Adds a script for computing optimal CAN-bus timing configurations. (#606

)

bin/can_osc_tolerance.py

@@ -0,0 +1,86 @@
+#!/usr/bin/python
+#
+# Prints information about possible CAN-bus timing configurations that fulfill
+# the needs for OpenLCB, i.e., those that are good enough for 300m long buses
+# at 125 kbps. The basis upon which this is built is AN1798 CAN Bit Timing
+# Requirements by NXP/Freescale
+# (https://www.nxp.com/docs/en/application-note/AN1798.pdf)
+#
+# Output is sorted by the osc tolerance decreasing. When configuring a CAN
+# controller, you should pick the first line that is possible to be configured
+# in your CAN controller, meaning the one which gives the best oscillator
+# tolerance.
+#
+# Interpreting output:
+# 
+# 0.98  :  ['len 310 m, prop 7 ps1 4 ps2 4 sjw 4, sample 75.0, o1 1.25 o2 0.98', 'len 320 m, prop 9 ps1 5 ps2 5 sjw 4, sample 75.0, o1 1 o2 0.98']
+#
+# Before colon: the oscillator tolerance in percent. This is to be interpreted
+# as +- this percentage at both source and destination (independently).
+#
+# len = max cable length
+# prop = number of TQ for propagation segment
+# ps1 = number of TQ for phase segment 1
+# ps2 = number of TQ for phase segment 2
+# sjw = number of TQ for (re)-synchronization jump width
+# sample = sample point within bit in %
+# o1 = osc tolerance limit from one constraint
+# o2 = osc tolerance limit from another constraint
+#
+# There are two entries in this line, which means that two different
+# configurations reach the same osc tolerance.
+
+
+from collections import defaultdict
+
+found = defaultdict(list)
+optimal = []
+
+MIN_LENGTH = 300
+
+def eval_timing(propseg, ps1, ps2, sjw):
+    global found, MIN_LENGTH, optimal
+    # number of time quanta in the bit period. SyncSeg is always 1.
+    ntq = 1 + propseg + ps1 + ps2
+    sample_point = 1.0 * (1 + propseg + ps1) / ntq
+    BAUD = 125000
+    sec_per_tq = 1.0 / BAUD / ntq
+    # 400 nsec for delay of trnasceiver and receiver. (100 nsec each twice for
+    # tx and rx)
+    TXRX_DELAY_SEC = 400e-9
+    # Cable propagation delay is 5 nsec per meter.
+    PROP_SEC_PER_METER = 5e-9
+    max_length = (propseg * sec_per_tq - TXRX_DELAY_SEC) / PROP_SEC_PER_METER / 2
+    # One formula says that the relative frequency * 2 over 10 bit time
+    # cannot be more than the SJW.
+    osc1_limit = sjw * 1.0 / (2 * 10 * ntq)
+    # Another formula says that over 13 bits time less ps2, we cannot have more
+    # drift than -ps1 or +ps2.
+    osc2_limit = min(ps1, ps2) / (2 * (13*ntq - ps2))
+    real_limit = min(osc1_limit, osc2_limit)
+    params = 'len %.0f m, prop %d ps1 %d ps2 %d sjw %d, sample %.1f, o1 %.3g o2 %.3g' % (max_length, propseg, ps1, ps2, sjw, sample_point * 100, osc1_limit * 100, osc2_limit * 100)
+    if max_length > MIN_LENGTH:
+        found[real_limit].append(params)
+    for x in optimal:
+        if x[0] > real_limit and x[1] > max_length:
+            return
+    optimal = [x for x in optimal if x[0] >= real_limit or x[1] >= max_length]
+    optimal.append([real_limit, max_length, params])
+
+def print_findings():
+    global found, optimal
+    for (tol, v) in sorted(found.items(), reverse=True)[:10]:
+        print('%-5.3g' % (tol * 100), ": ", v)
+    print('\n\noptimal:')
+    for x in sorted(optimal, reverse=True):
+        print('%-5.3g' % (x[0] * 100), ": ", x[2])
+
+
+# enumerates all possibilities        
+for propseg in range(1, 16):
+    for ps1 in range(1, 8):
+        for ps2 in range(1, 8):
+            for sjw in range(1, 1+min(4, min(ps1, ps2))):
+                eval_timing(propseg, ps1, ps2, sjw)
+
+print_findings()