Use crystal oscilator for system clock

1 files changed, 25 insertions, 0 deletions

diff --git a/gpio.s b/gpio.s
new file mode 100644
index 0000000..3b98ba0
--- /dev/null
+++ b/gpio.s
@@ -0,0 +1,25 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.equ RESETS_BASE,     0x4000c000
+.equ RESET_OFST,      0x0
+.equ RESET_DONE_OFST, 0x8
+
+.equ ATOMIC_CLEAR,    0x3000
+
+.type setup_gpio, %function
+.global setup_gpio
+
+setup_gpio:
+  // clear reset
+  ldr r1, =(RESETS_BASE + ATOMIC_CLEAR)
+  movs r0, 0x20 // IO_BANK0 is bit 5
+  str r0, [r1, RESET_OFST]
+  ldr r1, =RESETS_BASE
+1:
+  ldr r2, [r1, RESET_DONE_OFST]
+  tst r0, r2 // IO_BANK0 is still bit 5
+  // wait for reset done
+  beq 1b
+  mov pc, lr