Problems programming PIC16F88

[fuzzzy]

HI,

I'm facing a strange problem.

I'm designing a 433 transmitter and receiver. I'm using a PIC16f88 both in transmitter and receiver. The first board I made hadn't any problem.In that version I used PIC16F88 SOIC300mil powered by lm7805 and LM317 for 12V. Then I made a new version with pic16F88 SSOP powered by lm7805 and buck regulator 12V. In receiver new version I can't program PIC. I can't understand why! Programmong configuration is the same of old version and the same for transimtter and receiver. When programming receiver i get an errorm message in veryfying device program. I can't erase o write this device.I can only read it. I'm trying programming it in ICSP mode by MPLAB ICD2. I tryed programming without LM7805 and without Buck regulator on PCB but i've always same problems. Circuit without Buck regulator is the same as old circuit i could program without problems.
I carefully verifyed also connection to MPLABICD and PIC. theyre correct(i can read PIC, i put a programmed PIC on PCB and i could read it, but not erase)

I don't know what to do!

Please help me.

 

[vinodkp]

may be you have the programming pins connected to the external circuit without any buffering, so when the pc sends the data it gets lost in your external circuit ( circuit external to the PIC) . you can test this by removing all the external circuit except for the PIC from the board and then try programming

 

[fuzzzy]

i tryed to remove all components but it doesn't work. In your opinion could be a problem the fact i connected only one pin of vdd and vss even if tssop package has got 2 pins for vdd and 2 for vss? this is the only difference wirh the previus working device.

 

[Alliegro]

All vss and vdd pins are to be connected. Also if i remember correctly, rb3 has to be grounded with 10k. there was lot of discussion on diy programmer board once. Are you using rb3?.. Or it could be just a hardware mode which microchip has implimented in the new batch of devices. Try both packages of f88 from a new batch.

 

[fuzzzy]

I'm not using RB3 for Low voltage programming so i don't need it grounded. Microchip forum answered me for my problem and suggested me to connect all Vdd and Vss Pins. i did it and everithing works fine now. programming is perfect. could anyone tell me why?? using a multimeter i can see vdd and vss pins are short circuited internally. i can't understand why.

thanks

 

[PaulHolland]

Hi, Check if your programmer is applying power to the device during programming. You can check this:
Programmer/Settings menu on MPLAB

Select the Power Tab on the top. and press update when the porgrammer is connected to your hardware !. If you see strange voltage levels you know what to look for.

regards,

Paul.

 

[Alliegro]

All though the vdd and vss pairs appear as short, but when the pic draws current during operation, the voltages are minutely different. It is as if a very low value resistor is in between the two vss or vdd pins. the more the current drawn, the more the difference in voltage. the resistors may be in the order of a few micro or milli ohms. you would only see a short with an ordinary multimeter.
This has been specifically been done for the noise isolation between analog and digital part inside the microcontroller.

 

[fuzzzy]

Ok i understand now. Have I to make particular connection externally or is it enough an external short circuit?