HotRIO slave

This module implements the core functionality of an HotRIO slave. It receives, decodes and retransmits an HotRIO frames stream. If a packet inside the stream matches the address of the slave, the payload is exchanged: the received payload is made available externally and a new payload is inserted.

This module uses an 8-bit interface with a PCS. The interface is made by a 8-bit bus plus a single bit that flags for k-characters.

On all the received packets, the parity bits are checked. If an error is detected, both parity bits are invalidated.

The logic is clocked by a single clock, thus this can be used in a PCS configured in a “Single Clock Domain” mode or, with the proper synchronization, in a “Dual Clock Domain” mode.

Ports

Name	Mode	Description	Type
clk_i	IN	Input clock signal, must be connected to the RX clock of the PCS	std_logic
rstn_i	IN	Internally synchronized, active low reset	std_logic
rx_pcs_data_i	IN	Input PCS signals	std_logic_vector(7 downto 0)
rx_pcs_kchar_i	IN	Input k-character flag	std_logic
tx_pcs_data_o	OUT	Output PCS signals	std_logic_vector(7 downto 0)
tx_pcs_kchar_o	OUT	Output k-character flag	std_logic
rx_hotrio_packet_o	OUT	Contains the last received HotRIO packet, even if it is not valid	hotrio_packet_t*
tx_hotrio_packet_i	IN	Contains the HotRIO packet to be transmitted	hotrio_packet_t*
packet_valid_o	OUT	Flag that indicates that a valid packet has been received and is available in the hotrio_packet output	std_logic
packet_sent_o	OUT	Flag that indicates that packet has been successfully sent	std_logic
packet_new_o	OUT	Flag that indicates that a packet has been received, even if it is not valid	std_logic
pulse_2us_o	OUT	Pulse that signals that a “2us” marker has been received	std_logic
pulse_1ms_o	OUT	Pulse that signals that a “1ms” marker has been received	std_logic
pulse_1s_o	OUT	Pulse that signals that a “1s” marker has been received	std_logic
sync_o	OUT	High when the state machine is synchronized with the incoming data	std_logic
cnt_err_kchar_o	OUT	Counts the times when an unexpected k-character is received	(CONFIG_DATA_WIDTH - 1 downto 0)
cnt_err_data_o	OUT	Counts the times when an unexpected character is received (example: non valid marker)	(CONFIG_DATA_WIDTH - 1 downto 0)
cnt_err_hrio_pid_o	OUT	Counts the number of non-HotRIO packets received	(CONFIG_DATA_WIDTH - 1 downto 0)
cnt_err_parity_o	OUT	Counts the number of parity errors (even or odd)	(CONFIG_DATA_WIDTH - 1 downto 0)
cnt_packets_received_o	OUT	Counts the number of received packets	(CONFIG_DATA_WIDTH - 1 downto 0)
cnt_valild_packets_received_o	OUT	Counts the number of valid received packets	(CONFIG_DATA_WIDTH - 1 downto 0)

* Defined in the Globals package

Dependencies

Name	Motivation
globals	Types definition and HotRIO constants.
signal_cdc	Used to synchronize the input reset

Implementation details

This module is implemented around a state machine

State machine

The state machine consists on seven states: SYNC, MARKER, PROTOCOL_ID, FRAME_ADDRESS, FRAME_PAYLOAD, FRAME_CONTROL and WAIT_FRAME.

After a reset, the state machine is in the SYNC state. In this state, it waits for a valid HotRIO marker. In this state, the sync_o output is kept low and all incoming data is simply forwarded. When a valid HotRIO marker is received, the state machine transitions to the PROTOCOL_ID state and sets sync_o to 1.

The MARKER state is used when the state machine is synchronized with the incoming data. In this state, two things are checked: the status of the k-character flag and the incoming value itself. If the k-character flag is not high, it means that the received value is not a marker. In this case the cnt_err_kchar_en_v counter is incremented and the state machine goes in the SYNC state. The incoming data is checked against the list of valid HotRIO markers, if the check succeeds, the corresponding output flag is raised following the scheme:

If comma character is received, no output flag is raised:

• pulse_2us_o <= ‘0’

• pulse_1ms_o <= ‘0’

• pulse_1s_o <= ‘0’

If “2-microsecond” marker is received, only the first flag is raised:

• pulse_2us_o <= ‘1’

• pulse_1ms_o <= ‘0’

• pulse_1s_o <= ‘0’

If the millisecond marker is received, the first two flags are raised as they overlap

• pulse_2us_o <= ‘1’

• pulse_1ms_o <= ‘1’

• pulse_1s_o <= ‘0’

The same logic is followed if the second marker flag is received

• pulse_2us_o <= ‘1’

• pulse_1ms_o <= ‘1’

• pulse_1s_o <= ‘1’

Also in this case, if the marker is not recognized, the cnt_err_data_en_v counter is increased and the SYNC state selected. On the other hand, the state machine continues on the PROTOCOL_ID state.

In the following states, only data is expected and no k-characters. So in every state a check is added to verify that the k-character flag is low. If a k-character is detected, cnt_err_kchar_en_v is incremented and the state machine goes back to SYNC.

The PROTOCOL_ID state, verifies that the receiving data is a frame of HotRIO. If the received byte corresponds with the HOTRIO_PROTOCOL_ID (defined in the globals) the state machine transitions to the FRAME_ADDRESS state. In the other case, the cnt_err_hrio_pid_en_v is increased and the state machine goes back to SYNC. During this state the packet to be sent is latched.

In the FRAME_ADDRESS state, the received byte is checked against the slave address. If the check succeeds, flg_addr_match_v is raised. Otherwise is not. Also in this state, the parity bits of the packet to be transmitted are calculated and the counter packets_counter_v is increased. This counter keeps track of the number of HotRIO packets in the frame.

The FRAME_PAYLOAD behavior changes depending on the flg_addr_match_v flag. If the flag is not set, the incoming data is simply forwarded for the length of the HotRIO payload (defined in the globals as HOTRIO_PAYLOAD_SIZE). On the other hand, if the flg_addr_match_v is set, the received bytes are saved to a buffer and the slave payload is sent to the pcs.

The FRAME_CONTROL calculates and checks the received parity bits. If this check is not successful, the transmitted parity bits are inverted to signal that an error occured. This is done even if the address was not previously recognized (If an error occured, it is possible that the address was corrupted). The behavior of the rest of the bits in the control byte is not yet defined.

If the flg_addr_match_v flag is high, the packet_new_o output is set to indicate that a new packet has been received and the cnt_packets_received_s is incremented. If the parity check succeeds, the packet_valid_o is set and the cnt_valild_packets_received_s incremented.

In the end, packets_counter_v is compared to the constant HOTRIO_PACKETS_PER_FRAME. If all packets in the frame have been received, the state machine transitions to the MARKER state, otherwise it goes to the FRAME_ADDRESS state.

The WAIT_FRAME state is used when a non-HotRIO frame is detected. During this state, the input is directly forwarded. The length of the frame is not specified, it waits for a k-character. If a valid HotRIO k-character s received, the PROTOCOL_ID state is selected. If an unknown k-character is received, the SYNC state is selected.

Note

To simplify the diagram, the connections in case of error are omitted. They consist in a connection from every state but SYNC, to SYNC.