From be26e8497b510d4bf441f9d9cd3207d02b058bc0 Mon Sep 17 00:00:00 2001
From: Sam Anthony <sam@samanthony.xyz>
Date: Sat, 8 Nov 2025 11:03:02 -0500
Subject: specify dac voltage in millivolts

---
 fw/dac.c                      | 30 +++++++++++++++++++++---------
 fw/dac.h                      | 16 ++++++++--------
 fw/tests/system/dac_systest.c |  5 ++++-
 3 files changed, 33 insertions(+), 18 deletions(-)

diff --git a/fw/dac.c b/fw/dac.c
index bfea8c8..a4dc12c 100644
--- a/fw/dac.c
+++ b/fw/dac.c
@@ -8,6 +8,11 @@
 
 #include "dac.h"
 
+#define min(a, b) (((a) < (b)) ? (a) : (b))
+
+// Nominal reference voltage in millivolts
+#define VREF_MV 5000
+
 // Configuration bits:
 // buffered, gain=1x, mode=active
 typedef enum {
@@ -30,7 +35,14 @@ dacInit(void) {
 }
 
 static void
-set(U8 dacNum, Conf conf, U16 level) {
+set(U8 dacNum, Conf conf, U16 mv) {
+	U16 level;
+
+	mv = min(mv, VREF_MV); // clamp to 5V
+
+	// D = 2^10 * Vout / Vref / (1000mV/V)
+	level = (U16)((U32)mv * (1u<<10u) / VREF_MV);
+
 	level <<= 2u; // D0 at bit 2
 	level = ((U16)conf & 0xF000) | (level & 0x0FFF); // set config bits
 
@@ -52,20 +64,20 @@ set(U8 dacNum, Conf conf, U16 level) {
 }
 
 void
-dacSet1a(U16 level) {
-	set(1u, CONFA, level);
+dacSet1a(U16 mv) {
+	set(1u, CONFA, mv);
 }
 
 void
-dacSet1b(U16 level) {
-	set(1u, CONFB, level);
+dacSet1b(U16 mv) {
+	set(1u, CONFB, mv);
 }
 
 void
-dacSet2a(U16 level) {
-	set(2u, CONFA, level);
+dacSet2a(U16 mv) {
+	set(2u, CONFA, mv);
 }
 void
-dacSet2b(U16 level) {
-	set(2u, CONFB, level);
+dacSet2b(U16 mv) {
+	set(2u, CONFB, mv);
 }
diff --git a/fw/dac.h b/fw/dac.h
index 1882c3e..e9bf54b 100644
--- a/fw/dac.h
+++ b/fw/dac.h
@@ -20,17 +20,17 @@
 void dacInit(void);
 
 // Set DAC1 VOUTA.
-// Only the lower 10 bits are used.
-void dacSet1a(U16 level);
+// Mv is the desired voltage in millivolts.
+void dacSet1a(U16 mv);
 
 // Set DAC1 VOUTB.
-// Only the lower 10 bits are used.
-void dacSet1b(U16 level);
+// Mv is the desired voltage in millivolts.
+void dacSet1b(U16 mv);
 
 // Set DAC2 VOUTA.
-// Only the lower 10 bits are used.
-void dacSet2a(U16 level);
+// Mv is the desired voltage in millivolts.
+void dacSet2a(U16 mv);
 
 // Set DAC2 VOUTB.
-// Only the lower 10 bits are used.
-void dacSet2b(U16 level);
+// Mv is the desired voltage in millivolts.
+void dacSet2b(U16 mv);
diff --git a/fw/tests/system/dac_systest.c b/fw/tests/system/dac_systest.c
index 47aefb9..50b600a 100644
--- a/fw/tests/system/dac_systest.c
+++ b/fw/tests/system/dac_systest.c
@@ -13,7 +13,10 @@ main(void) {
 	spiInit();
 	dacInit();
 
-	dacSet1a(252u); // 1.23V
+	dacSet1a(123); // 0.123V
+	dacSet1b(345); // 0.345V
+	dacSet2a(1230); // 1.230V
+	dacSet2b(3450); // 3.450V
 
 	for (;;) {
 
-- 
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