82B21200 200A
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可编程逻辑器件PLD(Programmable logic Device)就是由用户进行编程实现所需逻辑功能的数字集成电路ASIC。可编程逻辑器件在现代电子工程设计中得到了广泛应用。它是在PAL,GAL等逻辑器件的基础上发展起来的,具有高密度,高速度,低功耗体系结构和逻辑单元,灵活以及运用范围宽等特点,同时还具有设计周期短,制造成本低,开发工具*,标准产品无需测试,质量稳定及可实时布线检验等优点。
现场总线技术广泛应用于工业测控局域网中,它可以实现较远距离、较快速度的信息传输,而且只需要双绞线作为传输媒介,简单可靠。但是,在该技术应用中总线控制器与微机之间的接口电路一般采用单片机来实现。采用这种方式将导致较长的研发周期和庞大的电路,且难于实现高速数据传输。本文利用CPLD成功地实现了嵌入式系统与CAN总线网络的通信,速度快,易于实现,而且非常稳定。
本系统采用的器件是Altera公司的CPLD,82B21200 200A属于MAX7000系列。Altera公司的CPLD器件主要包括MAX系列、ACEX系列、FLEX系列和APEX系列。其中MAX系列器件为高速实
际应用提供了非常高的性价比,这基于它*的MAX架构。MAX7000系列提供了速度zui快的可编程逻辑器件解决方案,它基于CMOS EEPROM工艺,传播延迟zui小为3.5ns。
2.CPLD实现接口电路
实时控制系统中,测控局域网利用CAN控制器实现微机间的数据传输。其基本的设计思想是:由于PC总线协议与CAN控制器不*,因而在它们中间设计了一个硬件电路作为接口来实现转换。PC地址与数据总线是分离的,而CAN控制器数据与地址线通过分时复用来实现地址与数据分离,刚好与单片机*。单片机技术虽然较成熟,但是,由于涉及单片机编程和开发的周期较长,且采用单片机系统需要较庞大的外围电路,不利于研制系统应用于嵌入式系统中。因此,用可编程逻辑器件设计接口电路,可以有效解决这些问题。
本系统采用“主-从”式点对点通信。系统硬件由两台PC104主机和两块CAN总线通信模块组成,如图1所示。主机采用CPU主频为300MHz的奔腾处理器;CAN总线控制器位SJA1000T;一带屏蔽的双绞线作为系统总线;通信波特率可用软件设置为1Mbps;为了提高通信的可靠性,用CAN控制器的报文筛选器对报文进行筛选,来对由于干扰而产生的错误报文进行滤波。
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