MCP2515: SPI转CAN接口高速协议详解

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MCP2515是一款专为CAN总线通信设计的高性能接口芯片，由Microchip Technology Inc.在2003年至2012年期间开发。该芯片的主要特点包括： 1. **CAN V2.0兼容性与高速支持**: MCP2515能够实现高达1Mbps的数据传输速率，支持标准和扩展数据帧以及远程帧，数据字段长度可变，范围从0到8字节。 2. **接收缓冲区与过滤功能**: 提供两个优先级存储的接收缓冲器，每个缓冲器有29位的过滤器，总共可以设置6个过滤条件。数据字节过滤仅对标准数据帧的前两个字节有效。 3. **多级发送缓冲与中断功能**: 三个发送缓冲区支持优先级和取消发送功能，确保消息发送的一次性尝试。中断输出可用于接收缓冲器满或特定事件发生时。 4. **高速SPI接口**: 采用灵活的SPI模式0、1，速度可达10MHz，适用于高效的数据交换。 5. **单次发送模式** (One-Shot mode): 确保每个消息只发送一次，避免重复传输。 6. **时钟输出与可编程分频**: 提供一个可编程分频的时钟输出，作为其他设备的时钟源，增加了系统的灵活性。 7. **SOF信号监测**: SOF信号可用作时间槽协议或诊断工具，用于早期检测总线性能下降。 8. **可配置中断输出与控制**：中断输出可以独立设置，也可用作通用目的输出，而请求发送（RTS）输入可以分别配置为控制每个发送操作的信号。 9. **发送控制选项**: RTS输入可以被配置为每个发送操作的控制信号，提供更精细的控制。 MCP2515是CAN总线应用的理想选择，它集成了多种高级功能，支持高效、可靠的数据传输和管理，适用于需要CAN通信的嵌入式系统设计，如工业自动化、汽车电子、物联网等场景。通过合理的配置和使用，开发人员能够充分利用MCP2515的优势，提高系统性能和可靠性。

 2003-2012 Microchip Technology Inc. DS21801G-page 15

MCP2515

3.0 MESSAGE TRANSMISSION

3.1 Transmit Buffers

The MCP2515 implements three transmit buffers. Each

of these buffers occupies 14 bytes of SRAM and are

mapped into the device memory map.

The first byte, TXBnCTRL, is a control register

associated with the message buffer. The information in

this register determines the conditions under which the

message will be transmitted and indicates the status of

the message transmission (see Register 3-2).

Five bytes are used to hold the standard and extended

identifiers, as well as other message arbitration

information (see Register 3-4 through Register 3-7).

The last eight bytes are for the eight possible data

bytes of the message to be transmitted (see

At a minimum, the TXBnSIDH, TXBnSIDL and

TXBnDLC registers must be loaded. If data bytes are

present in the message, the TXBnDm registers must

also be loaded. If the message is to use extended

identifiers, the TXBnEIDm registers must also be

loaded and the TXBnSIDL.EXIDE bit set.

Prior to sending the message, the MCU must initialize

the CANINTE.TXInE bit to enable or disable the

generation of an interrupt when the message is sent.

3.2 Transmit Priority

Transmit priority is a prioritization within the MCP2515

of the pending transmittable messages. This is

independent from, and not necessarily related to, any

prioritization implicit in the message arbitration scheme

built into the CAN protocol.

Prior to sending the SOF, the priority of all buffers that

are queued for transmission is compared. The transmit

buffer with the highest priority will be sent first. For

example, if transmit buffer 0 has a higher priority setting

than transmit buffer 1, buffer 0 will be sent first.

If two buffers have the same priority setting, the buffer

with the highest buffer number will be sent first. For

example, if transmit buffer 1 has the same priority

setting as transmit buffer 0, buffer 1 will be sent first.

There are four levels of transmit priority. If

TXBnCTRL.TXP<1:0> for a particular message buffer

is set to 11, that buffer has the highest possible priority.

If TXBnCTRL.TXP<1:0> for a particular message buf-

fer is 00, that buffer has the lowest possible priority.

3.3 Initiating Transmission

In order to initiate message transmission, the

TXBnCTRL.TXREQ bit must be set for each buffer to

be transmitted. This can be accomplished by:

• Writing to the register via the SPI write command

• Sending the SPI RTS command

• Setting the TX

nRTS pin low for the particular

transmit buffer(s) that are to be transmitted

If transmission is initiated via the SPI interface, the

TXREQ bit can be set at the same time as the TXP

priority bits.

When TXBnCTRL.TXREQ is set, the TXBnCTRL.ABTF,

TXBnCTRL.MLOA and TXBnCTRL.TXERR bits will be

cleared automatically.

Once the transmission has completed successfully, the

TXBnCTRL.TXREQ bit will be cleared, the

CANINTF.TXnIF bit will be set and an interrupt will be

generated if the CANINTE.TXnIE bit is set.

If the message transmission fails, the

TXBnCTRL.TXREQ will remain set. This indicates that

the message is still pending for transmission and one

of the following condition flags will be set:

• If the message started to transmit but encoun-

tered an error condition, the TXBnCTRL.TXERR

and the CANINTF.MERRF bits will be set and an

interrupt will be generated on the INT

pin if the

CANINTE.MERRE bit is set

• If the message is lost, arbitration at the

TXBnCTRL.MLOA bit will be set

3.4 One-Shot Mode

One-Shot mode ensures that a message will only

attempt to transmit one time. Normally, if a CAN

message loses arbitration, or is destroyed by an error

frame, the message is retransmitted. With One-Shot

mode enabled, a message will only attempt to transmit

one time, regardless of arbitration loss or error frame.

One-Shot mode is required to maintain time slots in

deterministic systems, such as TTCAN.

Note: The TXBnCTRL.TXREQ bit must be clear

(indicating the transmit buffer is not

pending transmission) before writing to

the transmit buffer.

Note: Setting the TXBnCTRL.TXREQ bit does

not initiate a message transmission. It

merely flags a message buffer as being

ready for transmission. Transmission will

start when the device detects that the bus

is available.

Note: If One-Shot mode is enabled

(CANCTRL.OSM), the above conditions

will still exist. However, the TXREQ bit will

be cleared and the message will not

attempt transmission a second time.

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