Treehopper C++ API
HardwareSpi Class Referenceabstract

Built-in SPI peripheral. More...

#include <HardwareSpi.h>

Inheritance diagram for HardwareSpi:
Spi

Public Member Functions

 HardwareSpi (TreehopperUsb &board)
 
bool enabled () override
 gets whether the SPI module is enabled More...
 
void enabled (bool) override
 sets whether the SPI module is enabled More...
 
virtual std::vector< uint8_t > sendReceive (std::vector< uint8_t > dataToWrite, SpiChipSelectPin *chipSelect=nullptr, ChipSelectMode chipSelectMode=ChipSelectMode::SpiActiveLow, double speed=6, SpiBurstMode burstMode=SpiBurstMode::NoBurst, SpiMode spiMode=SpiMode::Mode00)=0
 Send/receive data out of this SPI port. More...
 

Detailed Description

Basic Usage

Once enabled(), you can use the hardware SPI module on Treehopper through the sendReceive() method, which is used to simultaneously transmit and/or receive data.

Background

SPI is a full-duplex synchronous serial interface useful for interfacing with both complex, high-speed peripherals, as well as simple LED drivers, output ports, and any other general-purpose input or output shift register.

Compared to I2, SPI is a simpler protocol, generally much faster, and less popular for modern peripheral ICs.

spi-overview.svg
Basic SPI interfacing

Pins

Treehopper supports SPI master mode with the following pins:

  • MISO (Master In, Slave Out): This pin carries data from the slave to the master.
  • MOSI (Master Out, Slave In): This pin carries data from the master to the peripheral
  • SCK (Serial Clock): This pin clocks the data into and out of the master and slave device.

Not all devices use all pins, but the SPI peripheral will always reserve the SCK, MISO, and MOSI pin once the peripheral is enabled, so these pins cannot be used for other functions.

Chip Select

Almost all SPI peripherals also use some sort of chip select (CS) pin, which indicates a valid transaction. Thus, the easiest way to place multiple peripherals on a bus is by using a separate chip select pin for each peripheral (since a peripheral will ignore SPI traffic without a valid chip select signal). Treehopper supports two different chip-select styles:

  • SPI mode: chip-select is asserted at the beginning of a transaction, and de-asserted at the end; and
  • Shift output mode: chip-select is strobed at the end of a transaction
  • Shift input mode: chip-select is strobed at the beginning of a transaction These styles support both active-low and active-high signal polarities.

SPI Mode

SPI does not specify a transaction-level protocol for accessing peripheral functions (unlike, say, SMBus for I2c does); as a result, peripherals that use SPI have wildly different implementations. Even basic aspects – when data is clocked, and the polarity of the clock signal – vary by IC. This property is often called the "SPI mode" of the peripheral; Treehopper supports all four modes:

  • Mode 0 (00): Clock is idle-low. Data is latched in on the clock's rising edge and data is output on the falling edge.
  • Mode 1 (01): Clock is idle-low. Data is latched in on the clock's falling edge and data is output on the rising edge.
  • Mode 2 (10): Clock is idle-high. Data is latched in on the clock's rising edge and data is output on the falling edge.
  • Mode 3 (11): Clock is idle-high. Data is latched in on the clock's falling edge and data is output on the rising edge.

Clock Speed

Treehopper supports SPI clock rates as low as 93.75 kHz and as high as 24 MHz, but we recommend a clock speed of 6 MHz for most cases. You will not notice performance gains above 6 MHz, since this is the fastest rate that Treehopper's MCU can place bytes into the SPI buffer; any faster and the SPI peripheral will have to wait for the CPU before transmitting the next byte.

Note
In the current firmware release, clock rates between 800 kHz and 6 MHz are disallowed. There appears to be a silicon bug in the SPI FIFO that can cause lock-ups with heavy USB traffic. We hope to create a workaround for this issue in future firmware updates.

Burst mode

If you only need to transmit or receive data from the device, Treehopper supports an SpiBurstMode flag, which can improve performance substantially (especially in the case of BurstTx, which eliminates the back-and-forth needed, reducing transaction times down to a few hundred microseconds).

Chaining Devices & Shift Registers

Treehopper's SPI module works well for interfacing with many types of shift registers, which typically have a single output state "register" that is updated whenever new SPI data comes in. Because of the nature of SPI, any existing data in this register is sent to the MISO pin (sometimes labeled "DO" — digital output — or, confusingly, "SO" — serial output). Thus, many shift registers (even of different types) can be chained together by connecting the DO pin of each register to the DI pin of the next:

spi-shift-register.svg
Many shift registers can share the SPI bus and CS line

Please note that most shift registers refer to their "CS" pin as a "latch enable" (LE) signal.

In the example above, if both of these shift registers were 8-bit, sending the byte array {0xff, 0x03} would send "0xff" to the right register, and "0x03" to the left one.

Treehopper.Libraries has support for many different peripherals you can use with the SPI peripheral, including shift registers. See the Libraries documentation for more details on all the library components. Examples of shift register library components include Treehopper.Libraries.Displays.LedShiftRegister, Treehopper.Libraries.IO.PortExpander.Hc166, Treehopper.Libraries.IO.PortExpander.Hc595.

Further Reading

Wikipedia has an excellent SPI article: Serial Peripheral Interface Bus

Constructor & Destructor Documentation

◆ HardwareSpi()

HardwareSpi ( TreehopperUsb board)
explicit

Member Function Documentation

◆ enabled() [1/2]

bool enabled ( )
overridevirtual

Implements Spi.

◆ enabled() [2/2]

void enabled ( bool  )
overridevirtual

Implements Spi.

◆ sendReceive()

virtual std::vector<uint8_t> sendReceive ( std::vector< uint8_t >  dataToWrite,
SpiChipSelectPin chipSelect = nullptr,
ChipSelectMode  chipSelectMode = ChipSelectMode::SpiActiveLow,
double  speed = 6,
SpiBurstMode  burstMode = SpiBurstMode::NoBurst,
SpiMode  spiMode = SpiMode::Mode00 
)
pure virtualinherited
Parameters
[in]dataToWritevector of input data
[in]chipSelectpin to use as chip-select, or nullptr. Defaults to nullptr.
[in]chipSelectModeThe ChipSelectMode to use if chipSelect != nullptr. Defaults to ChipSelectMode::SpiActiveLow.
[in]speedThe speed, in MHz, to clock the data at. Defaults to 6.
[in]burstModeThe burst mode to use. Defaults to BurstMode::NoBurst.
[in]spiModeThe SPI mode to use. Defaults to SpiMode::Mode00.
Returns
A std::vector<uint8_t> of the received data

The documentation for this class was generated from the following file: