- A FLASH Bootloader for PIC16 and PIC18 Devices--硬譯（一）

兩種器件系列的代碼略有不同，但是功能本質(zhì)是一樣的。該目標(biāo)實(shí)現(xiàn)的壓力在于爭(zhēng)取最大的性能和功能, 同時(shí)要求最少的代碼空間。

FIRMWARE Basic Operation（固件基礎(chǔ)操作）

圖1  BootLoader 功能塊圖

Figure 1 summarizes （總結(jié)）the essential firmware design of the bootloader. 

圖1總結(jié)了bootloader的基本固件設(shè)計(jì)。

Data is received through the USART module, configured in Asynchronous mode for compatibility（兼容性） 

with  RS-232 and passed through the transmit/receive engine. 

數(shù)據(jù)通過(guò)USART（串口）模塊接收，配置為異步模式，通過(guò)發(fā)送/接收引擎下載，比如RS232。

The engine filters and parses the data, storing the information into a data buffer in RAM. 

引擎過(guò)濾并解析數(shù)據(jù), 然后存儲(chǔ)信息到在RAM的數(shù)據(jù)緩沖區(qū)中。

The command interpreter evaluates the command information within the buffer to determine what
should be done (i.e., Is the data written into a memory unit? Is data read from a memory unit? 

Does the firmware version need to be read?). 

命令解釋器評(píng)估在緩沖區(qū)的命令信息來(lái)確定應(yīng)該做什么。（比如：該數(shù)據(jù)是在存儲(chǔ)單元以寫(xiě)進(jìn)去的，

還是數(shù)據(jù)從該存儲(chǔ)單元讀?。抗碳姹臼欠裥枰x?。浚?/span>

Once the operation is performed, data is passed back to the transmit/receive engine to be transmitted 

back to the source, closing the software flow control loop.

一旦操作方式被確定，被傳遞到發(fā)送/接收引擎的數(shù)據(jù)會(huì)被返送到宿主機(jī)，并關(guān)閉軟件流控制回路。

COMMUNICATIONS （通信）
The microcontroller’s USART module is used to receive and transmit data; it is configured as a UART to
be compatible with RS-232 communications. 
微控制器的USART模塊被用于接收和傳送數(shù)據(jù)，它被確定用于UART去兼容與RS-232的通信。
The device can be set up in an application to bootload from a computer through its standard serial interface.
該器件能夠通過(guò)它的標(biāo)準(zhǔn)串口從電腦引導(dǎo)加載建立起一個(gè)應(yīng)用程序。
The following communications settings are used:
? 8 data bits
? No parity
? 1 STOP bit
下列通信設(shè)置將會(huì)被用到：
? 8個(gè)數(shù)據(jù)位
? 無(wú)校驗(yàn)位
? 1個(gè)停止位

The baud rate setting is variable depending on the application. Baud rate selection is discussed later.

波特率設(shè)置根據(jù)應(yīng)用程序是可變的。波特率的選擇稍后討論。

THE RECEIVE/TRANSMIT BUFFER（ 接收和傳送緩沖區(qū)）
All data is moved through a buffer (referred to as the Receive/Transmit Buffer). 　　
所有的數(shù)據(jù)都是通過(guò)一個(gè)緩沖區(qū)移動(dòng)(稱(chēng)為接收/發(fā)送緩沖區(qū))。
The buffer is a maximum of 255 bytes deep. 
緩沖區(qū)大小的最大值為255字節(jié)。
This is the maximum packet length supported by the protocol.
這個(gè)最大包長(zhǎng)度有被協(xié)議支持。
 However, some devices may not support the largest packet size due to memory limitations. 
但是，一些器件由于內(nèi)存大小并不支持該最大包長(zhǎng)度。

Note: The actual packet length supported by a particular device depends on the size of its data memory.
注:  實(shí)際的包長(zhǎng)度所支持的 特定器件取決于它的數(shù)據(jù)存儲(chǔ)區(qū)的大小。

Figure 2 shows an example of the mapping of the buffer within the PIC18F452.
圖2 顯示了一個(gè)示例關(guān)于PIC18F452內(nèi)的緩沖區(qū)映射。

圖2 數(shù)據(jù)內(nèi)存在PIC18F452的利用

A useful feature of the receive/transmit buffer is that it retains its memory between packets, thus 

allowing very fast repeat and replication operations. That is, if an empty packet is sent, the data