HIGH IMPEDANCE DETECT
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2002-Feb-08
Many times in digital systems, there is a need for a normal port logic. This is unchanged from previous three state input. This is a digital input that can detect 1) microcontrollers. B shows the software programmable beiig driven high, 2) being driven low, 3) not b&g driven pull-up resistor and C shows the software programmable (high impedance state). This is sometimes done using pull-down resistor. The high impedance state can be an A/D converter, but this can be an expensive solu- detected by using the following procedure: tion. Here we show a simple method of detecting the 1. enable pull-up, disable pull-down, read port input three states that is relatively inexpensive in silicon area. 2. disable pull-up, enable pull-down, read port input This is an improvement to the normal port logic used 3. use the table in Figure 2 to determine the state of the on a microcontroller. Referring to Figure 1, A shows the input B j- %tY uP A I 1' PAD REGULAR PORT LOGIC Data DDR PDWN (pull - down enable) Figure 1 72 0 Motorola. inc. ,993 0 M MO-LA INC. Technical Developments Volume 19 June 1993 First Read Second Read of data bit of data bit Data (PUP=l, PDWN=O) (PUP=O, PDWN=l) 0 0 LOW 0 1 ERROR 1 0 HIGH IMPEDANCE 1 1 HIGH Figure 2 If the input stays low with the pull-up resistor ena- bled, the input is driven low. If the input stays high with the pull-down resistor enabled, the input is driven high. If the input follows both the pull-up and the pull-down resistors, the input is high impedance. If the input fol- lows neither the pull-up nor pull-down, the state at the input must have changed between the two reads of the pin. This last case has been defined as an error condition.