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AT89C51ED2-RDTUM Datasheet(PDF) 86 Page - ATMEL Corporation |
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AT89C51ED2-RDTUM Datasheet(HTML) 86 Page - ATMEL Corporation |
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86 / 137 page  86 4235K–8051–05/08 AT89C51RD2/ED2 20. ONCE® Mode (ON- Chip Emulation) The ONCE mode facilitates testing and debugging of systems using AT89C51RD2/ED2 without removing the circuit from the board. The ONCE mode is invoked by driving certain pins of the AT89C51RD2/ED2; the following sequence must be exercised: • Pull ALE low while the device is in reset (RST high) and PSEN is high. • Hold ALE low as RST is deactivated. While the AT89C51RD2/ED2 is in ONCE mode, an emulator or test CPU can be used to drive the circuit. Table 20-1 shows the status of the port pins during ONCE mode. Normal operation is restored when normal reset is applied. Table 20-1. External Pin Status During ONCE Mode ALE PSEN Port 0 Port 1 Port 2 Port 3 Port I2 XTALA1/2 XTALB1/2 Weak pull- up Weak pull- up Float Weak pull- up Weak pull- up Weak pull- up Float Active Active |
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