diff --git a/apps/ledpwm1/Makefile b/apps/ledpwm1/Makefile
new file mode 100644
index 0000000..948a0b7
--- /dev/null
+++ b/apps/ledpwm1/Makefile
@@ -0,0 +1,65 @@
+# File:   Makefile
+# Author: M. P. Hayes, UCECE
+# Date:   12 Sep 2010
+# Descr:  Makefile for ledpwm1
+
+# Definitions.
+CC = avr-gcc
+CFLAGS = -mmcu=atmega32u2 -Os -Wall -Wstrict-prototypes -Wextra -g -I../../drivers/avr -I../../drivers -I../../utils
+OBJCOPY = avr-objcopy
+SIZE = avr-size
+DEL = rm
+
+
+# Default target.
+all: ledpwm1.out
+
+
+# Compile: create object files from C source files.
+ledpwm1.o: ledpwm1.c ../../drivers/avr/system.h ../../drivers/led.h ../../utils/pacer.h ../../utils/spwm.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+pio.o: ../../drivers/avr/pio.c ../../drivers/avr/pio.h ../../drivers/avr/system.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+system.o: ../../drivers/avr/system.c ../../drivers/avr/system.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+timer.o: ../../drivers/avr/timer.c ../../drivers/avr/system.h ../../drivers/avr/timer.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+led.o: ../../drivers/led.c ../../drivers/avr/pio.h ../../drivers/avr/system.h ../../drivers/led.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+pacer.o: ../../utils/pacer.c ../../drivers/avr/system.h ../../drivers/avr/timer.h ../../utils/pacer.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+spwm.o: ../../utils/spwm.c ../../drivers/avr/system.h ../../utils/spwm.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+
+
+
+# Link: create output file (executable) from object files.
+ledpwm1.out: ledpwm1.o pio.o system.o timer.o led.o pacer.o spwm.o
+	$(CC) $(CFLAGS) $^ -o $@ -lm
+	$(SIZE) $@
+
+
+# Create hex file for programming from executable file.
+ledpwm1.hex: ledpwm1.out
+	$(OBJCOPY) -O ihex ledpwm1.out ledpwm1.hex
+
+
+# Target: clean project.
+.PHONY: clean
+clean: 
+	-$(DEL) *.o *.out *.hex
+
+
+# Target: program project.
+.PHONY: program
+program: ledpwm1.hex
+	dfu-programmer atmega32u2 erase; dfu-programmer atmega32u2 flash ledpwm1.hex; dfu-programmer atmega32u2 start
+
+
diff --git a/apps/ledpwm1/Makefile.test b/apps/ledpwm1/Makefile.test
new file mode 100644
index 0000000..2b7fdbd
--- /dev/null
+++ b/apps/ledpwm1/Makefile.test
@@ -0,0 +1,55 @@
+# File:   Makefile
+# Author: M. P. Hayes, UCECE
+# Date:   11 Sep 2010
+# Descr:  Makefile for ledpwm1
+
+CC = gcc
+CFLAGS = -Wall -Wstrict-prototypes -Wextra -g -I../../drivers/test -I../../drivers -I../../utils
+
+DEL = rm
+
+
+# Default target.
+all: ledpwm1
+
+
+# Compile: create object files from C source files.
+ledpwm1-test.o: ledpwm1.c ../../drivers/led.h ../../drivers/test/system.h ../../utils/pacer.h ../../utils/spwm.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+led-test.o: ../../drivers/led.c ../../drivers/led.h ../../drivers/test/avrtest.h ../../drivers/test/pio.h ../../drivers/test/system.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+mgetkey-test.o: ../../drivers/test/mgetkey.c ../../drivers/test/mgetkey.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+pio-test.o: ../../drivers/test/pio.c ../../drivers/test/avrtest.h ../../drivers/test/pio.h ../../drivers/test/system.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+system-test.o: ../../drivers/test/system.c ../../drivers/test/avrtest.h ../../drivers/test/mgetkey.h ../../drivers/test/pio.h ../../drivers/test/system.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+timer-test.o: ../../drivers/test/timer.c ../../drivers/test/system.h ../../drivers/test/timer.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+pacer-test.o: ../../utils/pacer.c ../../drivers/test/system.h ../../drivers/test/timer.h ../../utils/pacer.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+spwm-test.o: ../../utils/spwm.c ../../drivers/test/system.h ../../utils/spwm.h
+	$(CC) -c $(CFLAGS) $< -o $@
+
+
+
+
+# Link: create executable file from object files.
