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AT89C51ED2-RDTIM Datasheet(PDF) 88 Page - ATMEL Corporation |
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AT89C51ED2-RDTIM Datasheet(HTML) 88 Page - ATMEL Corporation |
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88 / 137 page  88 AT89C51RD2/ED2 4235E–8051–04/04 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 cer- tain 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 63 shows the status of the port pins during ONCE mode. Normal operation is restored when normal reset is applied. Table 63. 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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