Abstract:
A low power bus hold circuit includes: a first inverter having an input coupled to a bus hold input node; and a second inverter having a first input coupled to a first output of the first inverter and a second input coupled to a second output of the first inverter, wherein the first and second outputs of the first inverter are separated by a resistor, and having an output coupled to the bus hold input node.

Description:
FIELD OF THE INVENTION 
   The present invention relates to electronic circuitry and, in particular, to low power bus-hold circuits. 
   BACKGROUND OF THE INVENTION 
   Bus hold is the ability of either an input pin or an I/O pin to retain the last valid logic state (voltage level) after the source driving it either enters the high impedance state, or is removed. There are four minimum current specifications that define Bus Hold. Two are sustaining currents that the node must source or sink without changing state, and two are overdrive currents that will cause the node to change state. IBHL is defined as “Bus Hold LOW sustaining current.” The bus hold circuit must sink at least this minimum LOW sustaining current when the input voltage is at the maximum level for a LOW logic state. IBHH is defined as “Bus Hold HIGH sustaining current.” The Bus Hold circuit must source at least this minimum HIGH sustaining current when the input voltage is at the minimum level for a HIGH logic state. IBHLO is defined as “Bus Hold LOW overdrive current.” An external driver must source at least IBHLO to switch the node from LOW to HIGH. IBHHO is defined as “Bus Hold HIGH overdrive current.” An external driver must sink at least IBHHO to switch the node from HIGH to LOW. 
   A prior art Bus-Hold circuit is shown in  FIG. 1 . The circuit of  FIG. 1  includes transistors MP 1 , MN 1 , MPH 1 , MNH 1 , diode D 1 , resistor R 3 A, and input io. One of the problems with the prior art bus-hold solution in  FIG. 1 , is that it turns “on” and “off” the current source of the bus-hold circuit at the same time, causing switching current in the current source. It also does not have a current limiting resistor in the switching path of the current source. This causes a higher power dissipation capacitance while increasing the bus-hold current range. 
   SUMMARY OF THE INVENTION 
   A low power bus hold circuit includes: a first inverter having an input coupled to a bus hold input node; and a second inverter having a first input coupled to a first output of the first inverter and a second input coupled to a second output of the first inverter, wherein the first and second outputs of the first inverter are separated by a resistor, and having an output coupled to the bus hold input node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a circuit diagram of a prior art bus-hold circuit; 
       FIG. 2  is a circuit diagram of a preferred embodiment low power bus-hold circuit; 
       FIG. 3  is a plot showing an example comparison of the prior art of  FIG. 1  with the preferred embodiment of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   A low power bus-hold circuit, according to the present invention, provides a smaller IBHHO/IBHLO current while maintaining the IBHH/IBHL minimum bus-hold current limit. This way an external driver can sink less current (IBHHO) to switch the input node from high to low and can source less current (IBHLO) to switch the input node from low to high, while preserving minimum high (IBHH) and minimum low (IBHL) sustaining current of the bus-hold circuit. This solution can also reduce the power dissipation capacitance of a bus-hold device by reducing the switching current in the bus-hold circuit. 
   The low power bus-hold circuit, according to the present invention, is shown in  FIG. 2 . The circuit of  FIG. 2  includes inverter INV 1  which includes PMOS transistor MP 1 , NMOS transistor MN 1 , and resistor R 1 ; inverter INV 2  which includes transistor MPH 1 , transistor MNH 1 , and diode D 1 ; resistors RP and RN; and input signal io. Signal io is attached to the input/IO of the bus-hold device. When signal io is in a LOW state, N-channel device MN 1  is in the OFF state and P-channel device MP 1  is in the ON state and turning N-channel device MNH 1  ON and P-channel device MPH 1  OFF. This causes the bus-hold to latch to a LOW state. When signal io is in a HIGH state, N-channel device MN 1  is in the ON state and P-channel device MP 1  is in the OFF state and turning N-channel device MNH 1  OFF and P-channel device MPH 1  ON. This causes the bus-hold to latch to a HIGH state. Device D 1  is a current blocking diode. 
   During a low to high transition N-channel device MN 1  goes to an ON state and quickly turns OFF the pull-down holding transistor MNH 1 . Resistor R 1  causes N-channel device MN 1  to turn ON the pull-up holding transistor MPH 1  at a slightly later time. The delayed turn on of transistor MPH 1  helps limit the IBHHO current while helping maintain the IBHH current. Since pull-down holding transistor MNH 1  was turned to the OFF state before pull-up holding transistor MPHI was turned ON, there was very little through current through transistors MPH 1 /MNH 1  to ground. During this transition, resistor R 1  not only slightly delays the turn on of pull-up holding transistor MPH 1 , but also becomes a current limiting resistor for the switching path formed by transistors MP 1  and MN 1 . 
   During a high to low transition P-channel device MP 1  goes to an ON state and quickly turns OFF the pull-up holding transistor MPH 1 . Resistor R 1  causes P-channel device MP 1  to turn ON the pull-down holding transistor MNH 1  at a slightly later time. The delayed turn ON of transistor MNH 1  helps limit the IBHLO current while helping maintain the IBHL current. Since pull-up holding transistor MPH 1  was turned to the OFF state before pull-down holding transistor MNH 1  was turned ON, there was very little through current through transistors MPH 1 /MNH 1  to ground. Again, during this transition, resistor R 1  not only slightly delays the turn on of pull-down holding transistor MNH 1  but also becomes a current limiting resistor for the switching path formed by transistors MP 1  and MN 1 . Limiting the current in the current switching paths helps reduce the power dissipation capacitance. 
   The low power bus-hold, according to the present invention, can have a lower IBHHO/IBHLO current while maintaining close IBHH/IBHL currents, as compared to the standard bus-hold. 
   When the bus-hold input is switching, this solution turns OFF the current source (sustaining current) in the bus-hold latch, then at a slightly later time turns ON the sustaining current by using a delay resistor Rl. This solution also has the current limiting resistors RP and RN in the switching path of the current source of the bus-hold. These advantages help reduce the power dissipation capacitance in a bus-hold device while optimizing the current range of the bus-hold circuit. 
     FIG. 3  is a plot showing an example comparison of the prior art of  FIG. 1  with the present invention of  FIG. 2 . The solid lines represent the present invention of  FIG. 2 . The dashed lines represent the prior art of  FIG. 1 . Input voltage VIL max is the maximum voltage level for a LOW logic state. Input voltage VIH min is the minimum voltage level for a HIGH logic state. 
   This solution reduces the power dissipation capacitance of a bus-hold device by reducing the switching current in the bus-hold circuit. It also provides a smaller IBHH/IBHLO current while maintaining the IBHH/IBHL minimum bus-hold current limit. This way an external driver can sink less current (IBHHO) to switch the input node from high to low and can source less current (IBHLO) to switch the input node from low to high, while preserving minimum high (IBHH) and minimum low (IBHL) sustaining current of the bus-hold circuit. This becomes a desirable solution when using a bus-hold in a device with very low power dissipation capacitance specifications. 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.