diff --git a/docs/library/pyb.SPI.rst b/docs/library/pyb.SPI.rst
index 5c732fccba226ecf26ba74f3bfa458b46b2929a9..70d6454015b881c8f1a619bea581a44514d0ee46 100644
--- a/docs/library/pyb.SPI.rst
+++ b/docs/library/pyb.SPI.rst
@@ -52,12 +52,29 @@ Methods
Turn off the SPI bus.
-.. method:: spi.init(mode, baudrate=328125, \*, polarity=1, phase=0, bits=8, firstbit=SPI.MSB, ti=False, crc=None)
+.. method:: spi.init(mode, baudrate=328125, \*, prescaler, polarity=1, phase=0, bits=8, firstbit=SPI.MSB, ti=False, crc=None)
Initialise the SPI bus with the given parameters:
- ``mode`` must be either ``SPI.MASTER`` or ``SPI.SLAVE``.
- ``baudrate`` is the SCK clock rate (only sensible for a master).
+ - ``prescaler`` is the prescaler to use to derive SCK from the APB bus frequency;
+ use of ``prescaler`` overrides ``baudrate``.
+ - ``polarity`` can be 0 or 1, and is the level the idle clock line sits at.
+ - ``phase`` can be 0 or 1 to sample data on the first or second clock edge
+ respectively.
+ - ``firstbit`` can be ``SPI.MSB`` or ``SPI.LSB``.
+ - ``crc`` can be None for no CRC, or a polynomial specifier.
+
+ Note that the SPI clock frequency will not always be the requested baudrate.
+ The hardware only supports baudrates that are the APB bus frequency
+ (see :meth:`pyb.freq`) divided by a prescaler, which can be 2, 4, 8, 16, 32,
+ 64, 128 or 256. SPI(1) is on AHB2, and SPI(2) is on AHB1. For precise
+ control over the SPI clock frequency, specify ``prescaler`` instead of
+ ``baudrate``.
+
+ Printing the SPI object will show you the computed baudrate and the chosen
+ prescaler.
.. method:: spi.recv(recv, \*, timeout=5000)
diff --git a/stmhal/spi.c b/stmhal/spi.c
index 78d70d1657115ac16dcbfcdc752b5b3a053b6921..9e2aeffe06f955efc74d2ed4597419181708c744 100644
--- a/stmhal/spi.c
+++ b/stmhal/spi.c
@@ -346,8 +346,9 @@ STATIC void pyb_spi_print(void (*print)(void *env, const char *fmt, ...), void *
// SPI2 and SPI3 are on APB1
spi_clock = HAL_RCC_GetPCLK1Freq();
}
- uint baudrate = spi_clock >> ((self->spi->Init.BaudRatePrescaler >> 3) + 1);
- print(env, "SPI(%u, SPI.MASTER, baudrate=%u", spi_num, baudrate);
+ uint log_prescaler = (self->spi->Init.BaudRatePrescaler >> 3) + 1;
+ uint baudrate = spi_clock >> log_prescaler;
+ print(env, "SPI(%u, SPI.MASTER, baudrate=%u, prescaler=%u", spi_num, baudrate, 1 << log_prescaler);
} else {
print(env, "SPI(%u, SPI.SLAVE", spi_num);
}
@@ -369,6 +370,7 @@ STATIC mp_obj_t pyb_spi_init_helper(const pyb_spi_obj_t *self, mp_uint_t n_args,
static const mp_arg_t allowed_args[] = {
{ MP_QSTR_mode, MP_ARG_REQUIRED | MP_ARG_INT, {.u_int = 0} },
{ MP_QSTR_baudrate, MP_ARG_INT, {.u_int = 328125} },
+ { MP_QSTR_prescaler, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0xffffffff} },
{ MP_QSTR_polarity, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 1} },