+ledpwm1: ledpwm1-test.o led-test.o mgetkey-test.o pio-test.o system-test.o timer-test.o pacer-test.o spwm-test.o
+	$(CC) $(CFLAGS) $^ -o $@ -lrt
+
+
+# Clean: delete derived files.
+.PHONY: clean
+clean: 
+	-$(DEL) ledpwm1 ledpwm1-test.o led-test.o mgetkey-test.o pio-test.o system-test.o timer-test.o pacer-test.o spwm-test.o
+
+
+
diff --git a/apps/ledpwm1/doc/Makefile b/apps/ledpwm1/doc/Makefile
new file mode 100644
index 0000000..c93d18c
--- /dev/null
+++ b/apps/ledpwm1/doc/Makefile
@@ -0,0 +1,45 @@
+# File:   Makefile
+# Author: M. P. Hayes, UCECE
+# Date:   11 Sep 2010
+# Descr:  Makefile for led5 docs
+
+# The scripts used to make the graphs require the program dot; this
+# is part of the graphviz package.
+
+DEL = rm
+
+all: file_dependencies.pdf module_dependencies.pdf makefile_dependencies.pdf build_dependencies.pdf callgraph.pdf
+
+file_dependencies.pdf: files.d 
+	../../../etc/graphdeps.py $< --out $@
+
+module_dependencies.pdf: modules.d 
+	../../../etc/graphdeps.py $< --modules --rotate --out $@
+
+makefile_dependencies.pdf: ../Makefile
+	../../../etc/graphdeps.py $< --out $@
+
+build_dependencies.pdf: ../Makefile
+	../../../etc/graphdeps.py $< --out $@ --showops
+
+callgraph.pdf: callgraph.d
+	../../../etc/graphdeps.py --calls --modules $< --out $@ --showops
+
+
+files.d: ../Makefile
+	(cd ..;	../../etc/makemake.py --relpath --files . . ../../drivers ../../drivers/avr ../../utils ../../extra --exclude system.h > doc/files.d)
+
+
+modules.d: ../Makefile
+	(cd ..;	../../etc/makemake.py --relpath --modules . . ../../drivers ../../drivers/avr ../../utils ../../extra --exclude system > doc/modules.d)
+
+
+callgraph.d: ../Makefile
+	(cd ..;	../../etc/makemake.py --cc="avr-gcc" --cflags="-Os -mmcu=atmega32u2" --relpath --calls . . ../../drivers ../../drivers/avr ../../utils ../../extra --exclude system.h > doc/callgraph.d)
+
+
+# Clean: delete derived files.
+.PHONY: clean
+clean: 
+	-$(DEL) *.d *.pdf
+
diff --git a/apps/ledpwm1/doc/README b/apps/ledpwm1/doc/README
new file mode 100644
index 0000000..39a9712
--- /dev/null
+++ b/apps/ledpwm1/doc/README
@@ -0,0 +1,16 @@
+Running make in this directory will generate a number of PDF graphs.
+In the callgraph, the arrows means "calls".  In the dependency graphs,
+the arrows means "requires" (or "depends upon").
+
+callgraph.pdf             This shows the callgraph, i.e., what functions each
+                          function in the program calls.
+module_dependencies.pdf   This shows the dependencies between the modules.
+file_dependencies.pdf     This shows the dependencies between the files.
+makefile_dependencies.pdf This shows the dependencies required by make when
+                          building the program.
+build_dependencies.pdf    This is like makefile_dependencies.pdf but shows
+                          the operations performed to generate the new file.
