diff --git a/bacnet-stack/ports/at91sam7s/main.c b/bacnet-stack/ports/at91sam7s/main.c
index 69750424..95cf3943 100644
--- a/bacnet-stack/ports/at91sam7s/main.c
+++ b/bacnet-stack/ports/at91sam7s/main.c
@@ -91,9 +91,13 @@ static inline void millisecond_timer(
 static inline void init(
     void)
 {
+    unsigned int pcsr;
+
     /* Initialize the Parallel I/O Controller A Peripheral Clock */
     volatile AT91PS_PMC pPMC = AT91C_BASE_PMC;
-    pPMC->PMC_PCER = pPMC->PMC_PCSR | (1 << AT91C_ID_PIOA);
+
+    pcsr = pPMC->PMC_PCSR;
+    pPMC->PMC_PCER = pcsr | (1 << AT91C_ID_PIOA);
 
     /* Set up the LEDs (PA0 - PA3) */
     volatile AT91PS_PIO pPIO = AT91C_BASE_PIOA;
diff --git a/bacnet-stack/ports/at91sam7s/rs485.c b/bacnet-stack/ports/at91sam7s/rs485.c
index db1b5806..a67e2278 100644
--- a/bacnet-stack/ports/at91sam7s/rs485.c
+++ b/bacnet-stack/ports/at91sam7s/rs485.c
@@ -65,9 +65,12 @@ static int RS485_Baud = 38400;
 void RS485_Initialize(
     void)
 {
+    unsigned int pcsr;
     /* Enable the USART0 clock in the Power Management Controller */
     volatile AT91PS_PMC pPMC = AT91C_BASE_PMC;
-    pPMC->PMC_PCER = pPMC->PMC_PCSR | (1 << AT91C_ID_US0);
+
+    pcsr = pPMC->PMC_PCSR;
+    pPMC->PMC_PCER = pcsr | (1 << AT91C_ID_US0);
 
     /* Disable and clear USART0 interrupt
        in AIC Interrupt Disable Command Register */
diff --git a/bacnet-stack/ports/at91sam7s/timer.c b/bacnet-stack/ports/at91sam7s/timer.c
index 8ef64672..bd91ef90 100644
--- a/bacnet-stack/ports/at91sam7s/timer.c
+++ b/bacnet-stack/ports/at91sam7s/timer.c
@@ -337,9 +337,12 @@ void Timer_Silence_Reset(
 void TimerInit(
     void)
 {
+    unsigned int pcsr;
     /* enable the Timer0 peripheral clock */
     volatile AT91PS_PMC pPMC = AT91C_BASE_PMC;
-    pPMC->PMC_PCER = pPMC->PMC_PCSR | (1 << AT91C_ID_TC0);
+
+    pcsr = pPMC->PMC_PCSR;
+    pPMC->PMC_PCER = pcsr | (1 << AT91C_ID_TC0);
     /* Set up the AIC  registers for Timer 0 */
     volatile AT91PS_AIC pAIC = AT91C_BASE_AIC;
     /* Disable timer 0 interrupt */