Compute time interval/resistance correlation for the TPL5110 system timer

tpl5110.py

#!/usr/bin/env python
'''
The TPL5110's time interval is configured by the applying resistance between the
DELAY pin and GND. The correspondance between time interval and resistance is
given starting on page 12 of this document:

http://www.ti.com/lit/ds/symlink/tpl5110.pdf

The documentation contains lookup tables for common intervals between 100ms to
2h. This script can also be used to calculate values between 1s and 2h.
'''
from math import sqrt

coefficients = [
  (   1,    5, ( 0.2253,  -20.7654,   570.5679)),
  (   5,   10, (-0.1284,   46.9861, -2651.8889)),
  (  10,  100, ( 0.1972,  -19.3450,   692.1201)),
  ( 100, 1000, ( 0.2617,  -56.2407,  5957.7934)),
  (1000, 7200, ( 0.3177, -136.2571, 34522.4680)),
]

def solve_r(T):
  '''Solve for resistance (ohms) given time interval (secs)'''
  for lo, hi, (a, b, c) in coefficients:
    if lo < T <= hi:
      return (100*(-b + sqrt(b**2 - 4*a*(c-100*T)))) / (2*a)
  raise Exception("Time value is out of range")
  

def solve_t(r):
  '''Solve for time interval (secs) given resistance (ohms)'''
  for lo, hi, (a, b, c) in coefficients:
    T = (a*r**2 + 100*b*r + 10000*c) / 1000000.0
    if lo < T <= hi:
      return T
  raise Exception("No time found for resistance value")


if __name__ == '__main__':
  time = 1*60*60  # one hour
  print(solve_r(time), 'ohms')
  print(solve_t(solve_r(time)), 'seconds')  # double check