diff --git a/configs/z80sim/defconfig b/configs/z80sim/defconfig
index 0603818d0bc..a993a6aeb1f 100644
--- a/configs/z80sim/defconfig
+++ b/configs/z80sim/defconfig
@@ -285,5 +285,5 @@ CONFIG_CUSTOM_STACK=n
 CONFIG_PROC_STACK_SIZE=1024
 CONFIG_PTHREAD_STACK_MIN=
 CONFIG_PTHREAD_STACK_DEFAULT=
-CONFIG_HEAP_SIZE=32768
-CONFIG_HEAP_BASE=CONFIG_DRAM_SIZE-CONFIG_HEAP_SIZE
+CONFIG_HEAP_SIZE=
+CONFIG_HEAP_BASE=
diff --git a/configs/z80sim/src/z80_irq.c b/configs/z80sim/src/z80_irq.c
index b6e81b32e1d..5fd15035984 100644
--- a/configs/z80sim/src/z80_irq.c
+++ b/configs/z80sim/src/z80_irq.c
@@ -76,10 +76,21 @@ void up_irqinitialize(void)
 
   current_regs = NULL;
 
-  /* And finally, enable interrupts */
+  /* Attach the timer interrupt -- There is not special timer interrupt
+   * enable in the simulation so it must be enabled here before interrupts
+   * are enabled.
+   *
+   * NOTE:  Normally, there are seperate enables for "global" interrupts
+   * and specific device interrupts.  In such a "normal" case, the timer
+   * interrupt should be attached and enabled in the the function up_timerinit()
+   */
+
+  irq_attach(Z80_IRQ_SYSTIMER, (xcpt_t)up_timerisr);
+
+  /* And finally, enable interrupts (including the timer) */
 
 #ifndef CONFIG_SUPPRESS_INTERRUPTS
-  irqrestore(TRUE);
+  irqrestore(Z80_C_FLAG);
 #endif
 }
 
@@ -93,7 +104,7 @@ void up_irqinitialize(void)
 
 void up_disable_irq(int irq)
 {
-  irqrestore(FALSE);
+  irqrestore(0);
 }
 
 /****************************************************************************
diff --git a/configs/z80sim/src/z80_timerisr.c b/configs/z80sim/src/z80_timerisr.c
index e56678d13e0..a088d14b862 100644
--- a/configs/z80sim/src/z80_timerisr.c
+++ b/configs/z80sim/src/z80_timerisr.c
@@ -91,5 +91,6 @@ int up_timerisr(int irq, FAR chipreg_t *regs)
 
 void up_timerinit(void)
 {
+  /* The timer interrupt was attached in up_irqinitialize -- see comments there */
 }