From: Brent Brown <brent@eds.co.nz>
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Date: Sun, 15 Nov 2020 17:03:32 -0800
Subject: Re: [PIC] An In-Circuit Serial Programmer application
Thread-Topic: [PIC] An In-Circuit Serial Programmer application
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On 16 Nov 2020 at 0:17, Don Kuenz wrote:

> =20
> >> Don wrote:
> >>> This project shows how to implement an In-Circuit Serial
> >>> Programmer application. It utilizes the ICSP's 5 VDC
> >>> (Vdd) pin to toggle the PIC microcontroller between
> >>> application mode and programming mode.
> >>>
> >>> https://crcomp.net/icsp/index.php
>=20
> > Chris wrote:
> >> Nice, but why not just use two Jumpers or a DPDT switch rather than ad=
d
> >> additional silicon?
> >> My preference with the more recent PIC's is to use PPS to assign the U=
SART
> >> to those pins.
> >> I then use the PICKit as a terminal for debugging as well as a Program=
mer.
> >> The serial interface may even have a use in the target application.
> >> Have the serial interface use the same pin assignments as the ICSP and=
 you
> >> have a nice 'Plug and Play' programming/debugging/communications port.
>=20
> Chris wrote:
> > P.S. - It also works well with a Bootloader.
> > use the ICSP interface to install the TinyBootloader or an equivalent w=
ith
> > the ICSP Dat and Clk lines set at the Rx and Tx lines and you have a ni=
ce
> > development device with Programming, Debugging and Communications all
> > achieved via a USB/Serial adapter - Great to hand out to students or us=
e
> > for casual experimenting as well as in target applications.
>=20
> There's four good reasons (or at least three and a half) for me to use=20
> additional silicon. Let's start with the half reason. If you look=20
> closely at Figure 4 you see a blue DIP switch near the top, middle. It's
> a drag to flip those switches each time. A mouse click is so much=20
> easier.
>     There's also a dearth of ICSP applications available on the Inet,
> probably for a very good reason. The wide, inviting, easy route with old
> MCUs is to simply dedicate RB6 and RB7 to ICSP. The third reason is my
> application allows all 8 bits of RB to be used.
>     The final reason is actually the best. It motivated you to talk
> about another way. It's time for me to step up to an MCU with a UART.

IMHO the ICSP circuit can be simpler yet remain effective i many cases... j=
ust a=20
single pull up resistor to VDD. Value of 10k works well for me, using ICD3 =
and=20
ICD4. The resistor must present not too great a load to the programmer duri=
ng HV=20
programming mode, nor push up VDD appreciably. Conversely it must be low=20
enough value to prevent unwanted MCLR pul downs by noise. A couple of basic=
=20
tips: keep traces short around ICSP, and don't expose VDD to the outside wo=
rld=20
where it can pick up noise. In most cases the RC reset circuit is no great =
loss with=20
most PIC's having fairly decent internal power up & brownout reset generato=
rs that=20
can be enabled.

Yes ~ a bootloader is indispensable once you have settled on a good one. Al=
l going=20
well you then only need to use ICSP once... so isolation becomes less impor=
tant. I=20
prefer a bootloader that doesn't require a button press, DIP switch, or jum=
per, to=20
initiate it... these are time consuming and annoying steps during developme=
nt=20
code/program/debug cycles (frequent for me), and a complication when doing =
field=20
updates. Ideal: bootloader runs for a brief time at reset and invokes loadi=
ng if=20
detected else runs application, optionally add hardware reset capability by=
 using=20
e.g. the RTS line, and/or have the application code invoke the bootloader a=
fter=20
receiving a serial escape sequence (unless this presents an unacceptable se=
curity=20
risk).

Nice work on the web page though, good circuit design/description. I skip r=
ead it=20
and only noticed a missing "be" in 4th pragraph "can not left unused".

--=20
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