(This is an 8-bit version, please switch to the corresponding branch for the 32-bit version.)
CDBUS is a simple protocol designed for serial ports and serial bus communication, commonly employed in RS-485 buses. It utilizes hardware arbitration and other mechanisms to avoid data conflicts, allowing nodes to freely transmit and receive data packets, thereby overcoming the limitations of single-master polling.
CDBUS has a 3-byte header: [src_addr, dst_addr, data_len], followed by user data, and concluding with a 2-byte CRC (identical to MODBUS CRC).
The CDBUS protocol at the byte level can be directly used for traditional serial communication, such as traditional UART, RS-232, RS-485, and USB virtual serial ports.
The full CDBUS protocol at the bit level requires dedicated hardware controllers (or software emulation) to achieve conflict avoidance, higher speeds, and strong real-time performance.
- It introduces an arbitration mechanism, similar to the CAN bus, to automatically avoid conflicts.
- Supporting dual baud rates achieves high-speed communication,
with the maximum baud rate during the high-speed phase up to:
sysclk÷ 3. (e.g. 50 Mbps for 150 MHzsysclk.) - Supports unicast, multicast and broadcast.
- The maximum user data size is 253 bytes.
- Hardware packing, unpacking, verification and filtering, saving your time and CPU usage.
- Compatible with traditional RS-485 hardware (arbitration function remains effective).
The protocol timing example, consisting of only one byte of user data:
(You can configure the lengths of idle time and transmission permit time.)
Tips:
- When a high-priority node needs to send unimportant data, we can dynamically increase the transmission permit time (TX_PERMIT_LEN).
Arbitration example:
Example waveforms for TX output and TX_EN pins:
The RX data sampling point for reception is at 1/2 bit; for TX readback, it's at 3/4 bit.
In CDBUS-A mode, if the low-speed portion takes longer, it can become a bottleneck for communication efficiency.
To address this, single-rate peer-to-peer bus communication can be achieved using CDBUS-BS mode:
- Different nodes have different configurations for the TX_PERMIT_LEN parameter, requiring sufficiently large differences to avoid conflicts.
- If any node has pending data frames waiting to be sent before the transmission permit point, transmission starts from that point.
- Otherwise, wait until the idle time exceeds MAX_IDLE_LEN. When there are pending data frames waiting for transmission, first send a break character to bring the bus out of the idle state.
The CDBUS-BS mode is suitable for high-speed applications with few nodes, and it is also suitable for software implementation.
| Register Name | Addr | Access | Default | Description (8-bit width by default if not specified) |
|---|---|---|---|---|
| VERSION | 0x00 | RD | 0x0e | Hardware version |
| SETTING | 0x02 | RD/WR | 0x10 | Configs |
| IDLE_WAIT_LEN | 0x04 | RD/WR | 0x0a | Waiting time to enter idle |
| TX_PERMIT_LEN_L | 0x05 | RD/WR | 0x14 | Waiting time to allows sending (10 bits) |
| TX_PERMIT_LEN_H | 0x06 | RD/WR | 0x00 | |
| MAX_IDLE_LEN_L | 0x07 | RD/WR | 0xc8 | Max idle waiting time in BS mode (10 bits) |
| MAX_IDLE_LEN_H | 0x08 | RD/WR | 0x00 | |
| TX_PRE_LEN | 0x09 | RD/WR | 0x01 | Enable TX_EN how long ahead than TX output (2 bits) |
| FILTER | 0x0b | RD/WR | 0xff | Local address |
| DIV_LS_L | 0x0c | RD/WR | 0x5a | Low-speed rate setting (16 bits) |
| DIV_LS_H | 0x0d | RD/WR | 0x01 | |
| DIV_HS_L | 0x0e | RD/WR | 0x5a | High-speed rate setting (16 bits) |
| DIV_HS_H | 0x0f | RD/WR | 0x01 | |
| INT_FLAG | 0x10 | RD | n/a | Status |
| INT_MASK | 0x11 | RD/WR | 0x00 | Interrupt mask |
| RX | 0x14 | RD | n/a | Read RX page |
| TX | 0x15 | WR | n/a | Write TX page |
| RX_CTRL | 0x16 | WR | n/a | RX control |
| TX_CTRL | 0x17 | WR | n/a | TX control |
| RX_ADDR | 0x18 | RD/WR | 0x00 | RX page read pointer (rarely used) |
| RX_PAGE_FLAG | 0x19 | RD | n/a | RX page flag |
| FILTER_M0 | 0x1a | RD/WR | 0xff | Multicast filter0 |
| FILTER_M1 | 0x1b | RD/WR | 0xff | Multicast filter1 |
SETTING:
| FIELD | DESCRIPTION |
|---|---|
| [6] | Full duplex mode |
| [5] | Break Sync mode |
| [4] | Enable arbitration |
| [3] | Save broken frame |
| [2] | CRC maintained by user |
| [1] | Invert tx pin output |
| [0] | Enable push-pull output for tx and tx_en pin |
| [6] | [5] | [4] | DESCRIPTION |
|---|---|---|---|
| 0 | 0 | 1 | CDBUS-A mode (default) |
| 0 | 1 | 0 | CDBUS-BS mode |
| 1 | 0 | 0 | Full-duplex mode |
| 0 | 0 | 0 | Traditional half-duplex mode |
TX_PRE_LEN:
Example waveforms for TX output and TX_EN pins (TX_PRE_LEN = 1 bit):
Unused for Arbitration mode and the break character automatically generated by BS mode.
