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| <h1 id="hal_spi">hal_spi</h1> |
| <p>SPI (Serial Peripheral Interface) is a synchronous 4-wire serial interface |
| commonly used to connect components in embedded systems.</p> |
| <p>For a detailed description of SPI, see <a href="https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus">Wikipedia</a>.</p> |
| <h3 id="description">Description</h3> |
| <p>The Mynewt HAL interface supports the SPI master functionality only. Future |
| version will support SPI slave functionality.</p> |
| <h3 id="definition">Definition</h3> |
| <p><a href="https://github.com/apache/incubator-mynewt-larva/blob/master/hw/hal/include/hal/hal_spi.h">hal_spi.h</a></p> |
| <h3 id="hal_spi-theory-of-operation">HAL_SPI Theory Of Operation</h3> |
| <p>SPI is called a 4-wire interface because of the 4 signals, MISO, MOSI, CLK, |
| and SS. The SS signal (slave select) is an active low signal that activates |
| a SPI slave device. This is how a master "addresses" a particular slave |
| device. Often this signal is also referred to as "chip select" as it |
| selects particular slave device for communications.</p> |
| <p>The Mynewt SPI HAL has blocking transfers. This means that the API call |
| to transfer a byte will wait until the byte completes transmissions before |
| the function returns.</p> |
| <p>Since SPI is generally a shared communications bus, the hal_spi API allows |
| SPI runtime configuration to be compatible with the data format and speed of the |
| slave device. See the <code>hal_spi_config</code> method in the API above for a |
| description of the available settings. These settings are critical to the |
| operation of the slave device.</p> |
| <p>The Mynewt SPI HAL does not include built-in SS signaling. Its up to the |
| hal_spi user to control their own SS pins. Typically applications will do |
| this with GPIO.</p> |
| <p>An example of this would look like this (error handling excluded for brevity):</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code> /* each device may have different settings */ |
| extern struct hal_spi_settings dev1; |
| extern struct hal_spi_settings dev2; |
| |
| gpio_init_out(SS_DEV_1, 1); |
| gpio_init_out(SS_DEV_2, 1); |
| hal_spi *pspi = hal_spi_init(SPI0); |
| |
| ... |
| /* write a single byte (0) to device 1 */ |
| hal_spi_config(pspi, &dev1); |
| gpio_clear(SS_DEV_1) |
| rc = hal_spi_transfer(pspi, 0) |
| gpio_set(SS_DEV_1) |
| |
| /* read a single byte from device 2 */ |
| hal_spi_config(pspi, &dev2); |
| gpio_clear(SS_DEV_2) |
| val = hal_spi_transfer(pspi, 0) |
| gpio_set(SS_DEV_2) |
| </code></pre></div> |
| |
| <p>Depending on the device's operation, it may be necessary or more efficient |
| to write or read multiple bytes in a single transaction. An example is |
| shown below</p> |
| <p>```no-highlight</p> |
| <div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%;"><span></span><code>void spi_write_buf(struct hal_spi *pspi, uint8_t *buf, int len) { |
| char *ptr = buf; |
| |
| hal_spi_config(pspi, &dev2); |
| gpio_clear(SS_DEV_2) |
| while(len-- > 0) { |
| val = hal_spi_transfer(pspi, (uint16_t) *ptr++) |
| } |
| gpio_set(SS_DEV_2) |
| } |
| </code></pre></div> |
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