Specification

SPI MASTER CORE SPECIFICATION

INTRODUCTION:

The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard, named by Motorola, that operates in full duplex  mode. Devices communicate in master/slave mode where the master device initiates the data frame.

SPI (Serial Peripheral Interface) Master core. Synchronous serial interfaces are widely used to provide economical board-level interfaces between different devices such as microcontrollers, DACs, ADCs and other. Although there is no single standard for a synchronous serial bus, there are industry-wide accepted guidelines based on two most popular implementations:

·         SPI (a trademark of Motorola Semiconductor)

·         Microwire/Plus (a trademark of National Semiconductor)

Many IC manufacturers produce components that are compatible with SPI and Microwire/Plus.

The SPI Master core is compatible with both above-mentioned protocols as master with some additional functionality.

At the hosts side, the core acts like a WISHBONE compliant slave device.

Features:

·         Full duplex synchronous serial data transfer

·         Variable length of transfer word up to 128 bits

·         MSB or LSB first data transfer

·         Rx and Tx on both rising or falling edge of serial clock independently

·         8 slave select lines

·         Fully static synchronous design with one clock domain

·         Technology independent Verilog

·         Fully synthesizable

WISHBONE interface signals

Port	Width	Direction	Description
wb_clk_i	1	Input	Master clock
wb_rst_i	1	Input	Synchronous reset, active high
wb_adr_i	5	Input	Lower address bits
wb_dat_i	32	Input	Data towards the core
wb_dat_o	32	Output	Data from the core
wb_sel_i	4	Input	Byte select signals
wb_we_i	1	Input	Write enable input
wb_stb_i	1	Input	Strobe signal/Core select input
wb_cyc_i	1	Input	Valid bus cycle input
wb_ack_o	1	Output	Bus cycle acknowledge output
wb_err_o	1	Output	Bus cycle error output
wb_int_o	1	Output	Interrupt signal output

Table 1: Wishbone interface signals

All output WISHBONE signals are registered and driven on the rising edge of wb_clk_i. All input WISHBONE signals are latched on the rising edge of wb_clk_i.

SPI external connections

Port	Width	Direction	Description
/ss_pad_o	8	Output	Slave select output signals
sclk_pad_o	1	Output	Serial clock output
mosi_pad_o	1	Output	Master out slave in data signal output
miso_pad_i	1	Input	Master in slave out data signal input

Table 2: SPI  external connections

Core Registers list

Name	Address	Width	Access	Description
Rx0	0x00	32	R	Data receive register 0
Rx1	0x04	32	R	Data receive register 1
Rx2	0x08	32	R	Data receive register 2
Rx3	0x0c	32	R	Data receive register 3
Tx0	0x00	32	R/W	Data transmit register 0
Tx1	0x04	32	R/W	Data transmit register 1
Tx2	0x08	32	R/W	Data transmit register 2
Tx3	0x0c	32	R/W	Data transmit register 3
CTRL	0x10	32	R/W	Control and status register
DIVIDER	0x14	32	R/W	Clock divider register
SS	0x18	32	R/W	Slave select register

Table 3: List of core registers

All registers are 32-bit wide and accessible only with 32 bits (all wb_sel_i signals must be active).

Control and status register [CTRL]

Bit #	31:14	13	12	11	10	9	8	7	6:0
Access	R	R/W	R/W	R/W	R/W	R/W	R/W	R	R/W
Name	Reserved	ASS	IE	LSB	Tx_NEG	Rx_NEG	GO_BSY	Reserved	CHAR_LEN

Table 6: Control and Status register

Reset Value: 0x00000000

ASS

If this bit is set, ss_pad_o signals are generated automatically. This means that slave select signal, which is selected in SS register is asserted by the SPI controller, when transfer is started by setting CTRL[GO_BSY] and is de-asserted after transfer is finished. If this bit is cleared, slave select signals are asserted and de-aserted by writing and clearing bits in SS register.

IE

If this bit is set, the interrupt output is set active after a transfer is finished. The Interrupt signal is deasserted after a Read or Write to any register.

LSB

If this bit is set, the LSB is sent first on the line (bit TxL[0]), and the first bit received from the line will be put in the LSB position in the Rx register (bit RxL[0]). If this bit is cleared, the MSB is transmitted/received first (which bit in TxX/RxX register that is depends on the CHAR_LEN field in the CTRL register).

Tx_NEG

If this bit is set, the mosi_pad_o signal is changed on the falling edge of a sclk_pad_o clock signal, or otherwise the mosi_pad_o signal is changed on the rising edge of sclk_pad_o.

Rx_NEG

If this bit is set, the miso_pad_i signal is latched on the falling edge of a sclk_pad_o clock signal, or otherwise the miso_pad_i signal is latched on the rising edge of sclk_pad_o.

GO_BSY

Writing 1 to this bit starts the transfer. This bit remains set during the transfer and is automatically cleared after the transfer finished. Writing 0 to this bit has no effect.

Divider register [DIVIDER]

Bit #	31:16	15:0
Access	R	R/W
Name	Reserved	DIVIDER

Table 7: Divider register

Reset Value: 0x0000ffff

DIVIDER

The value in this field is the frequency divider of the system clock wb_clk_i to generate the serial clock on the output sclk_pad_o. The desired frequency is obtained according to the following equation:

fclk = fwb_clk_i / (DIVIDER + 1 )*2

  Slave select register [SS]

Bit #	31:8	7:0
Access	R	R/W
Name	Reserved	SS

Table 8: Slave Select register

Reset Value: 0x00000000

SS

If CTRL[ASS] bit is cleared, writing 1 to any bit location of this field sets the proper ss_pad_o line to an active state and writing 0 sets the line back to inactive state. If CTRL[ASS] bit is set, writing 1 to any bit location of this field will select appropriate ss_pad_o line to be automatically driven to active state for the duration of the transfer, and will be driven to inactive state for the rest of the time.