■ 问题提出

在 带有128KB缓存的AD7606模拟采集板 后面更新了不带板内缓冲区的C51程序。采集板与计算机之间存在两种实时数据通道：

• 通过USB-RS232转接串口，波特率为460800
• 通过WiFi-UART模块，波特率为460800，但是实际传输速率要小于460800

为了能够利用 STC WiFi下载程序 界面作为接受数据的缓冲界面，这样就需要能够利用ASCII码来高效传输二进制数据。

传统的方式通过C51的printf函数，将二进制转换成ASCII字符串通过串口发送出去，虽然这样有很强的可读性，但是存在着很大的转换效率低下：

• 通常一个两字节（16bit）的二进制，最大需要6个字节（带有符号位）。比如有符号整数 0xd8ef : -10000。
• 为了分割连续两个数字，其间还需增加一个空格（0x20）

因此，最大需要是踹死7个字节来传输一个16bit的数据。传输效率浪费了：$\eta _{ascii} = {7 \over 2}$

■ Base64编码方式

在网页 What is Base64 Encoding and How does it work? | Base64Encoder 给出了Base64编码的具体方式。它将连续三个字节（24bit）转换成四个6bit组，按照下面的表格找到对应的ASCII码。

这样的转换效率为$\eta _{base64} = {4 \over 3}$
通过Base64的转换比普通的ASCI字符效率提高了：
$\eta _\Delta = {{\eta _{ascii} } \over {\eta _{base64} }} = {{7/2} \over {4/3}} = {{21} \over 8} = 2.625$

在实际实现中，将原来的 “62”编码为$+$，修改为$*$，这样在下载程序界面可以避免换行的。整个显示是铺满Mem0界面。

■ C51编码程序和Python解码程序

1. C51的编码¹

/*
**==============================================================================
** BASE64ASCII.C:             -- by Dr. ZhuoQing, 2020-06-04
**
**==============================================================================
*/
//------------------------------------------------------------------------------
#define BASE64ASCII_GLOBALS        1              // Define the global variables
#include "BASE64ASCII.H"
#include "c51basic.H"
//------------------------------------------------------------------------------
void Base64ASCIISendChar(unsigned char ucChar) {SendChar(ucChar);
}
//------------------------------------------------------------------------------
void Base64ASCIIInit(void) {g_ucBase64ASCIIBufferPoint = 0;
}
//------------------------------------------------------------------------------
void Base64ASCIIPushByte(unsigned char ucByte) {g_ucBase64ASCIIBuffer[g_ucBase64ASCIIBufferPoint ++] = ucByte;if(g_ucBase64ASCIIBufferPoint >= 3) {Base64ASCIIFlushBuffer();}
}
unsigned char Base64ASCII6BitChar(unsigned char ucBit) {if(ucBit < 26) return 'A' + ucBit;if(ucBit < 52) return (ucBit - 26) + 'a';if(ucBit < 62) return (ucBit - 52) + '0';if(ucBit == 62) return '*';if(ucBit == 63) return '/';return '=';
}
//------------------------------------------------------------------------------
void Base64ASCIIFlushBuffer(void) {unsigned char ucDim[4];unsigned char ucChar, ucNibble;if(g_ucBase64ASCIIBufferPoint == 0) return;ucChar = g_ucBase64ASCIIBuffer[0];ucNibble = ucChar << 4;ucDim[0] = Base64ASCII6BitChar(ucChar >> 2);ucChar = g_ucBase64ASCIIBuffer[1];ucDim[1] = Base64ASCII6BitChar((ucNibble & 0x30) | (ucChar >> 4));ucNibble = (ucChar & 0xf) << 2;ucChar = g_ucBase64ASCIIBuffer[2];ucDim[2] = Base64ASCII6BitChar(ucNibble | (ucChar >> 6));ucDim[3] = Base64ASCII6BitChar(ucChar & 0x3f);if(g_ucBase64ASCIIBufferPoint == 1) {ucDim[2] = '=';ucDim[3] = '=';} else if(g_ucBase64ASCIIBufferPoint == 2) {ucDim[3] = '=';}Base64ASCIISendChar(ucDim[0]);Base64ASCIISendChar(ucDim[1]);Base64ASCIISendChar(ucDim[2]);Base64ASCIISendChar(ucDim[3]);g_ucBase64ASCIIBufferPoint = 0;
}
//==============================================================================
//                END OF THE FILE : BASE64ASCII.C
//------------------------------------------------------------------------------

/*
**==============================================================================
** BASE64ASCII.H:            -- by Dr. ZhuoQing, 2020-06-04
**
**  Description:
**
**==============================================================================
*/
#ifndef __BASE64ASCII__
#define __BASE64ASCII__
//------------------------------------------------------------------------------
#ifdef BASE64ASCII_GLOBALS#define BASE64ASCII_EXT
#else#define BASE64ASCII_EXT extern
#endif // BASE64ASCII_GLOBALS
//------------------------------------------------------------------------------
//==============================================================================
void Base64ASCIISendChar(unsigned char ucChar);
//------------------------------------------------------------------------------
void Base64ASCIIInit(void);
BASE64ASCII_EXT unsigned char g_ucBase64ASCIIBuffer[3];
BASE64ASCII_EXT unsigned char g_ucBase64ASCIIBufferPoint;
void Base64ASCIIPushByte(unsigned char ucByte);
void Base64ASCIIFlushBuffer(void);
unsigned char Base64ASCII6BitChar(unsigned char ucbit);
//==============================================================================
//             END OF THE FILE : BASE64ASCII.H
//------------------------------------------------------------------------------
#endif // __BASE64ASCII__

