28017 Parallax Inc, 28017 Datasheet - Page 2

• Manufacturer Part Number: 28017
• Description: KIT DEMO ACCELEROMETER MEMSIC
• Manufacturer: Parallax Inc
• Series: BASIC Stamp®r

Specifications of 28017

• Sensor Type: Accelerometer, 2 Axis
• Sensing Range: ±2g
• Interface: I²C
• Sensitivity: 512 counts/g
• Voltage - Supply: 2.7 V ~ 5.25 V
• Embedded: No
• Utilized Ic / Part: BASIC Stamp® or Javelin Modules
• Product: Microcontroller Accessories
• Flash: 512 KB
• Timers: 10 x 8 bit
• Operating Supply Voltage: 5 V
• Board Size: 69.34 mm x 46.99 mm x 21.84 mm
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

How It Works
Internally, the Memsic 2125 contains a small heater. This heater warms a "bubble" of air within the
device. When gravitational forces act on this bubble it moves. This movement is detected by very
sensitive thermopiles (temperature sensors) and the onboard electronics convert the bubble position
[relative to g-forces] into pulse outputs for the X and Y axis.
The pulse outputs from the Memsic 2125 are set to a 50% duty cycle at 0 g. The duty cycle changes in
proportion to acceleration and can be directly measured by the BASIC Stamp. Figure 2 shows the duty
cycle output from the Memsic 2125 and the formula for calculating g force.
The T2 duration is calibrated to 10 milliseconds at 25
convert the formula to the following BASIC Stamp routine:
Read_X_Force:
The T1 duration (Memsic output) is captured by PULSIN in the variable xRaw . Since each BASIC Stamp
module has its own speed and will return a different raw value for the pulse, the factor called Scale (set
by the compiler based on the BASIC Stamp module installed) is used to convert the raw output to
microseconds. This will allow the program to operate properly with any BASIC Stamp 2-series module.
At this point the standard equation provided by Memsic can be applied, adjusting the values to account
for the pulse-width in microseconds.
hence the final multiplication. The result is a signed value representing g-force in milli-g's (1/1000th g).
2
PULSIN Xin, HiPulse, xRaw
xRaw = xRaw */ Scale
xGForce = ((xRaw / 10) - 500) * 8
RETURN
Figure 1. Essential Memsic 2125 Connections
© Parallax, Inc. • Memsic 2125 Accelerometer Demo Kit (#28017) • 09/2004
Figure 2. Memsic 2125 Pulse Output
A(g) = ((T1 / T2) – 0.5) / 12.5%
Fortunately, one divided by divided by 0.125 (12.5%) is eight,
°
C (room temperature). Knowing this, we can