8 files changed, 264 insertions, 0 deletions
diff --git a/hexedit/Makefile b/hexedit/Makefile
new file mode 100644
index 0000000..95f49ae
--- /dev/null
+++ b/hexedit/Makefile
@@ -0,0 +1,17 @@
+all: build
+
+build: echo.uf2
+
+echo.uf2: echo.elf
+	../elf/elf2uf2 echo.elf echo.uf2
+
+objects = main.o xosc.o clocks.o gpio.o uart.o
+
+echo.elf: $(objects)
+	arm-none-eabi-ld -T pico_ram_only.ld -o echo.elf $(objects)
+
+$(objects): %.o: %.s
+	arm-none-eabi-as -o $@ $<
+
+clean:
+	rm echo.elf echo.uf2 *.o
diff --git a/hexedit/clocks.s b/hexedit/clocks.s
new file mode 100644
index 0000000..83bc9ff
--- /dev/null
+++ b/hexedit/clocks.s
@@ -0,0 +1,31 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.equ CLOCKS_BASE, 0x40008000
+.equ CLK_REF_CTRL_OFST,  0x30
+.equ CLK_SYS_CTRL_OFST,  0x3c
+.equ CLK_PERI_CTRL_OFST, 0x48
+
+.type setup_clocks, %function
+.global setup_clocks
+
+setup_clocks:
+
+  ldr r1, =CLOCKS_BASE
+
+  // Reference clock
+  movs r0, 0x2 // src = xosc
+  str r0, [r1, CLK_REF_CTRL_OFST]
+
+  // System clock
+  movs r0, 0x0 // src = clk_ref
+  str r0, [r1, CLK_SYS_CTRL_OFST]
+
+  // Peripheral clock
+  movs r0, 1 // set enable
+  lsls r0, 11
+  adds r0, 0x4 << 5 // src = xosc
+  str r0, [r1, CLK_PERI_CTRL_OFST]
+
+  bx lr
diff --git a/hexedit/gpio.s b/hexedit/gpio.s
new file mode 100644
index 0000000..32e2c37
--- /dev/null
+++ b/hexedit/gpio.s
@@ -0,0 +1,23 @@
+.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:
+  ldr r1, =(RESETS_BASE + ATOMIC_CLEAR)
+  movs r0, 0b1 << 5 // IO_BANK0
+  str r0, [r1, RESET_OFST]
+  ldr r1, =RESETS_BASE
+1:
+  ldr r2, [r1, RESET_DONE_OFST]
+  tst r2, r0
+  beq 1b
+  bx lr
diff --git a/hexedit/hexedit.s b/hexedit/hexedit.s
new file mode 100644
index 0000000..0bc03a9
--- /dev/null
+++ b/hexedit/hexedit.s
@@ -0,0 +1,25 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.type hexedit, %function
+.global hexedit
+
+hexedit:
+  ldr r2, =0x20000100
+  movs r1, 0
+getchar:
+  bl uart_recv
+  cmp r0, 'g
+  beq stop
+  subs r0, '0 // The ASCII char '0'
+  bmi next
+  lsls r1, 4
+  adds r1, r0
+  b getchar
+next:
+  ldr r0, [r2, 0]
+  adds r2, 4
+  b hexedit
+stop:
+  b 0x20000100
diff --git a/hexedit/main.s b/hexedit/main.s
new file mode 100644
index 0000000..5e294d4
--- /dev/null
+++ b/hexedit/main.s
@@ -0,0 +1,16 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.type main, %function
+.global main
+
+main:
+  bl start_xosc
+  bl setup_clocks
+  bl setup_gpio
+  bl setup_uart
+1:
+  bl uart_recv
+  bl uart_send
+  b 1b
diff --git a/hexedit/pico_ram_only.ld b/hexedit/pico_ram_only.ld
new file mode 100644
index 0000000..eb2450e
--- /dev/null
+++ b/hexedit/pico_ram_only.ld
@@ -0,0 +1,13 @@
+ENTRY(main)
+
+MEMORY {
+  FLASH(rx) : ORIGIN = 0x10000000, LENGTH = 2M
