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1、程序目的：介绍AT91SAM7S256-EK TWI的基本用法： 2、 功能说明:通过TWI实现I2C

<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x10</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_IMR">US_IMR</a></font></td><td><font size="-1">Interrupt Mask Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x14</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_CSR">US_CSR</a></font></td><td><font size="-1">Channel Status Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x18</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_RHR">US_RHR</a></font></td><td><font size="-1">Receiver Holding Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x1C</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_THR">US_THR</a></font></td><td><font size="-1">Transmitter Holding Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x20</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_BRGR">US_BRGR</a></font></td><td><font size="-1">Baud Rate Generator Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x24</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_RTOR">US_RTOR</a></font></td><td><font size="-1">Receiver Time-out Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x28</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_TTGR">US_TTGR</a></font></td><td><font size="-1">Transmitter Time-guard Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x40</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_FIDI">US_FIDI</a></font></td><td><font size="-1">FI_DI_Ratio Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x44</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_NER">US_NER</a></font></td><td><font size="-1">Nb Errors Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x4C</b></font></td><td><font size="-1"><a href="AT91SAM7A3_USART.html#US_IF">US_IF</a></font></td><td><font size="-1">IRDA_FILTER Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x100</b></font></td><td><font size="-1">US_RPR (<a href="#PDC_RPR">PDC_RPR</a>)</font></td><td><font size="-1">Receive Pointer Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x104</b></font></td><td><font size="-1">US_RCR (<a href="#PDC_RCR">PDC_RCR</a>)</font></td><td><font size="-1">Receive Counter Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x108</b></font></td><td><font size="-1">US_TPR (<a href="#PDC_TPR">PDC_TPR</a>)</font></td><td><font size="-1">Transmit Pointer Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x10C</b></font></td><td><font size="-1">US_TCR (<a href="#PDC_TCR">PDC_TCR</a>)</font></td><td><font size="-1">Transmit Counter Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x110</b></font></td><td><font size="-1">US_RNPR (<a href="#PDC_RNPR">PDC_RNPR</a>)</font></td><td><font size="-1">Receive Next Pointer Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x114</b></font></td><td><font size="-1">US_RNCR (<a href="#PDC_RNCR">PDC_RNCR</a>)</font></td><td><font size="-1">Receive Next Counter Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x118</b></font></td><td><font size="-1">US_TNPR (<a href="#PDC_TNPR">PDC_TNPR</a>)</font></td><td><font size="-1">Transmit Next Pointer Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x11C</b></font></td><td><font size="-1">US_TNCR (<a href="#PDC_TNCR">PDC_TNCR</a>)</font></td><td><font size="-1">Transmit Next Counter Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x120</b></font></td><td><font size="-1">US_PTCR (<a href="#PDC_PTCR">PDC_PTCR</a>)</font></td><td><font size="-1">PDC Transfer Control Register</font></td></tr>
<tr><td align="CENTER" bgcolor="#FFFFCC"><font size="-1"><b>0x124</b></font></td><td><font size="-1">US_PTSR (<a href="#PDC_PTSR">PDC_PTSR</a>)</font></td><td><font size="-1">PDC Transfer Status Register</font></td></tr>
</null></table><br><table border=1 cellpadding=0 cellspacing=0 width="100%"><null><th bgcolor="#FFFFCC"><font size="-1"><b>Function</b></font></th><th bgcolor="#FFFFCC"><font size="-1"><b>Description</b></font></th><tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_DisableIt">AT91F_US_DisableIt</a></b></font></td><td><font size="-1">Disable USART IT</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_EnableIt">AT91F_US_EnableIt</a></b></font></td><td><font size="-1">Enable USART IT</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_Close">AT91F_US_Close</a></b></font></td><td><font size="-1">Close USART: disable IT disable receiver and transmitter, close PDC</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_Error">AT91F_US_Error</a></b></font></td><td><font size="-1">Return the error flag</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_ResetTx">AT91F_US_ResetTx</a></b></font></td><td><font size="-1">Reset Transmitter and re-enable it</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_SendFrame">AT91F_US_SendFrame</a></b></font></td><td><font size="-1">Return 2 if PDC has been initialized with Buffer and Next Buffer, 1 if PDC has been initializaed with Next Buffer, 0 if PDC is busy</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_ResetRx">AT91F_US_ResetRx</a></b></font></td><td><font size="-1">Reset Receiver and re-enable it</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_ReceiveFrame">AT91F_US_ReceiveFrame</a></b></font></td><td><font size="-1">Return 2 if PDC has been initialized with Buffer and Next Buffer, 1 if PDC has been initializaed with Next Buffer, 0 if PDC is busy</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_Baudrate">AT91F_US_Baudrate</a></b></font></td><td><font size="-1">Caluculate baud_value according to the main clock and the baud rate</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_DisableRx">AT91F_US_DisableRx</a></b></font></td><td><font size="-1">Disable Receiver</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_TxReady">AT91F_US_TxReady</a></b></font></td><td><font size="-1">Return 1 if a character can be written in US_THR</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_GetChar">AT91F_US_GetChar</a></b></font></td><td><font size="-1">Receive a character,does not check if a character is available</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_SetIrdaFilter">AT91F_US_SetIrdaFilter</a></b></font></td><td><font size="-1">Set the value of IrDa filter tregister</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_RxReady">AT91F_US_RxReady</a></b></font></td><td><font size="-1">Return 1 if a character can be read in US_RHR</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_SetBaudrate">AT91F_US_SetBaudrate</a></b></font></td><td><font size="-1">Set the baudrate according to the CPU clock</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_Configure">AT91F_US_Configure</a></b></font></td><td><font size="-1">Configure USART</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_SetTimeguard">AT91F_US_SetTimeguard</a></b></font></td><td><font size="-1">Set USART timeguard</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_DisableTx">AT91F_US_DisableTx</a></b></font></td><td><font size="-1">Disable Transmitter</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_PutChar">AT91F_US_PutChar</a></b></font></td><td><font size="-1">Send a character,does not check if ready to send</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_EnableTx">AT91F_US_EnableTx</a></b></font></td><td><font size="-1">Enable sending characters</font></td></tr>
<tr><td bgcolor="#FFFFCC"><font size="-1"><b><a href="lib_AT91SAM7A3_h.html#AT91F_US_EnableRx">AT91F_US_EnableRx</a></b></font></td><td><font size="-1">Enable receiving characters</font></td></tr>