some more turbo tape autocalibration fixes

c64/kernal/tape/tape_turbo_sync.s

@@ -16,26 +16,12 @@
 tape_turbo_sync_header:
 
 
-#if CONFIG_TAPE_TURBO_AUTOCALIBRATE
-
-	// Initial pulse threshold
-	lda #$BF // auto-calibration under VICE often returns this value
-	sta IRQTMP+0
-
-	// Zero temporary stack area
-	lda #$00
-	ldy #$03
-!:
-	sta STACK, y
-	dey
-	bpl !-
-
-#endif // CONFIG_TAPE_TURBO_AUTOCALIBRATE
+#if !CONFIG_TAPE_TURBO_AUTOCALIBRATE
 
 	// Synchronize with start of sync sequence
 	jsr tape_turbo_sync_first
 
-	// Perform synchronization
+	// Perform synchronization, double loop, total $C0 * $04 iterations
 	ldx #$C0
 !:
 	phx_trash_a
@@ -51,20 +37,57 @@ tape_turbo_sync_header:
 	dex
 	bne !--
 
-	// FALLTROUGH
+#else // CONFIG_TAPE_TURBO_AUTOCALIBRATE
 
-#if CONFIG_TAPE_TURBO_AUTOCALIBRATE
+	// Initial pulse threshold
+	lda #$BF // auto-calibration under VICE often returns this value
+	sta IRQTMP+0
 
-tape_turbo_sync_calibrate:
+	// Zero temporary stack area
+	lda #$00
+	ldy #$03
+!:
+	sta STACK, y
+	dey
+	bpl !-
 
-	ldy #$80
+	// Synchronize with start of sync sequence
+	jsr tape_turbo_sync_first
+
+	// Perform synchronization, get measurements from 128 bytes
+	ldx #$10
 
 	// FALLTROUGH
 
-tape_turbo_sync_calibrate_loop:
+tape_turbo_sync_outer_loop:
+
+    // Set .Y (inner loop counter) to $28-$38, average $30; $10 * $30 equals $C0 * $04
+	// We want a bit of irregularity with byte selection for calibration
+	txa
+	pha                                          // store .X
+	clc
+	adc #$27
+	tay
+	
+	// FALLTROUGH
+
+tape_turbo_sync_inner_loop:
+
+	cpy #$09
+	bcc !+                                       // if inner couter is 8 or lower, get this byte for measurement
+
+	// Get byte without measurements
+	jsr tape_turbo_get_byte
+	cmp #$02
+	bne tape_turbo_sync_header                   // branch if failure
+	beq tape_turbo_sync_header_next_iter         // branch always
+	
+!:
+	// Get byte and take measurements
 
-	// Skip 5 bits
 	phy_trash_a
+
+	// Skip 5 bits, they should be all 0
 	ldy #$04
 !:
 	jsr tape_turbo_get_bit
@@ -72,13 +95,22 @@ tape_turbo_sync_calibrate_loop:
 	dey
 	bpl !-
 
+	// Get measurements from 3 bits
 	jsr tape_turbo_sync_measure_0
 	jsr tape_turbo_sync_measure_1
 	jsr tape_turbo_sync_measure_0
 
 	ply_trash_a
+
+	// FALLTROUGH
+
+tape_turbo_sync_header_next_iter:
+
 	dey
-	bne tape_turbo_sync_calibrate_loop
+	bne tape_turbo_sync_inner_loop
+	plx_trash_a
+	dex
+	bne tape_turbo_sync_outer_loop
 
 	// Calculate new pulse threshold as arithmetic mean
 	// We accumulated 256 zeros and 128 ones, so calculating average value is easy
@@ -90,10 +122,10 @@ tape_turbo_sync_calibrate_loop:
 	adc STACK+1
 	sta IRQTMP+0
 
-	// FALLTROUGH
-
 #endif // CONFIG_TAPE_TURBO_AUTOCALIBRATE
 
+	// FALLTROUGH
+
 tape_turbo_sync_payload:
 
 	jsr tape_turbo_sync_first
@@ -111,7 +143,7 @@ tape_turbo_sync_payload:
 	dex
 	bne !-
 
-	rts 
+	rts
 
 
 #endif // CONFIG_TAPE_TURBO