Implementation of SPI Protocol in FPGA

Transcription

1 Implementation of SPI Protocol in FPGA Veda Patil 1, Vijay Dahake 2, Dharmesh Verma 3, Elton Pinto 4 1,2 (Electronics & Telecommunication, Rama rao Adik Institute of technology/ Mumbai, India) 3,4 (Electronics & Telecommunication, Society of Applied Microwave Electronics Engineering & Research/Mumbai, India) Abstract: In this paper, Serial Peripheral Interface (SPI) protocol is implemented in Field Programmable Gate Array (FPGA). Both the components SPI Master and SPI Slave are implemented using state machine diagram. The coding is done in Very high speed integrated circuits Hardware Descriptive Language (VHDL). The simulated data is shown and Received data is analyzed by receiving an acknowledgment. Keywords: Clock Phase, Clock Polarity, FPGA, MISO, MOSI, SPI master, SPI Slave, VHDL. 1. Introduction Serial communication is the process of sending data one bit at a time, sequentially, over a communication channel. Serial communication is used for all long-haul communication and most computer networks, where the cost of cabling and synchronization difficulties makes parallel communication impractical. A serial connection requires fewer interconnecting cables (e.g., wires/fibers) and hence occupies less space. The extra space allows for better isolation of the channel from its surroundings. Serial buses are becoming more common even at shorter distances, as improved signal integrity and transmission speeds in newer serial technologies have begun to outweigh the parallel bus's advantage of simplicity and to outstrip its disadvantages. Serial links can be clocked considerably faster than parallel links in order to achieve a higher data rate. Crosstalk is less of an issue, because there are fewer conductors in proximity. In many cases, serial is a better option because it is cheaper to implement. Many Integrated Circuits have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore less expensive. 2. Serial Peripheral Interface (SPI) Protocol The 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 clock and data frame. Multiple slave devices are allowed with individual slave select (chip select) lines. SPI is often referred to as SSI (Synchronous Serial Interface). The SPI bus interface consists four logic signals namely Serial Clock (SCLK), Master Output Slave In (MOSI), Master In Slave out (MISO) and Slave Select (SS). Figure1. SPI Protocol Interface To begin a communication, the bus master first configures the clock, using a frequency less than or equal to the maximum frequency the slave device supports. Such frequencies are commonly in the range of MHz the master then transmits the appropriate chip select bit for the desired chip to logic 0. Logic 0 is transmitted because the chip s elect line is active low, meaning its off state is logic 1; on is asserted with logic 0. If a waiting period is required then the master must wait for at least that period of time before starting to issue clock cycles. Transmissions normally involve two shift registers of some given word size, such as eight bits, one in the master and one in the slave; they are connected in a ring. Data is usually shifted out with the most significant bit first, while shifting a new least significant bit into the same reg ister. After that register has been shifted out, the master and slave have exchanged register values. Then each device takes that value and is written to memory. If there is more data to exchange, the shift registers are loaded with new data and the process repeats. Transmissions may involve any number of clock cycles. When there is no more data to be transmitted, the master stops toggling its clock. Normally, it then deselects the slave. Issn (online) January 2013 Page 142

2 Figure 2. SPI Timing diagram 3. Clock Polarity and Phase In addition to setting the clock frequency, the master must also configure the clock polarity and phase with respect to the data. The timing applies to both the master and the slave to communicate articulately. The combinations of Clock polarity (CPOL) and Clock Phase (CPHA) give rise to different modes. Table 1.Different modes of clock Mode CPOL CPHA Mode 0: CPOL=0 i.e. SCLK is normally low and the first clock edge is a rising edge & CPHA=0, the data is captured on the clock's rising edge (low to high transition) and data is received on the clock s falling edge (high to low clock transition) Mode 1: CPOL=0 i.e. SCLK is normally low and the first clock edge is a rising edge & CPHA=1, data is captured on the clock's falling edge and data is propagated on a rising edge. Mode 2: CPOL=1 i.e. SCLK is normally high and the first clock edge is a falling edge & CPHA=0, data is captured on clock's falling edge and data is received on a rising edge. Mode 3: CPOL=1 i.e. SCLK is normally high and the first clock edge is a falling edge & CPHA=1, data is captured on clock's rising edge and data is received on a falling edge. For all CPOL and CPHA modes, the initial clock value must be stable before the chip select line goes active. The MOSI and MISO signals are usually stable (at their reception points) for the half cycle until the next clock transition. SPI master and slave devices may well sample data at different points in that half cycle. This adds mo re flexibility to the communication channel between the master and slave. Figure 3. Timing diagram showing clock polarity and clock phase. Issn (online) January 2013 Page 143

