mimxrt/machine_spi: Allow a setting of -1 for cs in the constructor.

In that case, no Pin will be configured for the CS signal, even if it is
internally still generated.  That setting allows to use any pin for CS,
which then must be controlled by the Python script.

Also make the default cs=-1 to match other ports (software CS).

docs/mimxrt/pinout.rst

@@ -322,7 +322,8 @@ Olimex RT1010Py             -                 CS0/-/SDO/SDI/SCK        SDCARD wi
 Seeed ARCH MIX     J4_12/-/J4_14/J4_13/J4_15  J3_09/J3_05/J3_08_J3_11
 =================  =========================  =======================  ===============
 
-Pins denoted with (*) are by default not wired at the board.
+Pins denoted with (*) are by default not wired at the board. The CS0 and CS1 signals
+are enabled with the keyword option cs=0 or cs=1 of the SPI object constructor.
 
 .. _mimxrt_i2c_pinout:
 

docs/mimxrt/quickref.rst

@@ -301,13 +301,19 @@ There are up to four hardware SPI channels that allow faster transmission
 rates (up to 30Mhz).  Hardware SPI is accessed via the
 :ref:`machine.SPI <machine.SPI>` class and has the same methods as software SPI above::
 
-    from machine import SPI
+    from machine import SPI, Pin
 
     spi = SPI(0, 10000000)
+    cs_pin = Pin(6, Pin.OUT, value=1)
+    cs_pin(0)
     spi.write('Hello World')
+    cs_pin(1)
 
 For the assignment of Pins to SPI signals, refer to
 :ref:`Hardware SPI pinout <mimxrt_spi_pinout>`.
+The keyword option cs=n can be used to enable the cs pin 0 or 1 for an automatic cs signal. The
+default is cs=-1. Using cs=-1 the automatic cs signal is not created. 
+In that case, cs has to be set by the script. Clearing that assignment requires a power cycle.
 
 Notes:
 

ports/mimxrt/machine_spi.c

@@ -77,7 +77,7 @@ static const iomux_table_t iomux_table[] = {
     IOMUX_TABLE_SPI
 };
 
-bool lpspi_set_iomux(int8_t spi, uint8_t drive, uint8_t cs) {
+bool lpspi_set_iomux(int8_t spi, uint8_t drive, int8_t cs) {
     int index = (spi - 1) * 5;
 
     if (SCK.muxRegister != 0) {
@@ -93,7 +93,7 @@ bool lpspi_set_iomux(int8_t spi, uint8_t drive, uint8_t cs) {
             IOMUXC_SetPinMux(CS1.muxRegister, CS1.muxMode, CS1.inputRegister, CS1.inputDaisy, CS1.configRegister, 0U);
             IOMUXC_SetPinConfig(CS1.muxRegister, CS1.muxMode, CS1.inputRegister, CS1.inputDaisy, CS1.configRegister,
                 pin_generate_config(PIN_PULL_UP_100K, PIN_MODE_OUT, drive, CS1.configRegister));
-        } else {
+        } else if (cs != -1) {
             mp_raise_ValueError(MP_ERROR_TEXT("The chosen CS is not available"));
         }
 
@@ -131,7 +131,7 @@ mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n
         { MP_QSTR_firstbit, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = DEFAULT_SPI_FIRSTBIT} },
         { MP_QSTR_gap_ns,   MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = -1} },
         { MP_QSTR_drive,    MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = DEFAULT_SPI_DRIVE} },
-        { MP_QSTR_cs,       MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0} },
+        { MP_QSTR_cs,       MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = -1} },
     };
 
     // Parse the arguments.
@@ -173,8 +173,11 @@ mp_obj_t machine_spi_make_new(const mp_obj_type_t *type, size_t n_args, size_t n
     }
     self->master_config->lastSckToPcsDelayInNanoSec = self->master_config->betweenTransferDelayInNanoSec;
     self->master_config->pcsToSckDelayInNanoSec = self->master_config->betweenTransferDelayInNanoSec;
-    uint8_t cs = args[ARG_cs].u_int;
+    int8_t cs = args[ARG_cs].u_int;
-    if (cs <= 1) {
+    // In the SPI master_config for automatic CS the value cs=0 is set already,
+    // so only cs=1 has to be addressed here. The case cs == -1 for manual CS is handled
+    // in the function spi_set_iomux() and the value in the master_config can stay at 0.
+    if (cs == 1) {
         self->master_config->whichPcs = cs;
     }
     LPSPI_MasterInit(self->spi_inst, self->master_config, BOARD_BOOTCLOCKRUN_LPSPI_CLK_ROOT);