diff --git a/FluxEngine.cydsn/CortexM3/ARM_GCC_541/Release/FluxEngine.hex b/FluxEngine.cydsn/CortexM3/ARM_GCC_541/Release/FluxEngine.hex
index c31b82c5..d00b5d68 100644
--- a/FluxEngine.cydsn/CortexM3/ARM_GCC_541/Release/FluxEngine.hex
+++ b/FluxEngine.cydsn/CortexM3/ARM_GCC_541/Release/FluxEngine.hex
@@ -13,35 +13,35 @@
 :4003000002F07AFD02A801F08AFF01F094FF13B05DF804EB04B0704710B5044601780648FFF7E5FF0420FFF7EFFE62782146BDE81040042001F080B83A36000007B50023B1
 :40034000ADF804308DF80600032301A88DF80530FFF7E2FF03B05DF804FB0000F8B51D4C0646FFF733FF637F03B156B91A48FFF7BEFFFFF75EFF012000236077636302F048
 :4003800099F83246616B1548FFF7B1FF114D0027636B9E4216D001F0D7FE00B16F63636B9E4205DD0020FFF72BFF6B6B013305E005DA0120FFF724FF6B6B013B6B6302F059
-:4003C000A1F8E5E7322002F05FF8BDE8F8400448FFF78DBF8081FF1F473600004E3600006B3600002DE9F04F9BB062B602F0F4F8B449042002F018F9B34801F0B1FEB34881
-:4004000002F0E4FBB24801F0E5FE02F0C5FA02F097F9002002F0B8FB01F000FF0221002000F0A4FFAB4D0321084602F04BF82E462C46DFF8C4B202F065F8AB7F73B12A6A5C
-:40044000EB689B1A41F28832934207D9002001F075FE002002F098FB0023AB7700F0BEFF18B99D48FFF743FF04E000F0BDFF0028F7D109E000F0B2FF0028FBD09748FFF7F7
-:4004800036FF032001F016F8032000F0B9FF0128D1D19348FFF764FE92490320FFF782FE94F838109048FFF722FF94F83830023B122B00F2F583DFE813F01300F3031C001E
-:4004C000F3032200F3034400F3036800F303A001F3033503F3035403F3035A03F303650303238DF828308DF829300A238DF82A3044E394F83A00FFF731FF7C4B3BE3FFF7AE
+:4003C000A1F8E5E7322002F05FF8BDE8F8400448FFF78DBF8081FF1F473600004E3600006B3600002DE9F04F9BB062B602F0F4F8B749042002F018F9B64801F0B1FEB64878
+:4004000002F0E4FBB54801F0E5FE02F0C5FA02F097F9002002F0B8FB01F000FF0221002000F0A4FFAE4D0321084602F04BF82E462C46DFF8D4B202F065F8AB7F73B12A6A46
+:40044000EB689B1A41F28832934207D9002001F075FE002002F098FB0023AB7700F0BEFF18B9A048FFF743FF04E000F0BDFF0028F7D109E000F0B2FF0028FBD09A48FFF7F1
+:4004800036FF032001F016F8032000F0B9FF0128D1D19648FFF764FE95490320FFF782FE94F838109348FFF722FF94F83830023B122B00F2F583DFE813F01300F3031C0015
+:4004C000F3032200F3034400F3036800F303A001F3033503F3035403F3035A03F303650303238DF828308DF829300B238DF82A3044E394F83A00FFF731FF7F4B3BE3FFF7AA
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 :400540001CE30220FFF7E0FD4FF000090DF1280A4FF480780027C8EB0903DA1907F80A200137402FF9D10220FFF7CEFD3A465146022000F061FFB8F10108EBD109F1010983
-:40058000B9F1400FE4D15A4BC7E294F83A0001F0F9FD606BFFF7E2FE02F09AFB554BDFF85C811A78002742F004021A701A7842F001021A701A7802F0FE021A701A7802F0B8
-:4005C000FE021A7002F088FB0220FFF79DFD41F6FF734FF480420121022002F0DBFA84F8780001F0FDFE08F807000137102FF8D1DFF80CA100270AF15D081FFA88F901378D
-:40060000102F14BF3A4600221AF8010F2244062392F82420402101F017FF4A4646F242419AF8000001F022FF09F14009102F1FFA89F9E4D100237372637A002BFCD000275B
-:40064000142239460AA8777202F0A2FB40230D934FF0FF333760736037723368DFF8A490334493F8241096F87800CDF8309001F06FFE96F8780001F02DFE012196F8780005
-:4006800001F000FECDF81090236813B96B7A002BFAD0002794F83BA0676094F80890B9F1000F08D101F02EFF6B7A83B3BAF1010A85F809902BD1042194F8780001F054FE3D
-:4006C0005CE000BF19010000F900000091000000C50000008081FF1F793600008C3600009881FF1FB881FF1F963600002C3600002E360000926400409C640040A481FF1F2D
-:40070000A381FF1F0086FF1F2B7A002BD3D1626823689A42F8D04FF0000962680AA808EB82124A440A92C9F140020B9200F0DAFA0B9A0137C2F1400209EB02030D9A5FFAC9
-:4007400083F95AB90220FFF7DFFC4022BC49022000F072FE049B0C9340230D93B9F13F0FDBD96268B74B01321340002BBEBF03F1FF3363F00F03013363608EE794F878002C
-:4007800001F002FE0028F9D10AA800F0DFFA0220FFF7BAFC337A63B90D9A402A06D0C2F1400292B2A649022000F046FE0220FFF7ABFC0D9A32F0400206D10220114600F0D5
-:4007C0003BFE0220FFF7A0FCFFF744FD237A33B19D48FFF78CFD0220FFF7B0FD06E09B4B09A81B88ADF82430FFF796FD627A3946237A9748FFF77BFD55E29648FFF777FDC5
+:40058000B9F1400FE4D15D4BC7E294F83A0001F0F9FD606BFFF7E2FE02F09AFB584BDFF86C811A78002742F004021A701A7842F001021A701A7802F0FE021A701A7802F0A2
+:4005C000FE021A7002F088FB0220FFF79DFD41F6FF734FF480420121022002F0DBFA84F8780001F0FDFE08F807000137102FF8D1DFF81CA100270AF15D081FFA88F901377D
+:40060000102F14BF3A4600221AF8010F2244062392F82420402101F017FF4A4646F242419AF8000001F022FF09F14009102F1FFA89F9E4D196F83B3033B100237372637A9C
+:40064000002BFCD000237372142200210AA802F09FFB40234FF0FF320D9300232360626023722368274F234493F8241094F878000C9701F06DFE94F8780001F02BFE01210C
+:4006800094F8780001F0FEFD04972368002BFCD000277760D6F80CA0237A7BB901F032FFA98F04E0EB68CAEB03038B4206D2626823689A422ED12B7A002BF3D094F808801C
+:4006C000042194F878005FFA88F801F04DFE54E019010000F900000091000000C50000008081FF1F793600008C3600009881FF1FB881FF1F963600002C3600002E3600006F
+:40070000926400400086FF1F9C640040A481FF1FA381FF1F4FF0000962680AA808EB82124A440A92C9F140020B9200F0DBFA0B9A0137C2F1400209EB02030D9A5FFA83F900
+:400740005AB90220FFF7E0FC4022BD49022000F073FE049B0C9340230D93B9F13F0FDBD96268B84B01321340002BBEBF03F1FF3363F00F03013363608EE794F8780001F0B3
+:4007800003FE0028F9D10AA800F0E0FA0220FFF7BBFC0D9B402B07D002204022A84900F04BFE0220FFF7B0FC0D9B022033F040021DBFC3F1400292B2A149114600F03CFEEF
+:4007C0000220FFF7A1FCFFF745FDB8F1000F06D09D48FFF78CFD0220FFF7B0FD06E09B4B09A81B88ADF82430FFF796FD627A3946237A9748FFF77BFD55E29648FFF777FDEF
 :40080000E76B17F03F0701D003204AE2012001F0BFFC95F83A0001F0B5FC02F059FA9BF80030DFF8348203F0FB038BF800309BF8003043F001038BF800309BF8003003F027
 :40084000FE038BF800309BF8003003F0FE038BF8003002F041FA686BFFF780FD01214FF4804341F6FF72084601F098FC85F8780001F0B6FD08F807000137102FF8D1DFF88D
 :40088000DC91002709F15D031FFA83F807930137102F14BF3A46002219F8010F2244052392F82420402101F0CFFD414646F24C4299F8000001F0DAFD08F14008102F1FFA56
-:4008C00088F8E4D10027B946BA46F36B4FF0FF389B09142239460AA837600593C6F80480377202F055FA402301200D9300F0E2FDCDF81880049701F005FE2268514B01322B
+:4008C00088F8E4D10027B946BA46F36B4FF0FF389B09142239460AA837600493C6F80480377202F055FA402301200D9300F0E2FDCDF81880059701F005FE2268514B01322B
 :400900001340002BBCBF03F1FF3363F00F036168B8BF01338B4200F0A380BAF1000F07D0237A002B40F0B0806B7A002B40F0AC800B9B002B34D1B9F1000F0BD07F22404926
 :400940005A540133402BFAD10A910B9328E0BAF1000F06D1012000F053FD01288046F6D107E0237A002B40F08F806B7A002BF1D08AE03349FFF716FC8146314B0B90404665
-:400980000A9300F097FDB9F13F0F07F1010706DD059BDB1BD3F1000949EB030900E0C1460B9BDBB12368079A0AA802EB83120D9BC3F1400313440C9300F0D7F90D9B6BB9FE
+:400980000A9300F097FDB9F13F0F07F1010706DD049BDB1BD3F1000949EB030900E0C1460B9BDBB12368079A0AA802EB83120D9BC3F1400313440C9300F0D7F90D9B6BB9FF
 :4009C0002A68204B01321340002BBEBF03F1FF3363F00F030133236040230D93636801333ED12B680F2B3BD14FF00008C5F8048001F002FC85F808806B6895F878002B44DD
 :400A000093F8241001F0A4FC95F8780001F062FC012195F8780001F035FC85F80980637A002BFCD04FF00008012086F8098001F0EFFB404601F0ACFBCDF8188013E000BF78
-:400A40000086FF1F0F000080A536000030360000BF360000D2360000A481FF1FA381FF1FBAF1000F05D0237A73B96B7A63B94FF0010A6368069A93423FF43DAF049B0133AB
-:400A800004936B68069336E701F0BCFB012001F07FFB002001F0BCFB042194F8780001F063FC94F8780001F06FFC0028F9D196F8780001F0FDFB737A327A02930123039279
-:400AC0000193CDF80090049B3A4605997A48FFF70EFCB9F1000F16D1059BBB420ADD012000F08EFC01288046F6D17449FFF75AFB3F2803DC012000F017FD04E0404600F04C
+:400A40000086FF1F0F000080A536000030360000BF360000D2360000A481FF1FA381FF1FBAF1000F05D0237A73B96B7A63B94FF0010A6368069A93423FF43DAF059B0133AA
+:400A800005936B68069336E701F0BCFB012001F07FFB002001F0BCFB042194F8780001F063FC94F8780001F06FFC0028F9D196F8780001F0FDFB737A327A02930123039278
+:400AC0000193CDF80090059B3A4604997A48FFF70EFCB9F1000F16D1049BBB420ADD012000F08EFC01288046F6D17449FFF75AFB3F2803DC012000F017FD04E0404600F04D
 :400B0000D9FC0137E8E7FFF7A5FB6D48FFF7EFFB237A0BB10220C4E06A4B1B8809A8ADF824302CE094F83A0001F02CFB606BFFF715FC6548FFF7DBFB00236372637A002B58
 :400B4000FCD0012001F064FB00237372637A002BFCD0002001F05CFB5C48FFF7C8FB5C4B09E000206077FFF7F9FB5A4B03E05A48FFF7F6FA594B1B88ADF828300AA8FFF72A
 :400B8000CBFB90E0A37F3BB1002001F0D7FA002001F0FAFF0023AB7701F07CFF002001F01FFF2A2701F04AFE002001F0EDFD3A4600210AA802F0ECF815238DF828308DF8DD
@@ -4615,12 +4615,12 @@
 :0200000490105A
 :04000000BC90ACAF55
 :0200000490303A
-:0200000004BC3E
+:0200000001A954
 :0200000490402A
 :4000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000C0
 :400040000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000080
 :400080000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000040
 :4000C0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
 :0200000490501A
-:0C00000000012E16106900002E3015259E
+:0C00000000012E16106900002E301212B4
 :00000001FF
\ No newline at end of file
diff --git a/FluxEngine.cydsn/main.c b/FluxEngine.cydsn/main.c
index d6db3ff9..26d3f9d2 100644
--- a/FluxEngine.cydsn/main.c
+++ b/FluxEngine.cydsn/main.c
@@ -343,12 +343,15 @@ static void cmd_read(struct read_frame* f)
     wait_until_writeable(FLUXENGINE_DATA_IN_EP_NUM);
     init_capture_dma();
 
-    /* Wait for the beginning of a rotation. */
+    /* Wait for the beginning of a rotation, if requested. */
         
-    index_irq = false;
-    while (!index_irq)
-        ;
-    index_irq = false;
+    if (f->synced)
+    {
+        index_irq = false;
+        while (!index_irq)
+            ;
+        index_irq = false;
+    }
     
     crunch_state_t cs = {};
     cs.outputptr = usb_buffer;
@@ -365,32 +368,27 @@ static void cmd_read(struct read_frame* f)
     /* Wait for the first DMA transfer to complete, after which we can start the
      * USB transfer. */
 
-    while ((dma_writing_to_td == 0) && !index_irq)
+    while (dma_writing_to_td == 0)
         ;
     dma_reading_from_td = 0;
     
     /* Start transferring. */
 
-    int revolutions = f->revolutions;
+    uint32_t start_time = clock;
     while (!dma_underrun)
     {
         CyWdtClear();
 
-        /* Have we reached the index pulse? */
-        if (index_irq)
-        {
-            index_irq = false;
-            revolutions--;
-            if (revolutions == 0)
-                break;
-        }
-        
         /* Wait for the next block to be read. */
         while (dma_reading_from_td == dma_writing_to_td)
         {
             /* On an underrun, give up immediately. */
             if (dma_underrun)
                 goto abort;
+            
+            /* Also finish if the sample session is over. */
+            if ((clock - start_time) >= f->milliseconds)
+                goto abort;
         }
 
         uint8_t dma_buffer_usage = 0;
@@ -416,26 +414,33 @@ static void cmd_read(struct read_frame* f)
         dma_reading_from_td = NEXT_BUFFER(dma_reading_from_td);
     }
 abort:;
+    bool saved_dma_underrun = dma_underrun;
     CyDmaChSetRequest(dma_channel, CY_DMA_CPU_TERM_CHAIN);
     while (CyDmaChGetRequest(dma_channel))
         ;
 
     donecrunch(&cs);
     wait_until_writeable(FLUXENGINE_DATA_IN_EP_NUM);
-    if (!dma_underrun)
+    /* If there's a complete packet waiting, send it. */
+    if (cs.outputlen != BUFFER_SIZE)
     {
-        if (cs.outputlen != BUFFER_SIZE)
-            USBFS_LoadInEP(FLUXENGINE_DATA_IN_EP_NUM, usb_buffer, BUFFER_SIZE-cs.outputlen);
+        USBFS_LoadInEP(FLUXENGINE_DATA_IN_EP_NUM, usb_buffer, BUFFER_SIZE);
         wait_until_writeable(FLUXENGINE_DATA_IN_EP_NUM);
     }
-    if ((cs.outputlen == BUFFER_SIZE) || (cs.outputlen == 0))
+    if ((cs.outputlen != 0) && (cs.outputlen != BUFFER_SIZE))
+    {
+        /* If there's a partial packet waiting, send it; this will also terminate the transfer. */
+        USBFS_LoadInEP(FLUXENGINE_DATA_IN_EP_NUM, usb_buffer, BUFFER_SIZE-cs.outputlen);
+    }
+    else
     {
+        /* Otherwise just terminate the transfer. */
         USBFS_LoadInEP(FLUXENGINE_DATA_IN_EP_NUM, NULL, 0);
-        wait_until_writeable(FLUXENGINE_DATA_IN_EP_NUM);
     }
+    wait_until_writeable(FLUXENGINE_DATA_IN_EP_NUM);
     deinit_dma();
 
-    if (dma_underrun)
+    if (saved_dma_underrun)
     {
         print("underrun after %d packets");
         send_error(F_ERROR_UNDERRUN);
diff --git a/lib/fluxsource/fluxsource.h b/lib/fluxsource/fluxsource.h
index ac3c3865..e824f6e0 100644
--- a/lib/fluxsource/fluxsource.h
+++ b/lib/fluxsource/fluxsource.h
@@ -27,8 +27,9 @@ class FluxSource
     virtual bool retryable() { return false; }
 };
 
-extern void setHardwareFluxSourceRevolutions(int revolutions);
+extern void setHardwareFluxSourceRevolutions(double revolutions);
 extern void setHardwareFluxSourceDensity(bool high_density);
+extern void setHardwareFluxSourceSynced(bool synced);
 
 #endif
 
diff --git a/lib/fluxsource/hardwarefluxsource.cc b/lib/fluxsource/hardwarefluxsource.cc
index 5420e184..57cdcd3e 100644
--- a/lib/fluxsource/hardwarefluxsource.cc
+++ b/lib/fluxsource/hardwarefluxsource.cc
@@ -3,13 +3,19 @@
 #include "fluxmap.h"
 #include "usb.h"
 #include "fluxsource/fluxsource.h"
+#include "fmt/format.h"
 
 FlagGroup hardwareFluxSourceFlags;
 
-static IntFlag revolutions(
+static DoubleFlag revolutions(
     { "--revolutions" },
     "read this many revolutions of the disk",
-    1);
+    1.25);
+
+static BoolFlag synced(
+    { "--sync-with-index" },
+    "whether to wait for an index pulse before started to read",
+    false);
 
 static IntFlag indexMode(
     { "--index-mode" },
@@ -29,6 +35,10 @@ class HardwareFluxSource : public FluxSource
     HardwareFluxSource(unsigned drive):
         _drive(drive)
     {
+        usbSetDrive(_drive, high_density, indexMode);
+        std::cerr << "Measuring rotational speed... " << std::flush;
+        _oneRevolution = usbGetRotationalPeriod();
+        std::cerr << fmt::format("{}ms\n", _oneRevolution / 1e6);
     }
 
     ~HardwareFluxSource()
@@ -40,9 +50,10 @@ class HardwareFluxSource : public FluxSource
     {
         usbSetDrive(_drive, high_density, indexMode);
         usbSeek(track);
-        Bytes crunched = usbRead(side, revolutions);
+        Bytes crunched = usbRead(side, synced, revolutions * _oneRevolution);
         auto fluxmap = std::make_unique<Fluxmap>();
         fluxmap->appendBytes(crunched.uncrunch());
+        std::cerr << "return fluxmap\n";
         return fluxmap;
     }
 
@@ -59,13 +70,19 @@ class HardwareFluxSource : public FluxSource
 private:
     unsigned _drive;
     unsigned _revolutions;
+    nanoseconds_t _oneRevolution;
 };
 
-void setHardwareFluxSourceRevolutions(int revolutions)
+void setHardwareFluxSourceRevolutions(double revolutions)
 {
     ::revolutions.setDefaultValue(revolutions);
 }
 
+void setHardwareFluxSourceSynced(bool synced)
+{
+    ::synced.setDefaultValue(synced);
+}
+
 std::unique_ptr<FluxSource> FluxSource::createHardwareFluxSource(unsigned drive)
 {
     return std::unique_ptr<FluxSource>(new HardwareFluxSource(drive));
diff --git a/lib/usb.cc b/lib/usb.cc
index 440ac6b6..621b6c6d 100644
--- a/lib/usb.cc
+++ b/lib/usb.cc
@@ -149,7 +149,7 @@ nanoseconds_t usbGetRotationalPeriod(void)
     usb_cmd_send(&f, f.f.size);
 
     auto r = await_reply<struct speed_frame>(F_FRAME_MEASURE_SPEED_REPLY);
-    return r->period_ms * 1000;
+    return r->period_ms * 1000000;
 }
 
 static int large_bulk_transfer(int ep, Bytes& bytes)
@@ -202,13 +202,16 @@ void usbTestBulkTransport()
     await_reply<struct any_frame>(F_FRAME_BULK_TEST_REPLY);
 }
 
-Bytes usbRead(int side, int revolutions)
+Bytes usbRead(int side, bool synced, nanoseconds_t readTime)
 {
     struct read_frame f = {
         .f = { .type = F_FRAME_READ_CMD, .size = sizeof(f) },
         .side = (uint8_t) side,
-        .revolutions = (uint8_t) revolutions
+        .synced = (uint8_t) synced
     };
+    uint16_t milliseconds = readTime / 1e6;
+    ((uint8_t*)&f.milliseconds)[0] = milliseconds;
+    ((uint8_t*)&f.milliseconds)[1] = milliseconds >> 8;
     usb_cmd_send(&f, f.f.size);
 
     auto fluxmap = std::unique_ptr<Fluxmap>(new Fluxmap);
diff --git a/lib/usb.h b/lib/usb.h
index 16941bb2..458a35bf 100644
--- a/lib/usb.h
+++ b/lib/usb.h
@@ -9,7 +9,7 @@ extern void usbRecalibrate();
 extern void usbSeek(int track);
 extern nanoseconds_t usbGetRotationalPeriod();
 extern void usbTestBulkTransport();
-extern Bytes usbRead(int side, int revolutions);
+extern Bytes usbRead(int side, bool synced, nanoseconds_t readTime);
 extern void usbWrite(int side, const Bytes& bytes);
 extern void usbErase(int side);
 extern void usbSetDrive(int drive, bool high_density, int index_mode);
diff --git a/protocol.h b/protocol.h
index 7e301799..de7c257e 100644
--- a/protocol.h
+++ b/protocol.h
@@ -3,7 +3,7 @@
 
 enum 
 {
-    FLUXENGINE_VERSION = 10,
+    FLUXENGINE_VERSION = 11,
 
     FLUXENGINE_VID = 0x1209,
     FLUXENGINE_PID = 0x6e00,
@@ -133,7 +133,8 @@ struct read_frame
 {
     struct frame_header f;
     uint8_t side;
-    uint8_t revolutions;
+    uint8_t synced;
+    uint16_t milliseconds;
 };
 
 struct write_frame
diff --git a/src/fe-rpm.cc b/src/fe-rpm.cc
index 0b12d473..aa2ea428 100644
--- a/src/fe-rpm.cc
+++ b/src/fe-rpm.cc
@@ -19,7 +19,7 @@ int mainRpm(int argc, const char* argv[])
     usbSetDrive(spec.drive, false, F_INDEX_REAL);
     nanoseconds_t period = usbGetRotationalPeriod();
     if (period != 0)
-        std::cout << "Rotational period is " << period/1000 << " ms (" << 60e6/period << " rpm)" << std::endl;
+        std::cout << "Rotational period is " << period/1000000 << " ms (" << 60e3/period << " rpm)" << std::endl;
     else
     {
         std::cout << "No index pulses detected from the disk. Common causes of this are:\n"