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T89C51RD2-RDTIL Datasheet(PDF) 48 Page - ATMEL Corporation |
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T89C51RD2-RDTIL Datasheet(HTML) 48 Page - ATMEL Corporation |
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48 / 104 page  48 T89C51RD2 4243G–8051–05/03 ONCE(TM) Mode (ON Chip Emulation) The ONCE mode facilitates testing and debugging of systems using T89C51RD2 with- out removing the circuit from the board. The ONCE mode is invoked by driving certain pins of the T89C51RD2; 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 T89C51RD2 is in ONCE mode, an emulator or test CPU can be used to drive the circuit. Table 30. shows the status of the port pins during ONCE mode. Normal operation is restored when normal reset is applied. Table 30. External Pin Status during ONCE Mode (a) "Once" is a registered trademark of Intel Corporation. ALE PSEN Port 0 Port 1 Port 2 Port 3 XTAL1/2 Weak pull-up Weak pull-up Float Weak pull-up Weak pull-up Weak pull-up Active |
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