+  SRAM(rwx) : ORIGIN = 0x20000000, LENGTH = 264K
+}
+
+SECTIONS {
+  .text : {
+    *(.text)
+    . = ALIGN(4);
+  } > SRAM
+}
diff --git a/hexedit/uart.s b/hexedit/uart.s
new file mode 100644
index 0000000..9804e53
--- /dev/null
+++ b/hexedit/uart.s
@@ -0,0 +1,116 @@
+.syntax unified
+.cpu cortex-m0plus
+.thumb
+
+.equ RESETS_BASE, 0x4000c000
+.equ RESET_OFST,      0x0
+.equ RESET_DONE_OFST, 0x8
+
+.equ IO_BANK0_BASE, 0x40014000
+.equ GPIO0_CTRL_OFST, 0x04
+.equ GPIO1_CTRL_OFST, 0x0c
+
+.equ UART0_BASE, 0x40034000
+.equ UARTDR_OFST,     0x00
+.equ UARTFR_OFST,     0x18
+.equ UARTIBRD_OFST,   0x24
+.equ UARTFBRD_OFST,   0x28
+.equ UARTLCR_H_OFST,  0x2c
+.equ UARTCR_OFST,     0x30
+
+.equ ATOMIC_SET,    0x2000
+.equ ATOMIC_CLEAR,  0x3000
+
+.type setup_uart, %function
+.global setup_uart
+
+setup_uart:
+
+  // Deassert reset
+  ldr r1, =(RESETS_BASE + ATOMIC_CLEAR)
+  movs r0, 0b1 // UART0
+  lsls r0, 22
+  str r0, [r1, RESET_OFST]
+  ldr r1, =RESETS_BASE
+1:
+  ldr r2, [r1, RESET_DONE_OFST]
+  tst r2, r0
+  beq 1b
+
+  // Configure and enable
+  ldr r1, =UART0_BASE
+  movs r0, 6
+  str r0, [r1, UARTIBRD_OFST]
+  movs r0, 33
+  str r0, [r1, UARTFBRD_OFST]
+  movs r0, 0b111 << 4 // 0b11 = word len 8 bits, 0b1 = FIFO enabled
+  str r0, [r1, UARTLCR_H_OFST]
+  ldr r1, =(UART0_BASE + ATOMIC_SET)
+  movs r0, 0b1 // UART enable
+  str r0, [r1, UARTCR_OFST]
+
+  // Configure GPIO 0 and 1 as TX and RX
+  ldr r1, =IO_BANK0_BASE
+  movs r0, 2 // UART function
+  str r0, [r1, GPIO0_CTRL_OFST]
+  str r0, [r1, GPIO1_CTRL_OFST]
+
+  bx lr
+
+.type uart_send, %function
+.global uart_send
+
+uart_send:
+  ldr r1, =UART0_BASE
+  movs r3, 0b1 << 5 // TX FIFO full
+1:
+  ldr r2, [r1, UARTFR_OFST]
+  tst r2, r3
+  bne 1b
+  strb r0, [r1, UARTDR_OFST]
+  bx lr
+
+.type uart_recv, %function
+.global uart_recv
+
+uart_recv:
+  ldr r1, =UART0_BASE
+  movs r3, 0b1 << 4 // RX FIFO empty
+1:
+  ldr r2, [r1, UARTFR_OFST]
+  tst r2, r3
+  bne 1b
+  ldrb r0, [r1, UARTDR_OFST]
+  bx lr
+
+.type send_hex, %function
+.global send_hex
+
+send_hex:
+  push {lr}
+  movs r4, r0
+  movs r0, '0
+  bl uart_send
+  movs r0, 'x
+  bl uart_send
+  movs r5, 8 // eight nibbles in a word
+0:
+  movs r0, 28 // rotate left 4
+  rors r4, r0
+  movs r0, 0xF // lowest nibble mask
+  ands r0, r4
+  cmp r0, 0x9 // number or letter?
+  bhi 1f
+  adds r0, '0
+  b 2f
+1:
+  adds r0, 'A - 0xA
+2:
+  bl uart_send
+  subs r5, 1
+  bne 0b
+  movs r0, '\r
+  bl uart_send
+  movs r0, '\n
+  bl uart_send
+  pop {pc}
diff --git a/hexedit/xosc.s b/hexedit/xosc.s
new file mode 100644
index 0000000..63760da
--- /dev/null
+++ b/hexedit/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 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, 31 // stable bit
+  beq 1b
+  bx lr