+
+callgraph.d               This shows the callgraph in text format.
+files.d                   This shows the file dependencies in text format.
+modules.d                 This shows the module dependencies in text format.
diff --git a/apps/ledpwm1/ledpwm1.c b/apps/ledpwm1/ledpwm1.c
new file mode 100644
index 0000000..4588842
--- /dev/null
+++ b/apps/ledpwm1/ledpwm1.c
@@ -0,0 +1,31 @@
+/** @file   ledpwm1.c
+    @author M.P. Hayes
+    @date   25 Sep 2013
+*/
+
+#include "system.h"
+#include "pacer.h"
+#include "led.h"
+#include "spwm.h"
+
+#define LOOP_RATE 100
+
+
+int main (void)
+{
+    spwm_t spwm1 = {.duty = 50, .period = 100};
+    
+    system_init ();
+
+    led_init ();
+
+    pacer_init (LOOP_RATE);
+    
+    while (1)
+    {
+        pacer_wait ();
+
+        led_set (LED1, spwm_update (&spwm1));
+    }
+    return 0;
+}
diff --git a/utils/spwm.c b/utils/spwm.c
new file mode 100644
index 0000000..7d99862
--- /dev/null
+++ b/utils/spwm.c
@@ -0,0 +1,48 @@
+/** @file   spwm.c
+    @author M. P. Hayes, UCECE
+    @date   25 Sep 2013
+    @brief  These routines are for software pulse width modulation.  They are
+    useful for modulating the luiminance of an LED or beeping a piezo
+    tweeter. 
+*/
+#include "system.h"
+#include "spwm.h"
+
+
+/** Set the period.  This must be greater than the duty.  */
+void
+spwm_period_set (spwm_t *spwm, spwm_period_t period)
+{
+    spwm->period = period;
+}
+
+
+/** Set the duty.  This must be less than the period.  */
+void
+spwm_duty_set (spwm_t *spwm, spwm_period_t duty)
+{
+    spwm->duty = duty;
+}
+
+
+/** Return whether spwm output should be high or low.
+    For example, to control the flashing of a LED use:
+
+    led_set (led, spwm_update (spwm));  */
+bool
+spwm_update (spwm_t *spwm)
+{
+    spwm->count++;
+    if (spwm->count >= spwm->period)
+        spwm->count = 0;
+
+    return spwm->count < spwm->duty;
+}
+
+
+/* Reset spwm.  */
+void
+spwm_reset (spwm_t *spwm)
+{
+    spwm->count = 0;
+}
diff --git a/utils/spwm.h b/utils/spwm.h
new file mode 100644
index 0000000..002873c
--- /dev/null
+++ b/utils/spwm.h
@@ -0,0 +1,45 @@
+/** @file   spwm.h
+    @author M. P. Hayes, UCECE
+    @date   25 Sep 2013
+    @brief  These routines are for software pulse width modulation.  They are
+    useful for modulating the luiminance of an LED or beeping a piezo
+    tweeter. 
+*/
+#ifndef SPWM_H
+#define SPWM_H
+
+#include "system.h"
+
+
+typedef uint8_t spwm_period_t;
+
+
+/* This structure is defined here so the compiler can allocate enough
+   memory for it.  However, its fields should be treated as
+   private.  */
+typedef struct spwm_struct
+{
+    spwm_period_t period;
+    spwm_period_t duty;
+    spwm_period_t count;
+} spwm_t;
+
+
+/** Set the period.  This must be greater than the duty.  */
+void spwm_period_set (spwm_t *spwm, spwm_period_t period);
+
+
+/** Set the duty.  This must be less than the period.  */
+void spwm_duty_set (spwm_t *spwm, spwm_period_t duty);
+
+
+/** Return whether spwm output should be high or low.
+    For example, to control the flashing of a LED use:
+
+    led_set (led, spwm_update (spwm));  */
+bool spwm_update (spwm_t *spwm);
+
+
+/* Reset spwm.  */
+void spwm_reset (spwm_t *spwm);
+#endif