diff --git a/soc/runtime/Makefile b/soc/runtime/Makefile
index 241448f37..673235928 100644
--- a/soc/runtime/Makefile
+++ b/soc/runtime/Makefile
@@ -1,6 +1,6 @@
 include $(MSCDIR)/software/common.mak
 
-OBJECTS := isr.o flash_storage.o clock.o elf_loader.o services.o session.o log.o test_mode.o kloader.o bridge_ctl.o mailbox.o ksupport_data.o kserver.o moninj.o main.o
+OBJECTS := isr.o flash_storage.o clock.o rtiocrg.o elf_loader.o services.o session.o log.o test_mode.o kloader.o bridge_ctl.o mailbox.o ksupport_data.o kserver.o moninj.o main.o
 OBJECTS_KSUPPORT := ksupport.o exception_jmp.o exceptions.o mailbox.o bridge.o rtio.o ttl.o dds.o
 
 CFLAGS += -Ilwip/src/include -Iliblwip
diff --git a/soc/runtime/main.c b/soc/runtime/main.c
index 3a639b145..74df38aea 100644
--- a/soc/runtime/main.c
+++ b/soc/runtime/main.c
@@ -24,6 +24,7 @@
 #include "kloader.h"
 #include "flash_storage.h"
 #include "clock.h"
+#include "rtiocrg.h"
 #include "test_mode.h"
 #include "kserver.h"
 #include "session.h"
@@ -250,13 +251,10 @@ int main(void)
     puts("ARTIQ runtime built "__DATE__" "__TIME__"\n");
 
     clock_init();
+    rtiocrg_init();
     puts("Press 't' to enter test mode...");
     blink_led();
 
-#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
-    rtio_crg_pll_reset_write(0);
-#endif
-
     if(check_test_mode()) {
         puts("Entering test mode.");
         test_main();
diff --git a/soc/runtime/rtiocrg.c b/soc/runtime/rtiocrg.c
new file mode 100644
index 000000000..08cabd5b2
--- /dev/null
+++ b/soc/runtime/rtiocrg.c
@@ -0,0 +1,64 @@
+#include <stdio.h>
+#include <generated/csr.h>
+
+#include "clock.h"
+#include "flash_storage.h"
+#include "rtiocrg.h"
+
+void rtiocrg_init(void)
+{
+    char b;
+    int clk;
+
+#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
+    rtio_crg_pll_reset_write(0);
+#endif
+    b = 'i';
+    clk = 0;
+    fs_read("startup_clock", &b, 1, NULL);
+    if(b == 'i')
+        printf("Startup RTIO clock: internal\n");
+    else if(b == 'e') {
+        printf("Startup RTIO clock: external\n");
+        clk = 1;
+    } else
+        printf("WARNING: unknown startup_clock entry in flash storage\n");
+
+    if(!rtiocrg_switch_clock(clk))
+        printf("WARNING: startup RTIO clock failed\n");
+}
+
+int rtiocrg_check(void)
+{
+#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
+    return rtio_crg_pll_locked_read();
+#else
+    return 1;
+#endif
+}
+
+int rtiocrg_switch_clock(int clk)
+{
+    int current_clk;
+
+    current_clk = rtio_crg_clock_sel_read();
+    if(clk == current_clk) {
+#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
+        busywait_us(150);
+        if(!rtio_crg_pll_locked_read())
+            return 0;
+#endif
+        return 1;
+    }
+#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
+    rtio_crg_pll_reset_write(1);
+#endif
+    rtio_crg_clock_sel_write(clk);
+#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
+    rtio_crg_pll_reset_write(0);
+    busywait_us(150);
+    if(!rtio_crg_pll_locked_read())
+        return 0;
+#endif
+    return 1;
+}
diff --git a/soc/runtime/rtiocrg.h b/soc/runtime/rtiocrg.h
new file mode 100644
index 000000000..df498f617
--- /dev/null
+++ b/soc/runtime/rtiocrg.h
@@ -0,0 +1,8 @@
+#ifndef __RTIOCRG_H
+#define __RTIOCRG_H
+
+void rtiocrg_init(void);
+int rtiocrg_check(void);
+int rtiocrg_switch_clock(int clk);
+
+#endif /* __RTIOCRG_H */
diff --git a/soc/runtime/session.c b/soc/runtime/session.c
index d3227a4d6..c0908639c 100644
--- a/soc/runtime/session.c
+++ b/soc/runtime/session.c
@@ -12,6 +12,7 @@
 #include "kloader.h"
 #include "exceptions.h"
 #include "flash_storage.h"
+#include "rtiocrg.h"
 #include "session.h"
 
 #define BUFFER_IN_SIZE (1024*1024)
@@ -128,32 +129,6 @@ static int check_flash_storage_key_len(char *key, unsigned int key_len)
     return 1;
 }
 
-static int switch_clock(int clk)
-{
-    int current_clk;
-
-    current_clk = rtio_crg_clock_sel_read();
-    if(clk == current_clk) {
-#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
-        busywait_us(150);
-        if(!rtio_crg_pll_locked_read())
-            return 0;
-#endif
-        return 1;
-    }
-#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
-    rtio_crg_pll_reset_write(1);
-#endif
-    rtio_crg_clock_sel_write(clk);
-#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
-    rtio_crg_pll_reset_write(0);
-    busywait_us(150);
-    if(!rtio_crg_pll_locked_read())
-        return 0;
-#endif
-    return 1;
-}
-
 static int process_input(void)
 {
     switch(buffer_in[8]) {
@@ -180,7 +155,7 @@ static int process_input(void)
                 submit_output(9);
                 break;
             }
-            if(switch_clock(buffer_in[9]))
+            if(rtiocrg_switch_clock(buffer_in[9]))
                 buffer_out[8] = REMOTEMSG_TYPE_CLOCK_SWITCH_COMPLETED;
             else
                 buffer_out[8] = REMOTEMSG_TYPE_CLOCK_SWITCH_FAILED;
@@ -561,13 +536,11 @@ void session_poll(void **data, int *len)
             *len = -1;
             return;
         }
-#ifdef CSR_RTIO_CRG_PLL_RESET_ADDR
-        if(!rtio_crg_pll_locked_read()) {
+        if(!rtiocrg_check()) {
             log("RTIO clock failure");
             *len = -1;
             return;
         }
-#endif
     }
 
     l = get_out_packet_len();