Nuts & Volts

■ PHOTO 4. Note that I checked the Use
DTR/RTS for Reset and Boot Loader box. I
clicked on the Read Device ID button and
the LPC2136 spit out the Part ID and Boot
Loader ID numbers. At this point, I can
do everything you see in the window.
The Buffer pulldown menu lets you
examine the contents of Flash and SRAM.
regulated power. I used an
LM1086CS-3.3 at the center of my
+3.3 VDC power source for the
LPC2136. As you can see in Photo 3, I
partitioned a chunk of the LM1086CS-
3.3 heatsink pad to accommodate an
LM1117MP-1.8 in the event that an
LPC2106 was in use instead of an
LPC2136.
The power supply subsystem for
the LPC2106 and LPC2136 is one
of those redundant items I alluded
to earlier. The only difference in
the power supply circuitry is the
inclusion of the +1.8 VDC voltage
regulator and its associated support
circuitry. Otherwise, there is no
rocket science involved in the
The Standard for checking
Capacitors in-circuit
92 April 2006
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LPC21XX power supply design.
The main oscillator circuitry of
the LPC2106 isn’t as sophisticated as
that of the LPC2136. The LPC2136 can
generate precision baud rates without
having to depend on the input clock
frequency being a frequency multiple
that is baud rate friendly.
To get the best performance balance
between baud rate generation
and speed without engaging the PLL,
the LPC2106 will generally be clocked
at 14.7456 MHz. On the other hand,
the LPC2136 can be clocked at 12
MHz and still provide decent baud
rate accuracy across the standard
baud rate range. Engaging the
LPC2136’s PLL with a 12 MHz input
Locate shorted or leaky
components or conditions
to the exact spot in-circuit
Still cutting up the pcb,
and unsoldering every
part trying to guess at
where the short is?
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clock will also allow the maximum
CPU clock rate of 60 MHz to be
attained. Our preliminary LPC2136
design is clocked at 12 MHz.
Thus far, we’ve made design
provisions to power and clock our
LPC2136. All we need to add to our
design at this point is a serial port
and a suitable LPC2136 reset circuit.
The serial port circuitry is nothing you
haven’t seen before. A three-volt
version of the venerable MAX232 —
the MAX3232 — is at the heart of the
LPC2136’s RS-232 subsystem.
Right now, we will only connect
one set of the MAX3232 drivers to the
LPC2136’s UART0 transmit and
receive pins. Connecting to UART0
instead of UART1 will enable us to
communicate with the LPC2136 using
a free LPC2000 Flash Utility from
Philips. However, to use the LPC2000
Flash Utility to its fullest, we must
design and construct an LPC2136
reset circuit that interacts with the
LPC2000 Flash Utility while, at the
same time, provides a stable reset
operation for the LPC2136.
The neat thing about this little
circuit we’re about to examine is that
is can be used for a multitude of
LCP2XXX devices including the
LPC2106. This is yet another one of
those redundant LPC2XXX subsystems.
Let’s take a look at how it works.
The LPC2000 Flash Utility uses
the PC serial port’s RTS modem control
signal to prepare the target ARM7
device to enter ISP mode following a
reset. The personal computer serial
port’s DTR modem control signal is
used to perform the reset operation.
Our design will eliminate the need for
the RTS signal by manually pulling the
LPC2136’s P0.14 line low using a voltage
divider made up of resistors R4
and R5. Resistor R5 is switched in and
out by one-half of DIP switch SW1.
When R5 is switched out of the
voltage divider, a logical high level is