MCU SPI接口与多个从设备通信

你好，下面是工程师的回复：

我认为40个外部器件在 SPI 引脚上的负载可能太大。 默认情况下、SPI 引脚配置为2mA 驱动强度。 您可以修改 TivaWare 库以改变为8mA 或12mA 等驱动强度。 您将需要使用更高的强度比如12mA，来确定它是否能够提供足够的上升/下降时间以及从器件所需的 SPI 频率。 

通常、你可以调用 GPIOPinTypeSSI 来配置 GPIO 引脚以实现 SSI 功能。 正如您在 SSI 功能中看到的、默认情况下、2mA 用作驱动强度。 您可以将驱动强度更改为更高的值、例如12mA 或8mA、然后重建外设驱动程序库。 或者、您可以将此文件复制到工作项目中，对其进行更新然后编译它。

//
//! Configures pin(s) for use by the SSI peripheral.
//!
//! \param ui32Port is the base address of the GPIO port.
//! \param ui8Pins is the bit-packed representation of the pin(s).
//!
//! The SSI pins must be properly configured for the SSI peripheral to function
//! correctly. This function provides a typical configuration for those
//! pin(s); other configurations may work as well depending upon the board
//! setup (for example, using the on-chip pull-ups).
//!
//! The pin(s) are specified using a bit-packed byte, where each bit that is
//! set identifies the pin to be accessed, and where bit 0 of the byte
//! represents GPIO port pin 0, bit 1 represents GPIO port pin 1, and so on.
//!
//! \note This function cannot be used to turn any pin into a SSI pin; it only
//! configures a SSI pin for proper operation. Note that a GPIOPinConfigure()
//! function call is also required to properly configure a pin for the SSI
//! function.
//!
//! \note A subset of GPIO pins on Tiva devices, notably those used by the
//! JTAG/SWD interface and any pin capable of acting as an NMI input, are
//! locked against inadvertent reconfiguration. These pins must be unlocked
//! using direct register writes to the relevant GPIO_O_LOCK and GPIO_O_CR
//! registers before this function can be called. Please see the ``gpio_jtag''
//! example application for the mechanism required and consult your part
//! datasheet for information on affected pins.
//!
//! \return None.
//
//*****************************************************************************
void
GPIOPinTypeSSI(uint32_t ui32Port, uint8_t ui8Pins)
{
//
// Check the arguments.
//
ASSERT(_GPIOBaseValid(ui32Port));

//
// Make the pin(s) be peripheral controlled.
//
GPIODirModeSet(ui32Port, ui8Pins, GPIO_DIR_MODE_HW);

//
// Set the pad(s) for standard push-pull operation.
//
GPIOPadConfigSet(ui32Port, ui8Pins, GPIO_STRENGTH_2MA, GPIO_PIN_TYPE_STD);
}