3 4. VHDL Coding And Result 4.1. SPI Master The number of slaves is declared in the ENTITY by the GENERIC parameter as slaves and the transmit and receive data bus widths are declared by GENERIC parameter d_width. Table below shows different input - output ports which are defined in the code. In the table M specifies the data width and N specifics the number of slaves. The clock and clk_div inputs define the frequency of SCLK (i.e. the SPI data rate). Clock is the system clock used to operate the synchronous logic inside the component. The clk_div integer input allows the user to set the relative speed at which the current transaction occurs. Clk_div is the number of clock periods between SCLK transitions. The enable pin latches in the standard logic values of clk_div, cpol and cpha at the start of each transaction. This allows communication with individual slaves using independent SPI speeds & modes. A low logic level on the busy output port indicates that the component is ready to accept a command. The component latches the settings, address, and data for a transaction on the first rising edge of clock where the enable input is asserted. On the following clock, the component asserts the busy signal and begins performing the transaction. Once complete, the component outputs the received data on the rx_data port. This data remains on the port until the component receives new data from a subsequent transaction. The component sets busy low to notify the user when the data is available, and the component is immediately ready for another instruction.the transition timing diagram is obtained by the simulator is given in the table 2. This SPI master is instantiated with one slaves and eight bit data width. It transmits the data to slave 2, which operates in mode 3 (CPOL= 1 and CPHA= 1 ).The master receives the data. Table 2. Port descriptions of SPI Master entity Port Width Mode Data Type Interface Description clock 1 in standard logic user logic System clock. reset_n 1 in standard logic user logic Asynchronous active low reset. enable 1 in standard logic user logic H: latches in settings, address, and data to initiate a transaction, L: no transaction is initiated. cpol 1 in standard logic user logic SPI clock polarity setting. cpha 1 in standard logic user logic SPI clock phase setting. cont 1 in standard logic user logic Continuous mode flag. clk_div 32 in integer user logic Speed setting. The integer input is the number of system clocks per 1/2 period of sclk. addr 32 in integer user logic Address of target slave. The slaves are assigned addresses starting with 0. tx_data M* in standard logic vector user logic Data to transmit. miso 1 in standard logic slave devices Master in, slave out data line. sclk 1 buffer standard logic slave devices SPI clock. ss_n N^ buffer standard logic vector slave devices Slave select signals. mosi 1 out standard logic slave devices Master out, slave in data line. busy 1 out standard logic user logic Busy / data ready signal. rx_data M* out standard logic vector user logic Data received from target slave. Issn (online) January 2013 Page 144

4 Figure 4. Simulated results of SPI Master 4.2. SPI Slave The mode that the slave operates in is defined by the GENERIC parameters cpol and cpha. The transmit and receive data bus width are declared by the GENERIC parameter d_width. Each transaction between the SPI slave component and the SPI master must consist of an 8-bit command, followed by a N-bit data transfer. The size N of the data transfer is determined by the GENERIC d_width parameter in the VHDL entity and is set by the user. The receive register is a buffer that holds data received from the master over the MOSI line. For the user logic to access the resulting data, the user must pulse the rx_reqinput when the component is not performing a transaction. The slave then outputs the receive register contents on the rx_data port. The transmit register is a buffer that holds the data intended to be transmitted by the slave to the master over the MISO lin e. The user logic fills this register by presenting the data on the tx_load_data port and pulsing the tx_load_en input when the component is not performing a transaction. The status register contains three status bits. The master can read, set, or clear these bits with transactions addressed to this register. During a transaction that reads or writes the status register, the first three data bits sent correspond to trdy, rrdy, and roe, respectively. The slave component ignores the remaining bits sent or received during the transaction. The user logic has access to these bits via the ports bearing their names. The user logic can also set or clear these bits by presenting the desired values on the st_load_trdy, st_load_rrdy, and st_load_roe inputs and pulsing the st_load_en input when the component is not performing a transaction (i.e. the busy output is low). The reset_n input port must have logic high for the SPI slave component to operate. A low signal on this port asynchronously resets the component. During reset, the miso output assumes a high impedance state, and the rx_data output port clears. The transmit, receive, transmit ready, receive ready, and receive overrun error registers all clear. Figure 5 illustrates a transaction with cpol = 0, cpha = 0, and d_width = 8. The slave is configured to operate in mode 0. While the slave is not busy, the user logic pulses the tx_load_en input while presenting the data on the tx_load_data bus. The trdy signal immediately goes high to indicate that new data exists to transmit. The master then initiates the transaction with the command , instructing the slave to load the subsequent MOSI data into the receive register and output the transmit register s contents to MISO. Once the last data bit is sent, the trdy signal goes low to indicate that the transmit register s current value has been sent to the master. Likewise, when the slave reads the last data bit into the receive register, the rrdy signal asserts to indicate that new data has been received. The user logic then pulses the rx_req input, and the slave responds by outputting the received data on the rx_data bus. The rrdy output then deasserts to indicate that the data contained in the receive register has already been read by the user logic. Issn (online) January 2013 Page 145

5 Table 3. Port descriptions of SPI Slave entity Port Width Mode Data Type Interface Description sclk 1 in standard logic master device SPI clock. mosi 1 in standard logic master device Master out, slave in data line. miso 1 out standard logic master device Master in, slave out data line. ss_n 1 in standard logic master device Active low slave select signal. busy 1 out standard logic user logic 1 during transactions with the master device, 0 when available to user logic reset_n 1 in standard logic user logic Active low asynchronous reset. tx_load_en 1 in standard logic user logic Latches tx_load_data into the transmit register (the slave must not be busy). tx_load_data M* in standard logic vector user logic Data to be latched into the transmit register. rx_req 1 in standard logic user logic Requests the last received data from the master (the slave must not be busy). rx_data M* out standard logic vector user logic Presents the last received data from the master when requested. trdy 1 buffer standard logic user logic, master device rrdy 1 buffer standard logic user logic, master device roe 1 buffer standard logic user logic, master device Transmit Ready. 1 when the slave has loaded data into the transmit register, but it has not yet been sent. Receive Ready. 1 when the slave has received data from the master, but it has not yet been requested. Receive Overrun Error. 1 when data received from the master is overwritten by new data from the master before it has been accessed by the user logic. st_load_en 1 in standard logic user logic Status Load Enable. Latches data into the trdy, rrdy, and roe registers from the user logic (the slave must not be busy). st_load_trdy 1 in standard logic user logic The trdy value that is latched in by st_load_en. st_load_rrdy 1 in standard logic user logic The rrdy value that is latched in by st_load_en. st_load_roe 1 in standard logic user logic The roe value that is latched in by st_load_en. Figure 4. Simulated results of SPI Slave Issn (online) January 2013 Page 146

6 5. Conclusion In this paper SPI Master and SPI Slave protocol has been implemented in VHDL. This SPI Master is a flexible programmable logic component that accommodates communication with a variety of slaves via single parallel interface. It allows communication with user specified number of slaves, which may require independent SPI modes, data widths, and serial clock speeds. The VHDL SPI slave offers a user definable mode and data width. It includes status signals for transmit ready, receive ready, and receive overrun error. References [1] Motorola MMC 2001 Reference Manual. [2] Motorola SPI Block Guide V03.06, [3] Microchip MCP2510 Application Note AN215. Issn (online) January 2013 Page 147