Abstract:
A CMOS low power Schmitt type input buffer for a dynamic random access memory (DRAM) circuit. This buffer is further characterized in that a falling edge on the input has better than average noise immunity and has a slightly longer propagation time through the buffer than a rising edge.

Description:
FIELD OF THE INVENTION 
     This invention relates to a CMOS electronic circuit such as a dynamic random access memory (DRAM), specifically to low power internal buffering of the DRAM inputs. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits such as DRAMs are often placed in electronic systems that are less than ideal from an electrical noise standpoint. Noise on the DRAM inputs can cause internal misfiring and malfunction of the circuit. General practice is to assume that systems will always be noisy, and that the DRAM input should be modified to operate properly in spite of the noise. 
     Negative transitions on some DRAM input signals (such as an address strobe, for example) are critical, since a mistriggered strobe can begin an accidental access cycle within the DRAM, possibly altering stored data. 
     A well known method to reduce DRAM noise sensitivity is to run an input through a Schmitt type input buffer internal to the chip. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the invention is a CMOS Schmitt type input buffer that has low power requirements. This buffer is characterized in that a falling edge on the input has better than average noise immunity and has a slightly longer propagation time through the buffer than a rising edge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of the inventive buffer. 
     FIG. 2 is a timing diagram showing the operation of the buffer. 
     FIG. 3 shows an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the invention includes input stage transistors Q1, Q2, Q3, and Q4, middle stage transistors Q5 and Q6, and output stage transistors Q7 and Q8, all connected as shown in FIG. 1. Q1, Q2, and Q7 are p-channel devices, and the rest are n-channel devices. VIN is an input terminal, and VOUT is an output terminal. Q1 and Q2 together have just enough gain to gate Q5 and Q7. Q5 has just enough gain to gate Q8. These low gains contribute to the low power requirement of the inventive buffer. Q7 and Q8 are sized sufficiently to drive a desired load on VOUT. 
     The operation of the preferred embodiment is now described with reference to FIG. 2, where waveforms of nodes VIN, A, B, and VOUT are shown (not to scale) over an operation cycle. 
     At time T1, VIN is driven low and the embodiment is at equilibrium. Nodes A and B are both high. At time T2, VIN begins a positive transition, simultaneously deactivating Q1 and Q2 and activating Q3 and Q4. Q4 and Q6 overcome weak pullup Q5, and nodes A and B together begin a negative transition. 
     At time T4, A and B activate Q7 and Q8, and VOUT undergoes a positive transition until equilibrium at time T5. 
     At time T6, a negative transition begins on VIN, activating Q1 and Q2 and deactivating Q3, Q4, and Q6. At time T7, Q1 and Q2 begin pulling up node A, activating Q5, which begins pulling up node B at time T8. Also, approximately at time T8, node A deactivates Q7, allowing VOUT to begin to fall as shown. At time T9, node B activates Q8, which begins to pull VOUT down. At time T10, VOUT is fully pulled down and equilibrium is reached as at time T1. 
     It should be noted that for a positive transition at VIN, output stage transistors Q7 and Q8 behave just like an ordinary CMOS buffer stage: Q7 turns on while Q8 turns off, Q7 and Q8 both being on for an instant during the transition. However, for a negative transition, Q7 is allowed to fully turn off before Q8 is activated. This reduces power consumption of the buffer and gives a cleaner negative transition on VOUT, but propagation time is increased due to the T7-T8 delay on node B. 
     An alternative embodiment is shown in FIG. 3, where transistor F1 is equivalent to Q1 and Q2, and F2 and F4 are equivalent to Q3, Q4, and Q6. In the preferred embodiment, one of the advantages of multiple transistors such as Q1 and Q2 is that adjustments can be made in the integrated circuit by metal optioning one of them in or out to adjust gain. 
     The same reasoning applies to Q4, with the added benefit that a change in Q4 affects Q3 and Q6 identically. 
     Although these embodiments are intended for use in a DRAM, this inventive buffer is ideal for any low power application where a noise immune negative transition is desired and where slightly slower propagation of such a transition is not a hindrance.