FILTERS:
Match from top to bottom:
| SRC_ADDR | DST_ADDR | FILTER | FILTER_Mx | Receive or not | Remarks |
|---|---|---|---|---|---|
| not care | not care | 255 | not care | Receive | Promiscuous mode |
| = FILTER | not care | != 255 | not care | Drop | Avoid loopback |
| != FILTER | 255 | not care | not care | Receive | Broadcast |
| != FILTER | != 255 | not care | any = DST_ADDR | Receive | Multicast |
| != FILTER | != 255 | = DST_ADDR | not care | Receive | Unicast |
| not care | != 255 | != DST_ADDR | all != DST_ADDR | Drop |
For example, addresses from 0xe0 to 0xfe can be reserved for multicast address.
The default value 0xff of FILTER_Mx means not enabled.
DIV_xx_x:
Baud rate divider value: DIV_xx[15:0] = sysclk ÷ baud_rate − 1
The minimum value is 2.
For single rate, DIV_HS needs to be set to the same value as DIV_LS.
INT_FLAG:
| FIELD | DESCRIPTION |
|---|---|
| [7] | 1: TX error: TX is 0, but RX is sampled as 1 |
| [6] | 1: TX collision detected |
| [5] | 1: TX page released by hardware |
| [4] | 1: RX error: frame broken |
| [3] | 1: RX lost: no empty page for RX |
| [2] | 1: Break character received |
| [1] | 1: RX page ready for read |
| [0] | 1: Bus in IDLE mode |
Reading this register will automatically clear bit7, bit6, bit4, bit3 and bit2.
INT_MASK:
Output of irq = ((INT_FLAG & INT_MASK) != 0).
RX_CTRL:
| FIELD | DESCRIPTION |
|---|---|
| [4] | Reset RX block |
| [1] | Switch RX page |
| [0] | Reset RX page read pointer |
TX_CTRL:
| FIELD | DESCRIPTION |
|---|---|
| [5] | Send break character |
| [4] | Abort TX |
| [1] | Switch TX page |
| [0] | Reset TX page write pointer |
RX_PAGE_FLAG:
A value of zero indicates that the frame in the current RX page is correct;
Non-zero indicates the pointer to the last received byte of the disrupted frame, including CRC.
Always zero if save broken frame is not enabled.
parameter DIV_LS = 346, // default: 115200 bps for 40MHz clk
parameter DIV_HS = 346
input clk, // core clock
input reset_n, // async active-low reset
input chip_select,
output irq, // interrupt output
// avalon-mm slave interface, read and write without latency
// support burst read and write (normally for REG_TX and REG_RX)
input [4:0] csr_address,
input csr_read,
output [7:0] csr_readdata,
input csr_write,
input [7:0] csr_writedata,
// connect to external PHY chip, e.g. MAX3485
input rx,
output tx,
output tx_en # Configuration
write(REG_SETTING, [0x11]) # Enable push-pull output
# TX
write(REG_TX, [0x0c, 0x0d, 0x01, 0xcd]) # Write frame without CRC
while (read(REG_INT_FLAG) & 0x20) == 0: # Make sure we can successfully switch to the next page
pass
write(REG_TX_CTRL, [0x03]) # Trigger sending by switching TX page
# RX
while (read(REG_INT_FLAG) & 0x02) == 0: # Wait for RX page ready
pass
header = read(REG_RX, len=3)
data = read(REG_RX, len=header[2])
write(REG_RX_CTRL, [0x03]) # Release RX pageInstall iverilog (>= v10) and cocotb, goto tests/ folder, run ./test_all.sh or ./test_all.sh test_xxx.py.
(You can checkout the waveform cdbus.vcd by GTKWave.)
The CDCTL controllers family incorporates the CDBUS IP Core, providing peripheral interfaces such as SPI, I2C, and PCIe.
E.g. The tiny CDCTL-Bx module supports SPI and I2C interfaces:
(The source code and gerber files for this module are fully open-source and located in the example/ directory.)
For a list of CDBUS-related ASIC chips, please refer to the wiki page.
More relevant projects that may interest you:
- CDBUS GUI: https://github.com/dukelec/cdbus_gui
- CDNET: https://github.com/dukelec/cdnet
This Source Code Form is subject to the terms of the Mozilla
Public License, v. 2.0. If a copy of the MPL was not distributed
with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
Notice: The scope granted to MPL excludes the ASIC industry.
The CDBUS protocol is royalty-free for everyone except chip manufacturers.
Copyright (c) 2017 DUKELEC, All rights reserved.