sylveos

commit af078277eb1cd892c681d040f3a5fa8892a405d9
parent 7ebb5bafd8a4a433d75c6d380efdeed9108e3d14
Author: Sylvia Ivory <git@sivory.net>
Date:   Fri,  6 Feb 2026 22:15:29 -0800

Software UART

Diffstat:
M
boot/root.zig
 | 
4
+++-
A
pi/devices/sw-uart.zig
 | 
70
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
M
pi/root.zig
 | 
12
++++++++++--
A
programs/sw-uart.zig
 | 
17
+++++++++++++++++

4 files changed, 100 insertions(+), 3 deletions(-)
diff --git a/boot/root.zig b/boot/root.zig
@@ -30,6 +30,9 @@ export fn kmain() void {
     uart.write_slice("Setting up stack\n");
     pi.setup_stacks();
 
+    uart.write_slice("Setting Cycle Counter\n");
+    pi.cycle_counter_init();
+
     uart.write_slice("Exception Vector:\n");
     for (0..16) |i| {
         const dst: *allowzero u32 = @ptrFromInt(i * @sizeOf(*u32));
@@ -39,7 +42,6 @@ export fn kmain() void {
     uart.write_slice("Enabling Interrupts\n");
     uart.enable_interrupts() catch {};
     pi.interrupts.enable_interrupts();
-    pi.cycle_counter_init();
 
     uart.write_slice("Entering program\n");
     uart.flush();
diff --git a/pi/devices/sw-uart.zig b/pi/devices/sw-uart.zig
@@ -0,0 +1,70 @@
+const std = @import("std");
+
+const interrupts = @import("../interrupts.zig");
+const mem = @import("../mem.zig");
+
+const clock = @import("./clock.zig");
+const gpio = @import("./gpio.zig");
+const pi = @import("../root.zig");
+
+pub const Error = error{ AlreadyInitialized, NotInitialized, InvalidBaud } || gpio.Error;
+
+var initialized = false;
+var interrupts_enabled = false;
+
+pub var delay: u32 = undefined;
+var tx: u8 = undefined;
+var rx: u8 = undefined;
+
+pub fn initialize(baud: u32, tx_pin: u8, rx_pin: u8) Error!void {
+    if (initialized) return Error.AlreadyInitialized;
+
+    try gpio.fn_sel(tx_pin, .output);
+    // try gpio.set_pull(tx_pin, .Up);
+    tx = tx_pin;
+
+    try gpio.fn_sel(rx_pin, .input);
+    // try gpio.set_pull(rx_pin, .Up);
+    rx = rx_pin;
+
+    try gpio.set_on(tx_pin);
+
+    mem.barrier(.Write);
+
+    // Cycles to delay
+    delay = (@divFloor(700_000_000, baud));
+    if (delay == 0) return Error.InvalidBaud;
+
+    initialized = true;
+}
+
+pub fn write_byte(byte: u8) !void {
+    if (!initialized) return Error.NotInitialized;
+
+    gpio.set_off(tx) catch {};
+    mem.barrier(.Write);
+
+    var deadline = pi.cycle_counter_read() + delay;
+    pi.cycle_wait_until(deadline);
+
+    for (0..8) |n| {
+        const state = (byte & (@as(u8, 1) << @truncate(n))) != 0;
+        gpio.write(tx, state) catch {};
+        mem.barrier(.Write);
+
+        deadline += delay;
+        pi.cycle_wait_until(deadline);
+    }
+
+    gpio.set_on(tx) catch {};
+    mem.barrier(.Write);
+
+    deadline += delay;
+    pi.cycle_wait_until(deadline);
+}
+
+pub fn write_slice(bytes: []const u8) !void {
+    for (bytes) |b| {
+        try write_byte(b);
+    }
+}
diff --git a/pi/root.zig b/pi/root.zig
@@ -1,3 +1,5 @@
+const std = @import("std");
+
 pub const Scheduler = @import("scheduler.zig");
 pub const PSR = @import("./psr.zig").PSR;
 
@@ -10,15 +12,15 @@ pub const devices = struct {
     pub const clock = @import("./devices/clock.zig");
     pub const gpio = @import("./devices/gpio.zig");
     pub const mini_uart = @import("./devices/mini-uart.zig");
+    pub const sw_uart = @import("./devices/sw-uart.zig");
     pub const timer = @import("./devices/timer.zig");
     pub const mailbox = @import("./devices/mailbox.zig");
 };
 
 pub inline fn cycle_counter_init() void {
-    const in: u32 = 1;
     asm volatile ("MCR p15, 0, %[in], c15, c12, 0"
         :
-        : [in] "r" (in),
+        : [in] "r" (1),
     );
 }
 
@@ -28,6 +30,12 @@ pub inline fn cycle_counter_read() u32 {
     );
 }
 
+pub inline fn cycle_wait_until(deadline: u32) void {
+    while (cycle_counter_read() < deadline) {
+        std.mem.doNotOptimizeAway(deadline);
+    }
+}
+
 pub inline fn set_sp(sp: *usize) void {
     asm volatile ("mov sp, %[value]"
         :
diff --git a/programs/sw-uart.zig b/programs/sw-uart.zig
@@ -0,0 +1,17 @@
+const std = @import("std");
+const pi = @import("pi");
+
+const mini_uart = pi.devices.mini_uart;
+const sw_uart = pi.devices.sw_uart;
+const clock = pi.devices.clock;
+
+pub fn main() !void {
+    try sw_uart.initialize(115200, 8, 9);
+
+    while (true) {
+        mini_uart.write_slice("Hello World!\n");
+        try sw_uart.write_slice("Hello World!\n");
+
+        clock.delay_s(1);
+    }
+}