EVB90615 Datasheet, PDF (17/30 Pages) Melexis Microelectronic Systems – Infra Red Thermometer

MLX90615
Infra Red Thermometer
8.6 PWM
The MLX90615 can be read via PWM or SMBus compatible interface. Selection of PWM output is done in
EEPROM configuration (factory default is SMBus). Object or ambient temperature can be read through PWM.
The PWM period is derived from the on-chip oscillator and is programmable in a low or high frequency.
Config Register[2:0]
PWM data PWM frequency
000
To
High
010
To
Low
100
Ta
High
110
Ta
Low
xx1
SMBus, PWM disabled
Temperature ranges for the PWM output are written in EEPROM 0x0, 0x1 – PWM Tmin and PWM Trange
(Tmax-Tmin), scale is 0.02°K/LSB. Note that in SMBus mode the EEPROM 0x0 is used for Slave address
SA.
Figure 14: PWM format
8.6.1 PWM format
The temperature reading can be calculated from the signal timing as:
Tout
=
⎡ 2t₂
⎢⎣ _T
*Trange_{⎥⎦⎤}
+ Tmin
,
where Tmin and Trange are the corresponding rescale coefficients in EEPROM for the selected temperature
output and T is the PWM period. Tout is To or Ta according to bit Config Register, 2.
The different time intervals t₁, t₂ and t₃ have the following functions:
t₁: Start buffer. During this time the signal is always high. t₁ = 0.125*T (T is the PWM period, refer to fig. 14).
t₂: Valid Data Output Band, 0 to 1/2T. PWM output data resolution is 10 bit.
t₃: Low time. The maximum duty cycle is limited to t₁+ t₂ = 0.625 This means that the PWM line will never go
static, allowing detection of fault on the line (disconnected device, short on the line).
Example:
To => Config Reg,2 = 0
Tomin = 0°C
=>
PWM Tmin [EEPROM] = 50 * (tomin + 273.15) = 355Bh
Tomax = Tomin + Trange = +50°C
=>
PWM Trange [EEPROM] = 50 * (torange) = 09C3h
Captured PWM high duration is 0.495*T => t2=(0.495 – 0.125)*T=0.370*T =>
measured object temperature = 2X0.370* (50°C -0°C)+0°C = +37.0°C.
3901090615
Rev 001
Page 17 of 30
Data Sheet
28/Aug/2008