{ MP_QSTR_phase, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = 0} },
{ MP_QSTR_dir, MP_ARG_KW_ONLY | MP_ARG_INT, {.u_int = SPI_DIRECTION_2LINES} },
@@ -387,17 +389,20 @@ STATIC mp_obj_t pyb_spi_init_helper(const pyb_spi_obj_t *self, mp_uint_t n_args,
SPI_InitTypeDef *init = &self->spi->Init;
init->Mode = args[0].u_int;
- // compute the baudrate prescaler from the requested baudrate
- // select a prescaler that yields at most the requested baudrate
- uint spi_clock;
- if (self->spi->Instance == SPI1) {
- // SPI1 is on APB2
- spi_clock = HAL_RCC_GetPCLK2Freq();
- } else {
- // SPI2 and SPI3 are on APB1
- spi_clock = HAL_RCC_GetPCLK1Freq();
+ // configure the prescaler
+ mp_uint_t br_prescale = args[2].u_int;
+ if (br_prescale == 0xffffffff) {
+ // prescaler not given, so select one that yields at most the requested baudrate
+ mp_uint_t spi_clock;
+ if (self->spi->Instance == SPI1) {
+ // SPI1 is on APB2
+ spi_clock = HAL_RCC_GetPCLK2Freq();
+ } else {
+ // SPI2 and SPI3 are on APB1
+ spi_clock = HAL_RCC_GetPCLK1Freq();
+ }
+ br_prescale = spi_clock / args[1].u_int;
}
- uint br_prescale = spi_clock / args[1].u_int;
if (br_prescale <= 2) { init->BaudRatePrescaler = SPI_BAUDRATEPRESCALER_2; }
else if (br_prescale <= 4) { init->BaudRatePrescaler = SPI_BAUDRATEPRESCALER_4; }
else if (br_prescale <= 8) { init->BaudRatePrescaler = SPI_BAUDRATEPRESCALER_8; }
@@ -407,19 +412,19 @@ STATIC mp_obj_t pyb_spi_init_helper(const pyb_spi_obj_t *self, mp_uint_t n_args,
else if (br_prescale <= 128) { init->BaudRatePrescaler = SPI_BAUDRATEPRESCALER_128; }
else { init->BaudRatePrescaler = SPI_BAUDRATEPRESCALER_256; }
- init->CLKPolarity = args[2].u_int == 0 ? SPI_POLARITY_LOW : SPI_POLARITY_HIGH;
- init->CLKPhase = args[3].u_int == 0 ? SPI_PHASE_1EDGE : SPI_PHASE_2EDGE;
- init->Direction = args[4].u_int;
- init->DataSize = (args[5].u_int == 16) ? SPI_DATASIZE_16BIT : SPI_DATASIZE_8BIT;
- init->NSS = args[6].u_int;
- init->FirstBit = args[7].u_int;
- init->TIMode = args[8].u_bool ? SPI_TIMODE_ENABLED : SPI_TIMODE_DISABLED;
- if (args[9].u_obj == mp_const_none) {
+ init->CLKPolarity = args[3].u_int == 0 ? SPI_POLARITY_LOW : SPI_POLARITY_HIGH;
+ init->CLKPhase = args[4].u_int == 0 ? SPI_PHASE_1EDGE : SPI_PHASE_2EDGE;
+ init->Direction = args[5].u_int;
+ init->DataSize = (args[6].u_int == 16) ? SPI_DATASIZE_16BIT : SPI_DATASIZE_8BIT;
+ init->NSS = args[7].u_int;
+ init->FirstBit = args[8].u_int;
+ init->TIMode = args[9].u_bool ? SPI_TIMODE_ENABLED : SPI_TIMODE_DISABLED;
+ if (args[10].u_obj == mp_const_none) {
init->CRCCalculation = SPI_CRCCALCULATION_DISABLED;
init->CRCPolynomial = 0;
} else {
init->CRCCalculation = SPI_CRCCALCULATION_ENABLED;
- init->CRCPolynomial = mp_obj_get_int(args[9].u_obj);
+ init->CRCPolynomial = mp_obj_get_int(args[10].u_obj);
}
// init the SPI bus