C517A_99 Datasheet, PDF (78/218 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C517A
6.1.4 Port Loading and Interfacing
The output buffers of ports 1 to 5 can drive TTL inputs directly. The maximum port load which still
guarantees correct logic output levels can be be looked up in the DC characteristics in the Data
Sheet of the C517A. 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 to 5 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 DC characteristics specify these
currents). Port 0 as well as port 1 programmed to analog input function, however, have floating
inputs when used for digital input.
Semiconductor Group
6-12