Use crystal oscilator for system clock

1 files changed, 23 insertions, 0 deletions

diff --git a/xosc.s b/xosc.s
new file mode 100644
index 0000000..ee58ec0
--- /dev/null
+++ b/xosc.s
@@ -0,0 +1,23 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.equ XOSC_BASE, 0x40024000
+.equ CTRL_OFST, 0x0
+.equ STATUS_OFST, 0x4
+.equ STARTUP_OFST, 0xc
+
+.type start_xosc, %function
+.global start_xosc
+
+start_xosc:
+  ldr r1, =XOSC_BASE
+  movs r0, 47 // startup delay = 47 for 12Mhz crystal
+  str r0, [r1, STARTUP_OFST]
+  ldr r0, =0x00fabaa0 // enable
+  str r0, [r1, CTRL_OFST]
+1:
+  ldr r0, [r1, STATUS_OFST]
+  lsrs r0, r0, 31 // poll status bit
+  beq 1b
+  mov pc, lr