From 61720e925d2624cfae9b596413de9f55b508ce62 Mon Sep 17 00:00:00 2001
From: giomba <giomba@glgprograms.it>
Date: Sun, 15 Oct 2023 15:09:10 +0200
Subject: [PATCH] Add busy loop delay module.

---
 src/delay.asm | 35 +++++++++++++++++++++++++++++++++++
 src/delay.h   | 25 +++++++++++++++++++++++++
 2 files changed, 60 insertions(+)
 create mode 100644 src/delay.asm
 create mode 100644 src/delay.h

diff --git a/src/delay.asm b/src/delay.asm
new file mode 100644
index 0000000..1870081
--- /dev/null
+++ b/src/delay.asm
@@ -0,0 +1,35 @@
+SECTION code
+
+PUBLIC _delay_loop
+_delay_loop:
+    ; function prologue: use IX to index parameters passed on the stack
+    push    ix          ; 15
+    ld      ix,$0000    ; 14
+    add     ix,sp       ; 15
+
+    ; save scratch registers
+    push    af          ; 11
+    push    de          ; 11
+
+    ; load loop counter into DE
+    ld      d,(ix+$05)  ; 19
+    ld      e,(ix+$04)  ; 19
+
+    ; delay loop (24 × n)
+_delay_loop_loop:
+    dec     de          ; 6
+    ld      a,d         ; 4
+    or      e           ; 4
+    jp      nz,_delay_loop_loop ; 10
+
+    ; restore scratch registers
+    pop     de          ; 10
+    pop     af          ; 10
+
+    ; restore IX
+    pop     ix          ; 14
+
+    ret
+
+
+
diff --git a/src/delay.h b/src/delay.h
new file mode 100644
index 0000000..08956e8
--- /dev/null
+++ b/src/delay.h
@@ -0,0 +1,25 @@
+#ifndef CEDA_DELAY_H
+#define CEDA_DELAY_H
+
+/*
+    Assuming a Z80 CPU running at 4MHz, the chosen assembly loop
+    permits busy loops withing the following specs:
+
+    min: 37 us
+    max: 393253 ms
+    precision: 6 us
+
+    For example, to wait for 50 us use the following.
+    This will actually busy wait for 47 us due to precision losses.
+        delay_us(US_TO_LOOPS(50));
+*/
+
+#include <stdint.h>
+
+#define CPU_FREQ 4000000UL // 4 MHz
+
+#define US_TO_LOOPS(t) ((((t)*CPU_FREQ / 1000000UL) - 148) / 24)
+
+void delay_loop(uint16_t loops);
+
+#endif // CEDA_DELAY_H