Some small PIC's (like 12Fxxx and 16F630/16F676) can configure their MCLR pin as input.
These chips will (in most cases) fail to get into programmingmode when the MCLR (reset) pin is indeed configured as input.
Giving Vpp a fraction before the power from the target PIC is active solve this problem (= Vpp-before-Vcc).
The Galva-Wisp shorts the target PIC's power very briefly (a few milliseconds), while the MCLR pin goes to Vpp (= 13 volt).
A Schottky diode and a 1000µF capacitor *) on the Galva-Wisp ensures that the Galva-Wisp still has power during this brief period.
It's important that the 2 LED's are a low current type, else capacitor C1 is to small.
In any case the power to the target PIC must tolerate this brief short,
it must not deliver more current than the TIP120 darlington transistor can handle (a few ampère) and it must recover fast.
A 7805 or 78L05 based power supply will do fine, some lab power supplies will not, because they are designed to "fold back" after an overcurrent situation and recover slowly.
In such cases a 10Ω resistor in series with the power supply might solve the problem.
It is recommended to place always a 10Ω resistor in the Vcc power line from a small PIC.
*) On the good old Wisp628 PIC programmer the Schottky diode and the 1000µF capacitor are not present.
You have to place these two components on the outsite from the Wisp628 by yourself.
More info about the Wisp628 programmer on Voti.
A TIP120, elco and Schottky diode are already present on the Wisp648.
The Wisp648 is the successor from the Wisp628.
See also the scheme for the Wisp628 under.
Scheme for the Galva-Wisp PIC programmer
It is recommended to place always a 10Ω resistor in the Vcc line from a small PIC.
Smaller PIC's (i.e. 12Fxxx and 16F630/16F676) don't have a LVP (PGM) pin for the white wire.
The white wire is simply not used.
Scheme for the Wisp628 PIC programmer from Voti
The Wisp628 does not have a 1000µF and no Schottky diode onboard, which the Galva-Wisp already have.
So when you use a Wisp628 then place these two components also in the additional circuit.
The Schottky diode isn't a critical one, i.e., the SB140 may also be a SB130 or equivalent.
Users with a Wisp648 from Voti have to do nothing because the above mentioned circuit is already built in this programmer.
The only thing they have to do is place a jumper over the two pins from jumper J1 (= connected), or to place jumper P-short to Ena (= Enable), if it is a newer Wisp648 version with a three pin header.
If the used feeding can handle the power-short (for the Vpp-before-Vcc), and an 7805 feeding can, then jumper J1 may always be placed.
The Wisp648 will then try first to recognize the PIC without the power-short, and if recognizing not happens, then it will try again wíth power-short.
The 10Ω resistor is only connected with the 100n capacitor and the PIC itself.
Other components should connected the normal way directly to +5V.
Only when the PIC must be programmed, then also the Galva-Wisp PIC programmer is (temporarily) connected after the 10Ω resistor (by placing the DIL-clip over the PIC).
As you can see in the scheme here above,
all three wires from the TIP120 darlington transistor are connected with the 15-pole SUB-D connector (pin 1, 2 en 7).
By this it is the easiest way to place this transistor (and the 1k resistor) in the connector itself.
(see photo underneath).
For the bigger PIC's I have another cable with an 18-pin DIL-clip (without the TIP120, then not necessary).
You can fix the TIP120 with some 10 seconds super glue.
The TIP120 can be placed in the 15-pole sub-D connector.
Cut the upper metal part from the TIP120, then it fits better in the connector package.
The TIP120 is connected between pin 1 (GND) and pin 2 (5V) from the connector (see little scheme).
Place an 1k resistor between the base from the TIP120 and pin 7 from the connector.
On this cable is an 8-pin DIL clip mounted, because the transistor is only necessary with the small PIC's.
8 The TIP120 with 1k resistor is here mounted on the inside from the connector.
|"Diodecable" with a separate shield on every wire.|
The 8-pins DIL-clip fits also on 14-pins and 20-pins PIC's.
The same 8-pins DIL-clip which is wired for the 12Fxxx PIC's, fits also on the 14-pins PIC's (i.e.16F630/676) and 20-pins PIC's (i.e.16F689/690/785). .
Place this clip on the side from pin 1 from the PIC.
The cable on the DIL-clip should not be longer than 80cm, recommended is a cable with a shield on each wire (see picture).
SMD SOIC PIC's
This photo shows a 12F629 in DIL- and one in a SOIC package.
For the SMD-PICs there are SOIC-clips available.
Use only a little bit soldertin when solder a PIC in SMD package.
To much solder becomes that the SOIC-clip doesn't fit and then you can't program the PIC anymore.