??? 05/25/05 12:16 Read: times |
#93884 - PSEN Responding to: ???'s previous message |
hi,
Jan Waclawek said:
PSEN is less of a problem as it is of the open-drain type and it won't pull down unless external cycle occurs.
No, PSEN is not open-drain. At least for Atmel devices this pin is specified as: ============================================================= At Reset: While the RST pin is high, ALE and PSEN are weakly pulled high. After RST is pulled low, it will take 1 to 2 machine cycles for ALE and PSEN to start clocking. Driving the ALE and PSEN pins to 0 while reset is active could cause the device to go into an indeterminate state. At ONCE mode While the device is in ONCE mode, the Port 0 pins go into a float state, and the other port pins and ALE and PSEN are weakly pulled high. At normal mode As PSEN is a an output port in normal operating mode after reset, user application should take care to release PSEN after falling edge of reset signal. ============================================================= As /PSEN is specified to drive /OE of external program memory so this pin has true push-pull output. For example, looking at DC Parameters of AT89C51RD2, we may see that for /PSEN pin the output current is specified upto ±7mA. So this pin is not open-drain definely. Regards, Oleg |