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Issue
99, October 1998
Networking
with DeviceNet—Part
2: A Weather Station Application
by
Jim Brady
Start
• Can
Chips
• Chip
Setup •
Real
Time
• Message
Flow
• Connections
• Timers
• Analog
Input Point
• Identity
Object
• Fragmented
Messages
• Getting
Physical
• Applying
DeviceNET
• Software,Sources
CHIP
SETUP
The
82527 has 15 mailboxes for CAN messages, each with 15
registers. Setting up a mailbox requires telling it what
its message identifier is and if it is send or receive.
Done properly, your program only gets an interrupt for
a message directed to your device.
The
82527 also has a group of registers that control message
filtering, interrupt masking, data rate, and sample timing.
There are some tricky ones that set the sample point within
a bit time as well as the limit on how much that sample
point can jump around.
There
is a tradeoff—you want to let it jump as much as
possible to accommodate oscillator tolerance, and you
also want the sample point to be close to the end of the
bit time to accommodate long cables. But you can’t
allow it to jump so much that it goes past the end of
a bit time.
After
a lot of calculation, I ended up sampling at 87% of the
way through a bit time, with the jump limit (SJW) equal
to 12% of a bit time. That accommodated the worst-case
cable length, with a jump width still large enough to
handle crystal errors of about ±0.2%, which is plenty
for any crystal. The Intel 82527 Architectural Overview
provides information for this calculation.
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