120 lines
4.7 KiB
Markdown
120 lines
4.7 KiB
Markdown
## 03 SPI
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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).
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![](spi-timing.png)
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### Chip specification
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| IN/OUT | wire | function |
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| ------ | -------- | ------------------------------------------- |
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| IN | in[7:0] | byte to be sent. |
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| IN | in[8] | =1 send byte and set CSX low |
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| IN | in[8] | =0 pull CSX high without sending byte |
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| IN | load | =1 initiates the transmission, when in[8]=1 |
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| OUT | out[15] | =0 chip is busy, =0 chip is ready |
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| OUT | out[7:0] | received byte (when out[15]=0) |
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| OUT | CSX | chip select not |
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| OUT | SDO | serial data out |
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| OUT | SCK | serial clock |
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| IN | SDI | serial data in |
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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].
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When load=1 and in[8]=1 CSX goes high without transmission of any bit.
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**Attention:** Sampling of SDO is done at rising edge of SCK and shifting is done at falling edge of SCK.
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### Proposed Implementation
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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].
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![](SPI.png)
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### Memory map
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The special function register `SPI` is mapped to memory map of HACK according to:
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| address | I/O device | R/W | function |
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| ------- | ---------- | --- | ----------------------------------------- |
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| 4100 | SPI | R | out[15]=1 if busy, out[7:0] received byte |
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| 4100 | SPI | W | start transmittion of byte in[7:0] |
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### cat.asm
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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.
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According to the datasheet of spi flash rom chip W25Q16BV the commands needed to read the flash rom chip are:
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| command | function |
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| ------------------- | ---------------------------------------------------------------------- |
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| 0xAB | wake up from deep power down (wait 3us) before launching next command. |
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| 0x03 0x04 0x00 0x00 | read data (command 0x03) starting at address 0x040000 (256k) |
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| 0xB9 | go in power down |
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### SPI in real hardware
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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).
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```
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set_io SPI_SDO 45 # iCE40-SDO
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set_io SPI_SDI 46 # iCE40-SDI
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set_io SPI_SCK 48 # iCE40-SCK
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set_io SPI_CSX 49 # iCE40-SS_B
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```
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***
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### Project
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* Implement `SPI.v` and test with testbench:
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```
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$ cd 03_SPI
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$ apio clean
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$ apio sim
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```
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* Compare output `OUT` of special function tregister `SPI` with `CMP`.
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![](spi_tb.png)
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* Add special function register`SPI` to `HACK.v` at memory address 4100.
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* Implement `cat.asm` and test in simulation:
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```
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$ cd 03_SPI
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$ make
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$ cd ../00_HACK
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$ apio clean
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$ apio sim
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```
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* Check the wake up command 0xAB:
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![](spi_wakeup.png)
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* Check command 0x03040000 (read from address 0x040000)
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![](spi_read040000.png)
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* Check reading of string "SPI!" output to UartTX.
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![](echo.png)
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* preload the SPI memory chip with some text file at address 0x040000.
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* build and upload HACK with `cat.asm` in ROM.BIN to iCE40HX1K-EVB.
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```
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$ echo SPI! > spi.txt
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$ iceprogduino -o 256k -w spi.txt
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$ cd 00_HACK
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$ apio clean
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$ apio upload
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$ tio /dev/ttyACM0
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```
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* Check if you see "SPI!" in your terminal.
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