2. Python解码程序

def Base64Ascii2Bit(ascii):ascii = int(ascii)if ascii >= ord('A') and ascii <= ord('Z'): return ascii - ord('A')if ascii >= ord('a') and ascii <= ord('z'): return ascii - ord('a') + 26if ascii >= ord('0') and ascii <= ord('9'): return ascii - ord('0') + 52if ascii == ord('*'): return 62if ascii == ord('/'): return 63return 0
def Base64Ascii2Byte(fourb):data = bytearray()bits0 = Base64Ascii2Bit(fourb[0])bits1 = Base64Ascii2Bit(fourb[1])bits2 = Base64Ascii2Bit(fourb[2])bits3 = Base64Ascii2Bit(fourb[3])data.append(bits0 * 4 + int(bits1 / 16))data.append((bits1 & 0xf) * 16 + int(bits2 / 4))data.append((bits2 & 0x3) * 64 + bits3)if fourb[2] == ord('=') and fourb[3] == ord('='):data = data[0:1]return dataif fourb[3] == ord('='):return data[0:2]return data
def Base64Ascii2Data(ascii):data = bytearray()length = len(ascii)for i in range(int(length / 4)):bytedata = Base64Ascii2Byte(ascii[i*4:i*4+4])if len(bytedata) > 0:data.extend(bytedata)valdim = [x*256+y for x,y in zip(data[0::2], data[1::2])]valdim = [(d & 0x7fff) - (d & 0x8000) for d in valdim]return valdim

■ 最大采样速率测试结果

经过测试，在 带有128KB缓存的AD7606模拟采集板 上，对八个通道进行周期为1毫秒的采集，实际上传速率为 16kByte/s。再乘以编码增益$\eta _{base64} = {4 \over 3}$，则实际上传速率为：21.3kByte/秒.

#------------------------------------------------------------
stm32cmd('ad5v')
stm32cmd('CLEAR')
stm32cmd('adascii 1000 8 1')
val = stm32val()[10]
while True:time.sleep(.2)valnew = stm32val()[10]if valnew == val: breakval = valnew
tspbeep(1200, 200)
stm32cmd("COPY")
time.sleep(.5)
pastestr = bytes(clipboard.paste(), 'utf-8')
data = Base64Ascii2Data(pastestr)
plt.plot(data[0::8], label='Channel1')
plt.plot(data[1::8], label='Channel2')
plt.xlabel("sample")
plt.ylabel("Voltage(V)")
plt.legend(loc='upper right')
plt.grid(True)
plt.show()

■ 结论

通过Base64编码，将原来的3个字节的数据编码成4个字节的ASCII字符，可以在适合ASCII存储显示的界面完成二进制数据的传递。这大大提高数据的传输效率。

//==============================================================================
//                    PROCESS THE DEBUG BUFFER
//------------------------------------------------------------------------------
void SerialDebugProcessBuffer(void) {unsigned int nNumber, nChannel, nWaitTime, i, j;int nData[8];unsigned int nNum;SerialDebugBuffer2Argument();if(g_ucSDANumber == 0) return;if(strcmp("hello",                (char *)STD_ARG[0]) == 0)printf("%s is ready !\r\n", VERSION_STRING);else IFARG0("ad") {sscanf(SDA(1), "%d", &nNumber);sscanf(SDA(2), "%d", &nChannel);sscanf(SDA(3), "%d", &nWaitTime);g_ucWaitMS = 0;for(i = 0; i < nNumber; i ++) {while(g_ucWaitMS != nWaitTime);g_ucWaitMS = 0;AD7606Convert();AD7606ReadData(nData, 4);for(j = 0; j < nChannel; j ++) {nNum = nData[j];SendChar((unsigned char)(nNum >> 8));SendChar((unsigned char)nNumber);}}}else IFARG0("adtext") {sscanf(SDA(1), "%d", &nNumber);sscanf(SDA(2), "%d", &nChannel);sscanf(SDA(3), "%d", &nWaitTime);g_ucWaitMS = 0;for(i = 0; i < nNumber; i ++) {while(g_ucWaitMS != nWaitTime);g_ucWaitMS = 0;AD7606Convert();AD7606ReadData(nData, 4);for(j = 0; j < nChannel; j ++) {nNum = nData[j];printf("%d ", nNum);}}} else IFARG0("adascii") {sscanf(SDA(1), "%d", &nNumber);sscanf(SDA(2), "%d", &nChannel);sscanf(SDA(3), "%d", &nWaitTime);g_ucWaitMS = 0;Base64ASCIIInit();for(i = 0; i < nNumber; i ++) {while(g_ucWaitMS != nWaitTime);g_ucWaitMS = 0;AD7606Convert();AD7606ReadData(nData, 4);for(j = 0; j < nChannel; j ++) {nNum = nData[j];Base64ASCIIPushByte((unsigned char)(nNum >> 8));Base64ASCIIPushByte((unsigned char)nNum);                }}Base64ASCIIFlushBuffer();} else IFARG0("ad5v") {OFF(AD7606_RANGE);        } else IFARG0("ad10v") {ON(AD7606_RANGE);}else printf("Error command : %s !\r\n", STD_ARG[0]);
}