round SPI freq down; check max freq

Author: dhalbert

ports/nrf/common-hal/busio/SPI.c

@@ -65,27 +65,27 @@ void spi_reset(void) {
     }
 }
 
-// Convert frequency to clock-speed-dependent value. Choose the nearest value, lower or higher.
+// Convert frequency to clock-speed-dependent value. Choose the next lower baudrate if in between
+// available baudrates.
 static nrf_spim_frequency_t baudrate_to_spim_frequency(const uint32_t baudrate) {
 
-    // Round requested baudrate to nearest available baudrate.
     static const struct {
         const uint32_t boundary;
         nrf_spim_frequency_t spim_frequency;
     } baudrate_map[] = {
 #ifdef SPIM_FREQUENCY_FREQUENCY_M32
-        { (16000000 + 32000000) / 2, NRF_SPIM_FREQ_32M },
+        { 32000000, NRF_SPIM_FREQ_32M },
 #endif
 #ifdef SPIM_FREQUENCY_FREQUENCY_M16
-        { ( 8000000 + 16000000) / 2, NRF_SPIM_FREQ_16M },
+        { 16000000, NRF_SPIM_FREQ_16M },
 #endif
-        { ( 4000000 +  8000000) / 2, NRF_SPIM_FREQ_8M },
-        { ( 2000000 +  4000000) / 2, NRF_SPIM_FREQ_4M },
-        { ( 1000000 +  2000000) / 2, NRF_SPIM_FREQ_2M },
-        { (  500000 +  1000000) / 2, NRF_SPIM_FREQ_1M },
-        { (  250000 +   500000) / 2, NRF_SPIM_FREQ_500K },
-        { (  125000 +   250000) / 2, NRF_SPIM_FREQ_250K },
-        {                         0, NRF_SPIM_FREQ_125K },
+        {  8000000, NRF_SPIM_FREQ_8M },
+        {  4000000, NRF_SPIM_FREQ_4M },
+        {  2000000, NRF_SPIM_FREQ_2M },
+        {  1000000, NRF_SPIM_FREQ_1M },
+        {   500000, NRF_SPIM_FREQ_500K },
+        {   250000, NRF_SPIM_FREQ_250K },
+        {        0, NRF_SPIM_FREQ_125K },
     };
 
     size_t i = 0;
@@ -97,7 +97,7 @@ static nrf_spim_frequency_t baudrate_to_spim_frequency(const uint32_t baudrate)
         }
         i++;
     } while (boundary != 0);
-    // Will get here only if baudrate == 0.
+    // Should not get here.
     return 0;
 }
 
@@ -168,22 +168,26 @@ void common_hal_busio_spi_deinit(busio_spi_obj_t *self) {
 }
 
 bool common_hal_busio_spi_configure(busio_spi_obj_t *self, uint32_t baudrate, uint8_t polarity, uint8_t phase, uint8_t bits) {
-  // nrf52 does not support 16 bit
-  if (bits != 8)
+    // nrf52 does not support 16 bit
+    if (bits != 8) {
       return false;
+    }
 
-  nrf_spim_frequency_set(self->spim_peripheral->spim.p_reg, baudrate_to_spim_frequency(baudrate));
+    // Set desired frequency, rounding down, and don't go above available frequency for this SPIM.
+    nrf_spim_frequency_set(self->spim_peripheral->spim.p_reg,
+                           baudrate_to_spim_frequency(MIN(baudrate,
+                                                          self->spim_peripheral->max_frequency_MHz * 1000000)));
 
-  nrf_spim_mode_t mode = NRF_SPIM_MODE_0;
-  if (polarity) {
-      mode = (phase) ? NRF_SPIM_MODE_3 : NRF_SPIM_MODE_2;
-  } else {
-      mode = (phase) ? NRF_SPIM_MODE_1 : NRF_SPIM_MODE_0;
-  }
+    nrf_spim_mode_t mode = NRF_SPIM_MODE_0;
+    if (polarity) {
+        mode = (phase) ? NRF_SPIM_MODE_3 : NRF_SPIM_MODE_2;
+    } else {
+        mode = (phase) ? NRF_SPIM_MODE_1 : NRF_SPIM_MODE_0;
+    }
 
-  nrf_spim_configure(self->spim_peripheral->spim.p_reg, mode, NRF_SPIM_BIT_ORDER_MSB_FIRST);
+    nrf_spim_configure(self->spim_peripheral->spim.p_reg, mode, NRF_SPIM_BIT_ORDER_MSB_FIRST);
 
-  return true;
+    return true;
 }
 
 bool common_hal_busio_spi_try_lock(busio_spi_obj_t *self) {

shared-bindings/busio/SPI.c

@@ -136,7 +136,7 @@ static void check_lock(busio_spi_obj_t *self) {
 
 //|   .. method:: SPI.configure(\*, baudrate=100000, polarity=0, phase=0, bits=8)
 //|
-//|     Configures the SPI bus. Only valid when locked.
+//|     Configures the SPI bus. The SPI object must be locked.
 //|
 //|     :param int baudrate: the desired clock rate in Hertz. The actual clock rate may be higher or lower
 //|       due to the granularity of available clock settings.
@@ -154,8 +154,8 @@ static void check_lock(busio_spi_obj_t *self) {
 //|      and 8MHz. On the nRF52840, 16MHz and 32MHz are also available, but only on the first
 //|      `busio.SPI` object you create. Two more ``busio.SPI`` objects can be created, but they are restricted
 //|      to 8MHz maximum. This is a hardware restriction: there is only one high-speed SPI peripheral.
-//|      If you pick a a baudrate other than one of these, the nearest available
-//|      baudrate will be chosen.
+//|      If you pick a a baudrate other than one of these, the nearest lower
+//|      baudrate will be chosen, with a minimum of 125kHz.
 STATIC mp_obj_t busio_spi_configure(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
     enum { ARG_baudrate, ARG_polarity, ARG_phase, ARG_bits };
     static const mp_arg_t allowed_args[] = {