From: James Wages <james@kiramek.com>
To: "piclist@mit.edu" <piclist@mit.edu>
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Date: Thu, 22 Jul 2021 16:56:45 -0700
Subject: Re: [PIC] Automotive RS-485 & PIC18F25K80
Thread-Topic: [PIC] Automotive RS-485 & PIC18F25K80
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I sincerely appreciate the 2 replies from Andrew K. and madscientistatlarge=
..  I would now like to present further specifics, followed by my individual=
 replies to both of you gentlemen.

The PIC18F25K80 datasheet is found here:
https://www.microchip.com/wwwproducts/en/PIC18F66K80

On the "interface" PCB I have been discussing (which connects to Toyota "Di=
splay Audio"), I see the following:
PIC pin 23 connects to DI on the ADM3078E chip.
PIC pin 25 connects to RE* & DE on the ADM3078E, which are tied together.
*RE "NOT"

As per the PIC Datasheet...
Pin 25 =3D RB4/AN9/C2INA/ECCP1/P1A/CTPLS/KBI0
Pin 23 =3D RB2/CANTX/C1OUT/P1C/CTED1/INT2

Pin 25 could be any 1 of the following:
RB4 =3D Digital I/O
AN9 =3D Analog Input
C2INA =3D Comparator 2 Input A
ECCP1 =3D Capture 1 input/Compare 1 output/PWM1 output
P1A =3D Enhanced PWM1 Output A
CTPLS =3D CTMU pulse generator output. (CTMU =3D Charge Time Measurement Un=
it, for capacitance changes)
KBI0 =3D Interrupt-on-change pin

Pin 23 could be any 1 of the following:
RB2 =3D Digital I/O
CANTX =3D CAN bus TX
C1OUT =3D Comparator 1 output
P1C =3D Enhanced PWM1 Output C
CTED1 =3D CTMU Edge 1 input (CTMU =3D Charge Time Measurement Unit, for cap=
acitance changes)
INT2 =3D External Interrupt 2

Madscientistatlarge, you suggested it could be a CAN protocol.  But if that=
 were true, I would expect both CANTX and CANRX to be used.  But while Pin-=
23 supports CANTX, Pin-25 does not support CANRX.  So if CAN is indeed used=
, you would have TX used by the PIC, but not RX.  And I would assume one wo=
uld want to use the PIC's ECAN Bus Module when doing CAN communication, for=
 both TX and RX. If you have further thoughts to share on this point, I wou=
ld love to hear them.

Andrew K., I am unfamiliar with the old Alpine MBUS protocol you mentioned.=
  However, by Googling I found this:
http://www.hohensohn.info/mbus/

Biggest Differences:
MBUS seems to use 10V pulses, but my interface circuit is 3.3V.
MBUS apparently uses millisecond width pulses (slow 325 baud) whereas the p=
ulse widths in my interface circuit seem to be as short as 5us.

I am now led to wonder if Toyota could be using their own custom protocol o=
r perhaps a modification of the old Alpine protocol.

Hmmm...

--James W.



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