Tal... It's for reverse current protection. Imagine you have one of those RadioCra= p multivoltage wall-warts and you set centre pin to the wrong polarity...you run the risk of frying stuff. The diode is like bypass capacitors -- A Good Thing=81. Circuits might well work without it, but for the sake of a few cents, you may as well protect your investment in parts. -marc On 15/6/03 21:49, "Tal" wrote: > Jinx, >=20 > What is the purpose of the diode parallel to the regulator ? Is it > necessary ? >=20 > I used three terminal regulators for years without a parallel diode. Is > it a serious mistake ? >=20 > Thanks, >=20 > Tal >=20 >> -----Original Message----- >> From: pic microcontroller discussion list >> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jinx >> Sent: Sunday, June 15, 2003 4:48 PM >> To: PICLIST@MITVMA.MIT.EDU >> Subject: Re: [PIC] 16F84 reading 2 digits from serial port >>=20 >>=20 >>> Can you specify the @ 9.8304MHz when you buy the PIC? >>=20 >> The 9.8304 is just an example of a hex value crystal. I >> normally keep 1/2 dozen values in stock - 32768Hz, 3.2768MHz, >> 9.8304MHz, 10MHz, 18.432MHz and 20MHz and buy others as >> needed. If one-offs then somewhere like Radiospares, or a >> local wholesaler for quantity >>=20 >>> I was using the PIC basic compiler to program the PIC. >> Which program >>> can I use to create HEX for the assembly below? >>=20 >> It was written/compiled using MPLAB/MPASM >>=20 >> Please change previous PortA definitions to - >>=20 >> #define rs porta,0 ;LCD RS >> #define en porta,1 ;LCD enable >> #define rw porta,2 ;LCD R/W >>=20 >> As I said, the program was cobbled together from other >> working programs, and there may be the odd small error like this >>=20 >> As for the timing, 9.8304 will get you exactly 1 minute >> intervals with Timer1 free-running >>=20 >> (9830400/4) / 65536 =3D 37.5 rollovers / sec =3D 2250 (08CAh) / min >>=20 >> You could either enable Timer1 IRQs or poll TMR1IF (without >> IRQs enabled), then use a 2-byte counter to count 08CA >> rollovers. A 32kHz xtal could be used on Timer1 instead for >> 0.5 rollovers/sec, 30/min >>=20 >>> What about the circuit and additional components that I >> need, can you >>> help me with this? You mentioned that you have a circuit where this >>> PIC can work. >>=20 >> Components not shown on this diagram are the capacitors for >> the MAX232 - you can find connections in the MAX232 pdf. >>=20 > http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1798 >=20 > To protect the regulator from reversed supply and shorts add the diodes. > Not actually necessary to make the circuit work but one silly mistake > can be costly. The bridge means you can use any old plug-pack with > either polarity. Add a filter cap after the bridge > (470uF) if it's an AC o/p plug-pack >=20 >> The only issue is the LCD. I wanted to use two of the LSD40165-20 to >> show the two digits. This will allow people to see the number at a >> distance >=20 > 4" 7-seg LED ? Hmmm, big ;-) >=20 > http://www.elexp.com/a_data/08lsd40165-20.pdf >=20 > Ah, that's a completely different ball game to an LCD. The circuit for > that is a little more complex. I'd suggest an MC14543 (BCD to 7-seg > convertor) and a current sink chip (one of the ULN 2000 range for > example. Or, as the current is in the low 10s of mA "possibly" you may > be able to do this with the PIC itself, but I'd recommend the PIC > driving a grounding transistor for each segment. You'll find LEDs at >=20 > http://www.piclist.com/techref/io/led/pic.htm >=20 > Keep the LCD circuit though, it'll come in handy one day, betcha >=20 > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu >=20 >=20 >=20 > -- > http://www.piclist.com hint: To leave the PICList > mailto:piclist-unsubscribe-request@mitvma.mit.edu >=20 >=20 >=20 =20 -------------------------------------------------- Marc Nicholas Geekythings Inc. C/416.543.4896 UNIX, Database, Security and Networking Consulting -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu