samd/machine_bitstream: Add the machine.bitstream() function.

The SAMD21 implementation is an adaption of @jimmo's code for STM32Lxx.
The only changes are the addresses and names of the port registers and the
timing parameters.

SAMD21: The precision is about +/-25ns at 48MHz clock frequency.  The first
two cycles are about 40-60 ns longer than set.  But still good enough to
drive a neopixel device.

SAMD51: The precision is about +/-30ns at 120MHz clock frequency.  Good
enough to drive a neopixel device.

Author: robert-hh

ports/samd/Makefile

@@ -87,10 +87,11 @@ LIBSTDCPP_FILE_NAME = "$(shell $(CXX) $(CXXFLAGS) -print-file-name=libstdc++.a)"
 LDFLAGS += -L"$(shell dirname $(LIBSTDCPP_FILE_NAME))"
 endif
 
-SRC_C = \
+SRC_C += \
 	clock_config.c \
 	help.c \
 	machine_adc.c \
+	machine_bitstream.c \
 	machine_dac.c \
 	machine_i2c.c \
 	machine_led.c \

ports/samd/machine_bitstream.c

@@ -0,0 +1,206 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2021 Jim Mussared
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+// This is a translation of the cycle counter implementation in ports/stm32/machine_bitstream.c.
+
+#include "py/mpconfig.h"
+#include "py/mphal.h"
+#include "clock_config.h"
+
+#if MICROPY_PY_MACHINE_BITSTREAM
+
+#if __CORTEX_M == 0
+
+// No cycle counter on M0, do manual cycle counting instead.
+
+// STM32F091 @ 48MHz
+#define NS_CYCLES_PER_ITER_HIGH (3)
+#define NS_CYCLES_PER_ITER_LOW (3)
+#define NS_OVERHEAD_CYCLES_HIGH (12)
+#define NS_OVERHEAD_CYCLES_LOW (15)
+
+uint32_t mp_hal_delay_ns_calc(uint32_t ns, bool high) {
+    uint32_t ncycles = (get_cpu_freq() / 1000000 * ns + 500) / 1000; // + 500 for proper rounding
+    uint32_t overhead = MIN(ncycles, high ? NS_OVERHEAD_CYCLES_HIGH : NS_OVERHEAD_CYCLES_LOW);
+    return MAX(1, MP_ROUND_DIVIDE(ncycles - overhead, high ? NS_CYCLES_PER_ITER_HIGH : NS_CYCLES_PER_ITER_LOW));
+}
+
+void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const uint8_t *buf, size_t len) {
+    volatile const uint32_t mask = 1 << (pin % 32);
+    volatile uint32_t *outclr = &PORT->Group[pin / 32].OUTCLR.reg;
+    volatile uint32_t *outset = &PORT->Group[pin / 32].OUTSET.reg;
+
+    // Convert ns to loop iterations [high_time_0, low_time_0, high_time_1, low_time_1].
+    for (size_t i = 0; i < 4; ++i) {
+        timing_ns[i] = mp_hal_delay_ns_calc(timing_ns[i], i % 2 == 0);
+    }
+
+    mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
+
+    // Measured timing for SAMD21 at 48MHz (cycle=20.83ns)
+    // timing_ns = (1,1,1,1)
+    //   high:  310
+    //   low:   375
+    //   high0: 375
+    //   low0:  400
+    // timing_ns = (500, 500, 500, 500)
+    //   high:  500
+    //   low:   500
+    //   high0: 565
+    //   low0:  540
+    // timing_ns = (1000, 1000, 1000, 1000)
+    //   high:  1000
+    //   low:   1000
+    //   high0: 1065
+    //   low0:  1040
+
+    // --> high is 12 + n*3 cycles
+    //     low is 15 + n*3 cycles
+
+    // NeoPixel timing (400, 850, 800, 450) (+/-150ns) gives timing_ns=(2, 9, 8, 3) which in cycles is
+    // (12 + 6, 15 + 27, 15 + 24, 12 + 9) = (18, 42, 39, 21)
+    // --> (375, 875, 812, 437) nanoseconds.
+    // Measured output on logic analyser is (375, 875, 815, 435) (+/-5ns at 200MHz)
+
+    // Note: the first high/low cycle is longer by 2-3 cycles (40-60ns).
+    // This is slightly outside spec, but doesn't seem to cause a problem.
+
+    __asm volatile (
+        // Force consistent register assignment.
+        // r6 = len
+        "ldr r6, %0\n"
+        // r4 = buf
+        "ldr r4, %1\n"
+        // r5 = timing_ms
+        "ldr r5, %2\n"
+
+        // Must align for consistent timing.
+        ".align 4\n"
+
+        // Don't increment/decrement before first iteration.
+        "b .outer2\n"
+        ".outer:\n"
+        // ++buf, --len
+        "  add r4, #1\n"
+        "  sub r6, #1\n"
+
+        // len iterations
+        ".outer2:\n"
+        "  cmp r6, #0\n"
+        "  beq .done\n"
+
+        // r0 = *buf
+        "  ldrb r0, [r4, #0]\n"
+
+        // 8 bits in byte
+        "  mov r7, #8\n"
+        "  .inner:\n"
+        //   *outset = mask
+        "    ldr r2, %3\n"
+        "    ldr r1, %5\n"
+        "    str r1, [r2, #0]\n"
+
+        //   r3 = (r0 >> 4) & 8    (r0 is 8 if high bit is 1 else 0)
+        "    mov r8, r6\n"
+        "    lsr r3, r0, #4\n"
+        "    mov r6, #8\n"
+        "    and r3, r6\n"
+        "    mov r6, r8\n"
+
+        //   r2 = timing_ns[r2]
+        "    ldr r2, [r5, r3]\n"
+        "    .loop1:\n sub r2, #1\n bne .loop1\n"
+
+        //   *outclr = mask
+        "    ldr r2, %4\n"
+        "    str r1, [r2, #0]\n"
+
+        //   r2 = timing_ns[r3 + 4]
+        "    add r3, #4\n"
+        "    ldr r2, [r5, r3]\n"
+        "    .loop2:\n sub r2, #1\n bne .loop2\n"
+
+        //   b >>= 1
+        "    lsl r0, r0, #1\n"
+
+        "    sub r7, #1\n"
+        //   end of inner loop
+        "    beq .outer\n"
+        //   continue inner loop
+        "    b .inner\n"
+
+        ".done:\n"
+        :
+        : "m" (len), "m" (buf), "m" (timing_ns), "m" (outset), "m" (outclr), "m" (mask)
+        : "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8"
+        );
+
+    MICROPY_END_ATOMIC_SECTION(atomic_state);
+}
+
+#else // > CORTEX_M0
+
+#define NS_TICKS_OVERHEAD (70)
+
+void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const uint8_t *buf, size_t len) {
+    uint32_t fcpu_mhz = get_cpu_freq() / 1000000;
+    uint32_t ticks_overhead = fcpu_mhz * NS_TICKS_OVERHEAD / 1000;
+    // Convert ns to us ticks [high_time_0, period_0, high_time_1, period_1].
+    for (size_t i = 0; i < 4; ++i) {
+        timing_ns[i] = fcpu_mhz * timing_ns[i] / 1000;
+        if (timing_ns[i] > ticks_overhead) {
+            timing_ns[i] -= ticks_overhead;
+        }
+        if (i % 2 == 1) {
+            // Convert low_time to period (i.e. add high_time).
+            timing_ns[i] += timing_ns[i - 1] - ticks_overhead;
+        }
+    }
+
+    mp_uint_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
+    DWT->CYCCNT = 0;
+
+    for (size_t i = 0; i < len; ++i) {
+        uint8_t b = buf[i];
+        for (size_t j = 0; j < 8; ++j) {
+            uint32_t start_ticks = mp_hal_ticks_cpu();
+            uint32_t *t = &timing_ns[b >> 6 & 2];
+            mp_hal_pin_high(pin);
+            while ((mp_hal_ticks_cpu() - start_ticks) < t[0]) {
+            }
+            b <<= 1;
+            mp_hal_pin_low(pin);
+            while ((mp_hal_ticks_cpu() - start_ticks) < t[1]) {
+            }
+        }
+    }
+    MICROPY_END_ATOMIC_SECTION(atomic_state);
+
+}
+
+#endif // > CORTEX_M0
+
+#endif // MICROPY_PY_MACHINE_BITSTREAM

ports/samd/modmachine.c

@@ -25,6 +25,7 @@
  */
 
 #include "py/runtime.h"
+#include "extmod/machine_bitstream.h"
 #include "extmod/machine_mem.h"
 #include "extmod/machine_pulse.h"
 #include "extmod/machine_i2c.h"
@@ -167,6 +168,7 @@ STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR_disable_irq),         MP_ROM_PTR(&machine_disable_irq_obj) },
     { MP_ROM_QSTR(MP_QSTR_enable_irq),          MP_ROM_PTR(&machine_enable_irq_obj) },
     { MP_ROM_QSTR(MP_QSTR_time_pulse_us),       MP_ROM_PTR(&machine_time_pulse_us_obj) },
+    { MP_ROM_QSTR(MP_QSTR_bitstream),           MP_ROM_PTR(&machine_bitstream_obj) },
 };
 STATIC MP_DEFINE_CONST_DICT(machine_module_globals, machine_module_globals_table);
 

ports/samd/mpconfigport.h

@@ -111,6 +111,7 @@
 #define MICROPY_PY_MACHINE_SPI              (1)
 #define MICROPY_PY_MACHINE_SOFTSPI          (1)
 #define MICROPY_PY_OS_DUPTERM               (3)
+#define MICROPY_PY_MACHINE_BITSTREAM        (1)
 #define MICROPY_PY_MACHINE_PULSE            (1)
 #define MICROPY_PY_MACHINE_PWM              (1)
 #define MICROPY_PY_MACHINE_PWM_INIT         (0)

ports/samd/mphalport.h

@@ -71,11 +71,11 @@ static inline mp_uint_t mp_hal_ticks_us(void) {
     #endif
 }
 
-#if defined (MCU_SAMD21)
+#if defined(MCU_SAMD21)
 
 #define mp_hal_ticks_cpu mp_hal_ticks_us
 
-#elif defined (MCU_SAMD51)
+#elif defined(MCU_SAMD51)
 static inline void mp_hal_ticks_cpu_enable(void) {
     CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
     DWT->CYCCNT = 0;

ports/samd/samd_soc.c

@@ -111,7 +111,7 @@ void samd_init(void) {
     SysTick_Config(get_cpu_freq() / 1000);
     init_us_counter();
     usb_init();
-    #if defined (MCU_SAMD51)
+    #if defined(MCU_SAMD51)
     mp_hal_ticks_cpu_enable();
     #endif
 }