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HD6433636SW Folha de dados(PDF) 67 Page - Renesas Technology Corp |
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HD6433636SW Folha de dados(HTML) 67 Page - Renesas Technology Corp |
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67 / 418 page ![]() 53 Bit Manipulation in a Register Containing a Write-only Bit Example 3: In this example, the port 6 control register PCR6 is accessed by a BCLR instruction. As in the examples above, P6 7 and P66 are input pins, with a low-level signal input at P67 and a high-level signal at P6 6. The remaining pins, P65 to P60, are output pins that output low-level signals. In this example, the BCLR instruction is used to change pin P6 0 to an input port. It is assumed that a high-level signal will be input to this input pin. [A: Prior to executing BCLR] P6 7 P6 6 P6 5 P6 4 P6 3 P6 2 P6 1 P6 0 Input/output Input Input Output Output Output Output Output Output Pin state Low level High level Low level Low level Low level Low level Low level Low level PCR6 00111111 PDR6 10000000 [B: BCLR instruction executed] BCLR #0, @PCR6 The BCLR instruction is executed designating PCR6. [C: After executing BCLR] P6 7 P6 6 P6 5 P6 4 P6 3 P6 2 P6 1 P6 0 Input/output Output Output Output Output Output Output Output Input Pin state Low level High level Low level Low level Low level Low level Low level High level PCR6 1 1 11111 0 PDR6 10000000 [D: Explanation of how BCLR operates] When the BCLR instruction is executed, first the CPU reads PCR6. Since PCR6 is a write-only register, the CPU reads a value of H'FF, even though the PCR6 value is actually H'3F. Next, the CPU clears bit 0 in the read data to 0, changing the data to H'FE. Finally, this value (H'FE) is written to PCR6 and BCLR instruction execution ends. As a result of this operation, bit 0 in PCR6 becomes 0, making P6 0 an input port. However, bits 7 and 6 in PCR6 change to 1, so that P6 7 and P66 change from input pins to output pins. |
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