self.SPIsetup();
self.MCPreset(); #Reset the chip.
- #Loopback mode for testing.
+ # We're going to set some registers, so we must be in config
+ # mode.
self.MCPreqstatConfiguration();
+
+ # If we don't enable promiscous mode, we'll miss a lot of
+ # packets. It can be manually disabled later.
+ self.poke8(0x60,0xFF); #TODO Does this have any unpleasant side effects?
+
+ # Now we need to set the timing registers. See chapter 5 of
+ # the MCP2515 datasheet to get some clue as to how this
+ # arithmetic of this works, as my comments here will likely be
+ # rambling, incoherent, and unchanged after I get the infernal
+ # thing working.
+
+ # First, we must chose a Time Quanta (QT) which is used to
+ # define the durations of these segments. Section 5.3
+ # suggests setting BRP<5:0> to 0x04 to get a TQ=500ns, as a 20
+ # MHz crystal gives a clock period of 50ns. This way, for 125
+ # kHz communication, the bit time must be 16 TQ.
+
+ # A bit consists of four parts:
+ # 1: SyncSeg 1 TQ
+ # 2: PropSeg 2 TQ
+ # 3: PhaseSeg1 (PS1) 7 TQ
+ # 4: PhaseSeg2 (PS2) 6 TQ
+
+ # CNF1 with a prescaler of 4 and a SJW of 1 TQ. SJW of 4
+ # might be more stable.
+ self.poke8(0x2a,0x04);
+
+ # CNF2 with a BLTMODE of 1, SAM of 0, PS1 of 7TQ, and PRSEG of 2TQ
+ self.poke8(0x29,
+ 0x80 | # BTLMODE=1
+ (6<<3) | # 6+1=7TQ for PHSEG1
+ (1) # 1+1=2TQ for PRSEG
+ );
+
+ #CNF3 with a PS2 length of 6TQ.
+ self.poke8(0x28,
+ 5 #5+1=6TQ
+ );
def MCPreset(self):
"""Reset the MCP2515 chip."""
self.SPItrans([0xC0]);