ALDL Updates was DIY-WB Output Voltage Scaling

[Malcolm Robb, LC 0112G]

Ludis is right, and I should have waited till I had the code in-front of me 
before posting. Sorry if I mislead anyone. What happens is...

The receive part of the UART generates an interrupt when a byte is received 
(from the scan tool). This interrupt routine collects received bytes until 
the complete command string is received - i.e. $F4,$57,$01,$00,$B4 . The 
receive code checks the format of the command, verifies that the checksum 
is correct (the $B4 in this example) and the message length (the $57). If 
everything is OK, it then sets up some pointers to the data table to be 
used in transmission depending on the mode byte (the $01), and writes the 
first byte to the transmiter in the UART (which will normally be $F4). The 
Transmit part of the UART then takes over.

The transmit part of the UART is double buffered. For those that don't 
know, this means that it contains a transmit register, and a holding 
register. The data actually being transmitted is sent from the transmit 
register. The user (CPU) can't directly access the transmit register. What 
happens is the CPU writes data to a holding register, and when the transmit 
register becomes empty, the data is then transferred automatically from the 
holding into the transmit register. This transfer generates an interrupt, 
which tells the CPU that it is safe to write another byte to the holding 
register. In this way, a continuous stream of transmitted data can be 
achieved, because the holding register always contains the next byte to be 
transmitted. In reality the CPU has ages to respond to the transmit 
interrupt and write the next byte to the holding register, because as long 
as it does this before the previous byte is transmitted, the stream will be 
continuous. Remember a transmitted byte is 10 bits long (8 data bits + 1 
start bit + 1 stop bit), and at 8192 baud, this takes 1.22mS to transmit.

What I did get right was the stuff about the heartbeat/ idle chatter (I 
think). BobR mentioned the kernel scheduler & the minor and major loops. 
I'll let him try and explain that coz it's a bit complicated to put into 
words, but basically there are 16 major loops, executed in order at a rate 
of one each 12.5mS. In each major loop, the ECU code checks to see if it 
should start to transmit a heartbeat frame. If it should, then it sets up 
the pointers to the ALDL transmit table to be used, and writes the first 
byte to be transmitted to the holding register. After that, the transmit 
interrupt code explained above causes the rest of the heartbeat frame to be 
transmitted automagically - as Ludis states. TurboP4.PDF in incomming 
explains this pretty well - section 5.6.1 onwards.

The fact that the major loops execute every 12.5mS means that you have to 
be careful how many bytes you put in the heartbeat frames. 12.5mS is only 
enough time to transmit 10 bytes. This includes the 3 header bytes and the 
checksum, so in reality, you've only got space for 6 meaningful data bytes. 
I haven't checked what happens if you specify more bytes than that, but it 
wouldn't surprise me if it all went to pot. Note also that 12.5mS is 80 
frames a second, which is more than you'll get just about any other way.

Cheers,
Malcolm Robb, LC 0112G - Hoping the WB-PCB group buy goes ahead.

-----Original Message-----
From:	Ludis Langens [SMTP:ludis at cruzers.com]
Sent:	Monday, September 03, 2001 10:13 PM
To:	Malcolm Robb, LC 0112G
Subject:	Re: ALDL Updates was DIY-WB Output Voltage Scaling

"Malcolm Robb, LC 0112G" wrote:
>
> This is MY understanding of MY ECU code. I haven't fully commented it 
yet,
> and can't pretend to understand it all. I expect someone else ( Ludis ?)
> can add to this, and particularly comment on what happens in other 
program
> masks.

I haven't studied the ALDL code in P4's in detail, so I can't add more
info that you posted.  However, it looks to me like in some P4 masks,
the ALDL is driven from UART interrupts.  So - as far as the main line
code is concerned, once it starts sending a packet, the rest of the
bytes will be sent out automagically.

--
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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