1. AC Coupled Point to Point Data Transmission on SPI chip
Mitch and Nandor
July 20, 2009

2. Driver and Rcvr Hookup (Fabbed and Working in TSMC’s .25um Process)
….Lots of strays….
….Different Voltages….

3. Rcvr with Hysteresis Input Impedance ~ 4k  ~vdd/2
INM
Out H H
H  Hysteresis switch

4. SPI Driver Program Current Drive Aligns Sig and Sig_bar To minimize
common mode distortion
Current Driver Switches
OutP
OutN IN

5. Transient Response Case I Correct Initial Hysteresis
Digital Input
Receiver Output
(matches with delay)
AC inputs to Rcvr.
Input settles to Hysteresis value

6. Transient Response Case II Wrong Initial Hysteresis
Driver Input
Pulse Exceeds Rcvr Input RC time const.
Rcvr Output
Correct Data
NO Data
Initial Hysteresis in direction of first edge
AC coupled Rcvr Inputs

7. Notes
1)To avoid violating our NDA we will develop and distribute a receiver and driver model using LTSPICE compatible parts.
2) Notice that when the hysteresis is phased correctly with the first data edge then transmission is flawless with any combination of data.
3) Data transmission fails if the first edge of data is in the same direction as the hysteresis, But if a logic pulse of several Rcvr RC time constants is sent ( say during a beam gap) the hsyteresis phase is corrected. Note that no problem is induced by the long pulse if hysteresis is not flipped.