|
Issue
99, October 1998
X-Y Graphing Data
Logger
by
Alberto Ricci Bitti
Start
• Machine
Muscle
• Using
Componentware
• Low-Power
Guys
• Flash
Risc Glue
• The Graphic
Engine
• Casio
Protocol
• Main
Code
• Inside
The Box
• Setting
up the Casio
• First
Graph
• Modeling
The Data
• Broader
Vision
• Software,
References & Sources
LOW-POWER
GUYS
Every successful project
relies on component selection, especially battery-operated
designs. Maxim’s MAX186 is a low-power 12-bit ADC.
It needs minimal external hardware and features a nice
internal 4.096-V reference that sizes each step to a handy
1 mV.
The MAX186 sports eight single-ended
inputs, which is more than enough for most applications.
The same inputs can be reconfigured as four bipolar and
pseudodifferential inputs by simply changing the command
word. It’s suitable for reading data directly from
a large variety of sensors (see Figure 1).
|
Figure 1—The
unit can accommodate a variety of sensors. Up
to eight sensors can be read at the at the same
time with 1-mV steps from 0 to 4096 mV. a—This
setup reads temperatures ranging 0–100°C
with a resolution of 0.1°C. b—The
light sensor is placed in front of an LED to
nonintrusively monitor a device’s on/off
status. The diodes reduce the maximum output
to a safe 3.6 V.
|
The MAX186 generates its
clock internally, and the entire operation is controlled
through a four-wire SPI, QSPI, or Microwire serial interface.
The device is put in standby mode via a three-level input
pin or with a software command word.
It consumes 1.5 mA typical
while operating, which drops to a mere 2 mA in full power-down
mode. There is also an interesting fast power-down mode
(not used in this design) that consumes 30 mA with a wake-up
time as short as 5 ms.
If 10 bits are enough for
your taste, you can replace the MAX186 with the MAX192.
It’s fully pin- and software-compatible with the
MAX186. Price aside, the only difference resides in the
precision of the two lower bits, which aren’t guaranteed
for the MAX192.
The chip needs a single 5-V
power supply. With such a low power requirement, the drain
of the power regulator itself becomes important. Ordinary
regulators such as the 78L05 can easily drain more current
than the whole circuit.
You need specific low-dropout
regulators like the LM2936 to get maximum battery life.
The LM2936 has a quiescent current of only 9 mA, and it’s
internally protected from reverse battery connection.
|
