mimxrt10xx: Use the proper "betweenTransferDelay" (et al) values

Set the betweenTransferDelay to the SCK low-time, to avoid long pauses
between bytes (transfers) while preventing the last SCK cycle in a byte
from being a runt pulse.

Compared to an earlier revision of this change, which just set the delays
all to zero, this doesn't break using an AirLift, which was sensitive
to the runt pulses (the simple loopback-wire test didn't detect the problem)

Author: jepler

ports/mimxrt10xx/common-hal/busio/SPI.c

@@ -238,15 +238,10 @@ bool common_hal_busio_spi_configure(busio_spi_obj_t *self,
         baudrate = 30000000; // "Absolute maximum frequency of operation (fop) is 30 MHz" -- IMXRT1010CEC.pdf
     }
 
-    LPSPI_Enable(self->spi, false);
-    uint32_t tcrPrescaleValue;
-    self->baudrate = LPSPI_MasterSetBaudRate(self->spi, baudrate, LPSPI_MASTER_CLK_FREQ, &tcrPrescaleValue);
-    self->spi->TCR = (self->spi->TCR & ~LPSPI_TCR_PRESCALE_MASK) | LPSPI_TCR_PRESCALE(tcrPrescaleValue);
-    LPSPI_Enable(self->spi, true);
-
     if ((polarity == common_hal_busio_spi_get_polarity(self)) &&
         (phase == common_hal_busio_spi_get_phase(self)) &&
-        (bits == ((self->spi->TCR & LPSPI_TCR_FRAMESZ_MASK) >> LPSPI_TCR_FRAMESZ_SHIFT)) + 1) {
+        (bits == ((self->spi->TCR & LPSPI_TCR_FRAMESZ_MASK) >> LPSPI_TCR_FRAMESZ_SHIFT)) + 1 &&
+        (baudrate == common_hal_busio_spi_get_frequency(self))) {
         return true;
     }
 
@@ -257,10 +252,22 @@ bool common_hal_busio_spi_configure(busio_spi_obj_t *self,
     config.cpol = polarity;
     config.cpha = phase;
     config.bitsPerFrame = bits;
+    // The between-transfer-delay must be equal to the SCK low-time.
+    // Setting it lower introduces runt pulses, while setting it higher
+    // wastes time.
+    config.betweenTransferDelayInNanoSec = 1000000000 / config.baudRate / 2;
 
     LPSPI_Deinit(self->spi);
     LPSPI_MasterInit(self->spi, &config, LPSPI_MASTER_CLK_FREQ);
 
+    // Recompute the actual baudrate so that we can set the baudrate
+    // (frequency) property.  We don't need to set TCR because it was
+    // established by LPSPI_MasterInit, above
+    uint32_t tcrPrescaleValue;
+    LPSPI_Enable(self->spi, false);
+    self->baudrate = LPSPI_MasterSetBaudRate(self->spi, baudrate, LPSPI_MASTER_CLK_FREQ, &tcrPrescaleValue);
+    LPSPI_Enable(self->spi, true);
+
     return true;
 }