCLR|------- TxD (2) | |4 | | +--- DTR (20) | |13 | | RB7 |---+--- CTS (5) | | | RB6 |------- RTS (4) | |12 | | Connect directly to | GND | serial interface. ------- 5| +---------- GND (7) | | power 0V supplyClamp diodes:
In fact, the clamp diodes to Vdd is not diodes, but transistors, and about 80% of the current flows into ground. Because of this, are Vdd not to be pulled higher than Vdd+0.5V, even all the inputs pulls high. (at 20mA from the inputs, flows max 3-5 mA to Vdd, and cause about 5.4V). The decoubling capacitor and LED's inside the power supply does that the power supply keep the 5.0V at the output. (The duty cycle of the programmer does that it pull more current out, than it lead into the supply, and then a stabilized power supply has a decoubling capacitor, that makes the power positive out from the regulator.)
Since the processor makers typical not reduce the power into the chip and makes it latch-up free, we do it. If a circuit need to be latch-up free, it must never hang in latch-up condition, even if we pull in the pins above any accepted level in any condition. As soon as the condition that increase latch-up possibility is over, should it work again - else is it not truely latch-up free.
How to do it:
Reducing to a maximum current into the power pins, do eliminate latch-up risc. If the chip has substrat contacts, it will need a current to be over a high level - and only if the maximum current is above this level, will the Vbe for the transistors be high enough to make it hang.
Sometimes signals never folow the processors specifications, because wires inlclude induction and cause high currents. But designers of chips know about these problems - and makes the capacities in the chip high enough to avoid the riscs. (hopefully)...
If wee need to guarentee latch-up free condition exist, then it is possible to do without current limitting components in the chip, but to do it external using a resistor in Vdd/Gnd.
The RS232 outputs is current limited, and it do typical not exceed +/- 15mA. PIC's handle up to +/- 20mA. Current in the clamping diodes may cause latch up and to prevent this effect we do need the resistor R to reduce the current from power supply. The typical Vdd current is about 3 to 4 mA, and the voltage drop on 0.4V is ok. The power resistor reduce maximum Vdd current to about only 50mA and then protects the PIC from meltdown if it is inserted in a bad way.
Download PIP-02 beta driver, JDM84V33.ZIP (5K).
Download PIC16C84 utilities, PGM84V34.ZIP (38K).
The utilities is inclusive source.
Extra lowcost for PIC16C84 only.
Simple programmer for PIC16C84 and 24Cxx only.
How to make a processor with the delay between instructions less than a half nano second in standard 1u CMOS. (GHz instruction frequence.)
Z80 trick: Use RESET, NMI and CLOCK to bootload a romless system (RAM only).
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