#spi module
All transactions for sending and receiving are most-significant-bit first and least-significant last.
For technical details of the underlying hardware refer to [metalphreak's ESP8266 HSPI articles](http://d.av.id.au/blog/tag/hspi/).

## High Level Functions
The high level functions provide a send & receive API for half- and
full-duplex mode. Sent and received data items are restricted to 1 - 32 bit
length and each data item is surrounded by (H)SPI CS inactive.

## spi.recv()
Receive data from SPI.

#### Syntax
`spi.recv(id, size[, default_data])`

#### Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `size` number of data items to be read
- `default_data` default data being sent on MOSI (all-1 if omitted)

#### Returns
String containing the bytes read from SPI.

####See also
[spi.send()](#spisend)

## spi.send()
Send data via SPI in half-duplex mode. Send & receive data in full-duplex mode.

#### Syntax
HALFDUPLEX:<br />
`wrote = spi.send(id, data1[, data2[, ..., datan]])`

FULLDUPLEX:<br />
`wrote[, rdata1[, ..., rdatan]] = spi.send(id, data1[, data2[, ..., datan]])`

#### Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `data` data can be either a string, a table or an integer number.<br/>Each data item is considered with `databits` number of bits.

#### Returns
- `wrote` number of written bytes
- `rdata` received data when configured with `spi.FULLDUPLEX`<br />Same data type as corresponding data parameter.

#### Example
```lua
=spi.send(1, 0, 255, 255, 255)
4       255     192     32      0
x = {spi.send(1, 0, 255, 255, 255)}
=x[1]
4
=x[2]
255
=x[3]
192
=x[4]
32
=x[5]
0
=x[6]
nil
=#x
5

_, _, x = spi.send(1, 0, {255, 255, 255})
=x[1]
192
=x[2]
32
=x[3]
0
```
#### See also
- [spi.setup()](#spisetup)
- [spi.recv()](#spirecv)

## spi.setup()
Set up the SPI configuration.
Refer to [Serial Peripheral Interface Bus](https://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus#Clock_polarity_and_phase) for details regarding the clock polarity and phase definition.

#### Syntax
`spi.setup(id, mode, cpol, cpha, databits, clock_div[, duplex_mode])`

#### Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `mode` select master or slave mode
	- `spi.MASTER`
	- `spi.SLAVE` - **not supported currently**
- `cpol` clock polarity selection
	- `spi.CPOL_LOW` 
	- `spi.CPOL_HIGH` - **not supported currently**
- `cpha` clock phase selection
	- `spi.CPHA_LOW`
	- `spi.CPHA_HIGH`
- `databits` number of bits per data item 1 - 32
- `clock_div` SPI clock divider, f(SPI) = f(CPU) / `clock_div`
- `duplex_mode` duplex mode
	-  `spi.HALFDUPLEX` (default when omitted)
	- `spi.FULLDUPLEX`

#### Returns
Number: 1

## Low Level Hardware Functions
The low level functions provide a hardware-centric API for application
scenarios that need to excercise more complex SPI transactions. The
programming model is built up around the HW send and receive buffers and SPI
transactions are initiated with full control over the hardware features.

## spi.get_miso()
Extract data items from MISO buffer after `spi.transaction()`.

#### Syntax
`data1[, data2[, ..., datan]] = spi.get_miso(id, offset, bitlen, num)`

#### Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `offset` bit offset into MISO buffer for first data item
- `bitlen` bit length of a single data item
- `num` number of data items to retrieve

####Returns
`num` data items

#### See also
[spi.transaction()](#spitransaction)

## spi.set_mosi()
Insert data items into MOSI buffer for `spi.transaction()`.

#### Syntax
`spi.set_mosi(id, offset, bitlen, data1[, data2[, ..., datan]])`

####Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `offset` bit offset into MOSI buffer for inserting data1 and subsequent items
- `bitlen` bit length of data1, data2, ...
- `data` data items where `bitlen` number of bits are considered for the transaction.

#### Returns
`nil`

#### See also
[spi.transaction()](#spitransaction)

## spi.transaction()
Start an SPI transaction, consisting of up to 5 phases:

1. Command
2. Address
3. MOSI
4. Dummy
5. MISO

#### Syntax
`spi.transaction(id, cmd_bitlen, cmd_data, addr_bitlen, addr_data, mosi_bitlen, dummy_bitlen, miso_bitlen)`

#### Parameters
- `id` SPI ID number: 0 for SPI, 1 for HSPI
- `cmd_bitlen` bit length of the command phase (0 - 16)
- `cmd_data` data for command phase
- `addr_bitlen` bit length for address phase (0 - 32)
- `addr_data` data for command phase
- `mosi_bitlen` bit length of the MOSI phase (0 - 512)
- `dummy_bitlen` bit length of the dummy phase (0 - 256)
- `miso_bitlen` bit length of the MISO phase (0 - 512) for half-duplex.<br />Full-duplex mode is activated with a negative value.

####Returns
`nil`

####See also
- [spi.set_mosi()](#spisetmosi)
- [spi.get_miso()](#spigetmiso)
