Add new 96ppq sequencer tick timer

3 files changed, 57 insertions, 28 deletions

diff --git a/Makefile b/Makefile
index 00e643d..960a8a6 100644
--- a/Makefile
+++ b/Makefile
@@ -27,7 +27,7 @@ INC_FLAGS   := $(addprefix -I,$(INC_DIRS))
 INC_FLAGS   += -I$(LIBGBA_SRC)
 
 # Output library names and executables.
-TARGET         := STEPPER-v1.7
+TARGET         := STEPPER-v1.8-dev-01
 ELF            := $(BUILD_DIR)/$(TARGET).elf
 BIN            := $(BUILD_DIR)/$(TARGET).gba
 
@@ -82,7 +82,7 @@ run: main
         mgba-qt $(BIN)
 
 deploy:
-        mount /dev/sdg1 /mnt
+        mount /dev/sdf1 /mnt
         cp $(BIN) /mnt
         umount /mnt
 
diff --git a/src/main.c b/src/main.c
index 4502dbd..2e64275 100644
--- a/src/main.c
+++ b/src/main.c
@@ -11,11 +11,30 @@ WITH REGARD TO THIS SOFTWARE.
 
 // TODO: A list of features I would like to get to implement in the near future.
 //
+// High priority:
+//  + Higher resolution clock to allow for microtiming and more accurate tempo.
+//  - Look back again at the emulator issues...
+//  - Fix any bugs we currently have
+//      - Possible that when we switched banks there is some weird ui behaviour.
+//  - Add an envelope to ch3, would need to work with a timer in order to make
+//    it work I think.
+//  - Hold L/R retriggers at short intervals?
+//  - Sync via MIDI with the Analogue cables.
+//  - Channel params should show if there are some already on all triggers and
+//    modify only the selected parameter, not all of them.
+//  - Should scale mode be toggleable?
+//  - Add clipboard sharing between banks.
+//  - Allow using B + dpad to nudge trigs. This will potentially mean switching
+//    to key release to enable trigs.
+//  - Make sure transposing a sequence past the keyboard limit doesn't affect
+//    the sequence and can be reversed.
+//  - Study saving overhauls for bootleg cartridges.
+//
+// Low priority:
+//
 // UI tweaks.
 //  - Add custom user themes
 //  - Animations for cursor movement/current step highlight. (A fade out maybe?)
-//  - Should the channel parameters reset after leaving them? Is it more or
-//    less confusing this way?
 //
 // Quality of life improvements.
 //  - When not on play mode, adjusting a note or a parameter triggers the sound.
@@ -27,24 +46,9 @@ WITH REGARD TO THIS SOFTWARE.
 //  - Add tap tempo for BPM.
 //  - Allow "marking" several trigs to be able to copy/paste them and/or adjust
 //    their parameters.
-//  - Add an envelope to ch3, would need to work with a timer in order to make
-//    it work I think.
-//  - Sync via MIDI via arduinoboy or something similar.
 //  - Per trig LFO? How would we go about this? There is at least one empty slot
 //    in all channels. LFO amount? LFO speed? Would need a dedicated page for
 //    configuring LFOs
-//  - Per-channel N steps to create polymeters?
-//  - Channel params should show if there are some already on all triggers and
-//    modify only the selected parameter, not all of them.
-//  - Should scale mode be toggleable?
-//  - Improve SRAM saving to make room for longer patterns and/or more banks.
-//  - Higher resolution clock to allow for microtiming and more accurate tempo.
-//  - Add clipboard sharing between banks.
-//  - Allow using B + dpad to nudge trigs. This will potentially mean switching
-//    to key release to enable trigs.
-//  - Channel parameters should only modify the parameter being edited, not all
-//    of them.
-//  - Make sure sync works with the same cable for in/out.
 
 
 #include "gba/gba.h"
diff --git a/src/sequencer.c b/src/sequencer.c
index 632a777..11c0f5a 100644
--- a/src/sequencer.c
+++ b/src/sequencer.c
@@ -346,21 +346,46 @@ play_step(void) {
     step_counter = (step_counter + 1) % 16;
 }
 
+static nseq_ticks = 0;
+void
+sequencer_tick(void) {
+    if (nseq_ticks++ < 24) {
+        return;
+    }
+    nseq_ticks = 0;
+
+    // NOTE: ACK interrupt before play_step???
+    play_step();
+}
+
 void
 set_time(int bpm) {
     if (settings.sync == SYNC_IN_LINK) {
         return;
     }
-    // The number of ticks of a 1024 cycle clock in a step based on the BPM can
-    // be calculated as:
-    //     X bpm -> 60000 / 4 / bpm = Y ms = Ye-3 s
-    //     Y ms  -> Ye-3 / 59.99e-9 / 1024 = Z ticks
-    // We have to operate on integer values, so the numbers have been
-    // precalculated to `n_ticks = 244181 / bmp`
-    int n_ticks = -244181 / bpm;
-    irs_set(IRQ_TIMER_2, play_step);
+    // We use a high resolution 96 PPQ clock for sequencing. With the following
+    // formula we can calculate the time (in seconds) based on the PPQ and BPM:
+    //
+    //      t = 60 / (PPQ * BPM)
+    //
+    // To translate this timer to a GBA timer running at 64 CPU cycles per tick
+    // we need to divide this by the constant Q = 3.97329943e-06. In theory, the
+    // period of this timer running a a frequency of 262.21kHz should be
+    // Q = 3.815us, but using this number resulted in a less accurate sequencer
+    // compared to the metronome of a DAW (Bitwig). Tuning Q using a japanese
+    // GBA-SP and an Analogue Pocket resulted in the previously mentioned value.
+    //
+    // If we factor out the BPM we can obtain a fixed constant:
+    //
+    //      C = 60 / (96 * 3.97329943e-06) = 157300
+    //
+    // Allowing us to calculate the clock timer as -C/BPM. Note that this is
+    // integer division and we will be losing the fractional part, resulting in
+    // some minor precision differences, but it seems to work well in practice.
+    int n_ticks = -157300 / bpm;
+    irs_set(IRQ_TIMER_2, sequencer_tick);
     TIMER_DATA_2 = n_ticks;
-    TIMER_CTRL_2 = TIMER_CTRL_IRQ | TIMER_CTRL_ENABLE | TIMER_CTRL_FREQ_3;
+    TIMER_CTRL_2 = TIMER_CTRL_IRQ | TIMER_CTRL_ENABLE | TIMER_CTRL_FREQ_1;
 }
 
 TriggerNote *