Nuts & Volts

■ PHOTO 1. This is a bird’s eye view of
the LPC2106 adapter board. Note the
three .1 μF bypass capacitors mounted
on the bottom side.
the LPC2136. In fact, the only major
differences between the LPC2106 and
the LPC2136 are the number of peripherals
and the power requirements.
Figure 2 shows us that the
LPC2136 is a larger microcontroller in
terms of peripherals, SRAM, and Flash
storage. The LPC2136 isn’t the Big
Daddy here and has a big brother —
the LPC2138 — which carries 512KB of
Flash and 32KB of SRAM. The LPC2136
matches the LPC2138’s SRAM size, but
only houses half of the LPC2138’s
available Flash program memory.
The give and take between the
LPC2106 and the LPC2136 lies in the
LPC2106’s larger SRAM size and
reduced peripheral count versus the
LPC2136’s smaller amount of available
SRAM and increased peripheral count.
This offset of SRAM versus peripheral
count correlates with the first rule of
embedded computing which states that
nothing is free. If you want more peripherals,
you must give up something else.
In this case, that something else
is SRAM. As you can see in the
LPC2106, the first rule of embedded
computing works in the opposite way
as peripherals are sacrificed for additional
SRAM. This give and take is a
good thing, as it produces multiple
variants of the LPC210X and LPC213X
microcontrollers, which allows for
more cost effective designs.
The LPC2106 is a 48-pin device.
Since Figure 2 shows us that the
LPC2136 has 16 more general-purpose
I/O pins and pairs of I 2 C, SPI, and
analog-to-digital converter interfaces,
it would be logical to deduce that the
LPC2136 is a physically larger part. The
extra peripheral general I/O interfaces
push the LPC2136’s pin count to 64.
Note, also, that the LPC2136 supports
an on-chip digital-to-analog converter
subsystem. Another advanced feature
of the LPC2136 is the battery-backup
capable real time clock.
Many of the hardware design
points that exist for the LPC2106 are
common to the LPC2136. However,
we must take the battery-backable
real time clock of the LPC2136 into
account if we plan to use it. Also,
there is no need for a +1.8 VDC power
source for the LPC2136, as it only
requires a +3.3 VDC power source.
We’ll get to know the LPC2106
and the LPC2136 internals better
when we begin the coding phases of
our ARM7 designs. So, let’s attack the
hardware design points and build up
some ARM7 hardware.
FABRICATING ARM7
MICROCONTROLLER
SYSTEMS
THE DESIGN CYCLE
We won’t be using custom printed
circuit boards at the system level
in this installment, as you may want
to create your own flavor of the
LPC21XX designs I’m about to present
to you. With that thought in mind,
I’ve decided to use some prototype
boards from other EDTP products that
contain +3.3 VDC power supplies and
ready-to-go RS-232 serial ports. Once
we’ve verified the basic operations of
the ARM7 parts, I’ll put the final
designs down onto some professional
printed circuit boards, which I will
make available to you via the EDTP
Electronics website.
Now that I’ve chosen to use
cast-away prototype boards, the first
obstacle we have to negotiate is how
we will mount the fine-pitched
LPC21XX devices. The LPC2106 is
tiny and its 48 pins are positioned
equally around seven square
millimeters. The LPC2136 is equally
difficult to breadboard, as it is not
much larger with its 64 pins being
■ PHOTO 2. Here’s a shot of the
LPC2136 adapter board top and bottom.
The far right bottom capacitor is an
0805 SMT 10 μF at 6.3V tantalum that
is filtering the Vref pin. The rest of the
capacitors are .1 μF 0805 SMT devices,
which are acting in the power supply
bypass role.
April 2006 89