C541U_99 Datasheet, PDF (58/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
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 C541U DC characteristics in
chapter 10. The corresponding parameters are V_OL and V_OH.
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 C541U DC characteristics
specify these currents). Port 0 has floating inputs when used for digital input.
Semiconductor Group
6-13
1999-04-01