CAN frame format doc

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+datafmt.ps: datafmt.ms
+	dformat $< | pic | tbl | groff -ms -Tps >$@
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+.TL
+Data Frame Format for the CAN Gauge Interface
+.AU
+Sam Anthony
+.AI
+.LP
+The Gauge Interface receives parameter values from other nodes in the vehicle via the CAN bus.
+It also accepts frames from a host PC for calibration.
+The format of these calibration, or
+.I control ,
+frames are the subject of this document.
+.NH 1
+Parameters
+.LP
+The Interface listens for 6 parameters on the bus:
+.nr step -1 1
+.IP \n+[step].
+Tachometer
+.IP \n+[step].
+Speedometer
+.IP \n+[step].
+Analog 1
+.IP \n+[step].
+Analog 2
+.IP \n+[step].
+Analog 3
+.IP \n+[step].
+Analog 4
+.LP
+Each of these parameters has an associated CAN ID and a calibration table; thus, there are 6 IDs and 6 tables.
+The Interface listens for these IDs and uses the tables to lookup output signal values to drive he gauges.
+Each table has 32 rows, each of which contains a key and a value.
+The keys and values are each 32-bit unsigned integers.
+Numbers are transmitted in big-endian order in frames' DATA FIELD.
+.NH 1
+Frames
+.LP
+In addition to the parameter frames, there are two types of
+.I control
+frame:
+.B "Table Control" ,
+and
+.B "ID Control" .
+.NH 2
+Table Control Frame
+.LP
+The Table Control Frame is used to read and write rows of the calibration tables.
+It is an extended frame with DLC=8.
+It may be either a DATA FRAME: to write to a table\(emor a REMOTE FRAME: to read from a table.
+.PP
+The Table Control Frame has extended ID
+.B 12720XXh .
+The LSB,
+.I X ,
+inticates which table, and which row of the table, to read or modify.
+The upper 3 bits of
+.I X
+indicate one of  the 6 tables [0, 5].
+The lower 5 bits of
+.I X
+indicate one of the table's 32 rows [0, 31].
+.begin dformat
+style bitwid 0.15
+Table Control ID
+	28-8 12720h
+	7-5 Table
+	4-0 Row
+.end
+.LP
+The DATA FIELD contains a row of the table.
+The row is composed of two 32-bit unsigned integers: a key and a value.
+They are transmitted in big-endian order.
+.begin dformat
+style bitwid 0.07
+style recspread 0
+Table Control DATA FIELD
+	7-0 D0
+	7-0 D1
+	7-0 D2
+	7-0 D3
+	7-0 D4
+	7-0 D5
+	7-0 D6
+	7-0 D7
+noname
+	31-0 Key
+	31-0 Val
+.end
+.LP
+Upon receiving a Table Control DATA FRAME, the Interface will write the key and value to the row of the table specified in the ID.
+.PP
+In the case of a REMOTE FRAME, the Interface will read the row of the table specified in the ID, and respond with a DATA FRAME containing the key and value of the row.
+.NH 2
+ID Control Frame
+.LP
+The ID Control Frame is used to read or modify the CAN ID associated with one of the 6 parameters.
+It is an extended frame with DLC=8.
+It may be either a DATA FRAME: to modify\(emor a REMOTE FRAME to read.
+.PP
+The ID Control Frame has extended ID
+.B 127210Xh .
+The least significant nibble,
+.I X ,
+indicates one of the 6 parameters [0, 5].
+.begin dformat
+style bitwid 0.15
+ID Control ID
+	28-4 127210h
+	3-0 X
+.end
+.LP
+The DATA FIELD contains the CAN ID of the parameter specified in
+.I X .
+It may be either a standard or an extended ID.
+In the case of a standard 11-bit ID, DLC=2 and the ID is sent as a 16-bit number.
+In the case of an extended 29-bit ID, DLC=4 and the ID is sent as a 32-bit number.
+The numbers are sent big-endian in both cases.
+DLC can be used to determine whether the ID is standard or extended.
+.begin dformat
+style bitwid 0.08
+style recspread 0
+ID Control DATA FIELD (11-bit Std. ID)
+	7-0 D0
+	7-0 D1
+noname
+	15-11
+	10-0 Std. ID
+.end
+.begin dformat
+style bitwid 0.08
+style recspread 0
+ID Control DATA FIELD (29-bit Ext. ID)
+	7-0 D0
+	7-0 D1
+	7-0 D2
+	7-0 D3
+noname
+	31-29
+	28-0 Ext. ID
+.end