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On-Chip Peripheral Components
C540U / C541U
6.1.3.2
Port Loading and Interfacing
The output buffers of ports 1, 2 and 3 can drive TTL inputs directly. The maximum port load which
still guarantees correct logic output levels can be looked up in the C540U/C541U DC characteristics
in chapter 10. The corresponding parameters are VOL and VOH.
The same applies to port 0 output buffers. They do, however, require external pullups to drive
floating inputs, except when being used as the address/data bus.
When used as inputs it must be noted that the ports 1, 2 and 3 are not floating but have internal
pullup transistors. The driving devices must be capable of sinking a sufficient current if a logic low
level shall be applied to the port pin (the parameters I TL and I IL in the C540U/C541U DC
characteristics specify these currents). Port 0 has floating inputs when used for digital input.
Semiconductor Group
6-11
1997-10-01