The special function register `SPI` memory mapped to address 4100 enables HACK to read/write bytes from the spi flash memory chip W25Q16BV situated on iCE40HX1K-EVB. The timing diagramm for SPI communication looks like the following diagram (we use CPOL=0 and CPHA=0).
When load=1 and in[8]=0 transmission of byte in[7:0] is initiated. CSX is goes low (and stays low even when transmission is completed). The byte is send to SDO bitwise together with 8 clock signals on SCK. At the same time the SPI receives a byte at SDI. During transmission out[15] is 1. After 16 clock cycles the transmission of one byte is completed. out[15] goes low and SPI outputs the received byte to out[7:0].
When load=1 and in[8]=1 CSX goes high without transmission of any bit.
**Attention:** Sampling of SDO is done at rising edge of SCK and shifting is done at falling edge of SCK.
### Proposed Implementation
Use a `Bit` to store the state (0 = ready, 1 = busy) which is output to out[15]. Use a counter `PC` to count from 0 to 15. Finally we need a `BitShift8L`. This will be loaded with the byte in[7:0] to be send. Another `Bit` will sample the SDI wire when SCK=0 and shift the stored bit into the `BitShift8L` when SCK=1. After 8 bits are transmitted the module cleares out[15] and outputs the received byte to out[7:0].
![](SPI.png)
### Memory map
The special function register `SPI` is mapped to memory map of HACK according to:
| 4100 | SPI | R | out[15]=1 if busy, out[7:0] received byte |
| 4100 | SPI | W | start transmittion of byte in[7:0] |
### cat.asm
To test HACK with SPI we need a little machine language programm `cat.asm`, which reads 4 consecutive bytes of SPI flash memory chip W25Q16BV of iCE40HX1K-EVB, starting at address 0x040000 (256k) and sends them to UART_TX.
According to the datasheet of spi flash rom chip W25Q16BV the commands needed to read the flash rom chip are:
| 0xAB | wake up from deep power down (wait 3us) before launching next command. |
| 0x03 0x04 0x00 0x00 | read data (command 0x03) starting at address 0x040000 (256k) |
| 0xB9 | go in power down |
### SPI in real hardware
The board iCE40HX1K-EVB comes with a SPI flash rom chip W25Q16BV. The chip is already connected to iCE40HX1K according `iCE40HX1K-EVB.pcf` (Compare with schematic [iCE40HX1K_EVB](../../doc/iCE40HX1K-EVB_Rev_B.pdf).
```
set_io SPI_SDO 45 # iCE40-SDO
set_io SPI_SDI 46 # iCE40-SDI
set_io SPI_SCK 48 # iCE40-SCK
set_io SPI_CSX 49 # iCE40-SS_B
```
***
### Project
* Implement `SPI.v` and test with testbench:
```
$ cd 03_SPI
$ apio clean
$ apio sim
```
* Compare output `OUT` of special function tregister `SPI` with `CMP`.
![](spi_tb.png)
* Add special function register`SPI` to `HACK.v` at memory address 4100.
* Implement `cat.asm` and test in simulation:
```
$ cd 03_SPI
$ make
$ cd ../00_HACK
$ apio clean
$ apio sim
```
* Check the wake up command 0xAB:
![](spi_wakeup.png)
* Check command 0x03040000 (read from address 0x040000)
![](spi_read040000.png)
* Check reading of string "SPI!" output to UartTX.
![](echo.png)
* preload the SPI memory chip with some text file at address 0x040000.
* build and upload HACK with `cat.asm` in ROM.BIN to iCE40HX1K-EVB.