Patent Publication Number: US-2023160959-A1

Title: Wafer scale testing using a 2 signal jtag interface

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of prior application Ser. No. 17/559,444, currently pending, scheduled to issue as U.S. Pat. No. 11,561,258 on Jan. 24, 2023; 
     Which was a divisional of prior application Ser. No. 17/115,136, filed Dec. 8, 2020, now U.S. Pat. No. 11,243,253, granted Feb. 8, 2022; 
     Which was a divisional of prior application Ser. No. 16/717,236, filed Dec. 17, 2019, now U.S. Pat. No. 10,901,033, granted Jan. 25, 2021; 
     Which was a divisional of prior application Ser. No. 16/228,137, filed Dec. 20, 2018, now U.S. Pat. No. 10,551,438, granted Feb. 4, 2020; 
     Which was a divisional of prior application Ser. No. 16/026,721, filed Jul. 3, 2018, now U.S. Pat. No. 10,209,305, granted Feb. 19, 2019; 
     Which was a divisional of prior application Ser. No. 15/863,147, filed Jan. 5, 2018, now U.S. Pat. No. 10,060,980, granted Aug. 28, 2018; 
     Which was a divisional of prior application Ser. No. 15/419,379, filed Jan. 30, 2017, now U.S. Pat. No. 9,897,654, granted Feb. 20, 2018; 
     Which was a divisional of prior application Ser. No. 15/282,187, filed Sep. 30, 2016, now U.S. Pat. No. 9,594,116, granted Mar. 14, 2017; 
     Which was a divisional of prior application Ser. No. 14/934,451, filed Nov. 6, 2015, now U.S. Pat. No. 9,482,717, granted Nov. 1, 2016; 
     Which was a divisional of prior application Ser. No. 14/446,873, filed Jul. 30, 2014, now U.S. Pat. No. 9,207,280, granted Dec. 8, 2015; 
     Which was a divisional of prior application Ser. No. 13/175,484, filed Jul. 1, 2011, now U.S. Pat. No. 8,826,091, granted Sep. 2, 2014; 
     Which is a divisional of prior application Ser. No. 13/023,237, filed Feb. 8, 2011, now U.S. Pat. No. 8,015,463, granted Sep. 6, 2011; 
     Which is a divisional of prior application Ser. No. 12/849,191, filed Aug. 3, 2010, now U.S. Pat. No. 7,904,774, granted Mar. 8, 2011; 
     Which is a divisional of prior application Ser. No. 12/493,926, filed Jun. 29, 2009, now U.S. Pat. No. 7,797,602, granted Sep. 14, 2010; 
     Which is a divisional of prior application Ser. No. 12/175,655, filed Jul. 18, 2008, now U.S. Pat. No. 7,571,365, granted Aug. 4, 2009; 
     Which is a divisional of prior application Ser. No. 11/565,979, filed Dec. 1, 2006, now U.S. Pat. No. 7,417,450, granted Aug. 26, 2008; 
     Which claims priority from Provisional Application No. 60/741,927, filed Dec. 2, 2005, all of which are incorporated herein by reference. 
     The following patent application publication includes subject matter which is related to the subject matter of this application. The patent application publication has been assigned to the assignee of this application, and is incorporated herein by reference. US Patent Application Publication, Pub. No. US 2006/0,236,174 A1. 
     The disclosure of this application also relates to U.S. Pat. No. 6,073,254, “Selectively accessing test access ports in a multiple test access port environment.” 
     The disclosure of this application also relates to U.S. Pat. No. 6,242,269, “Parallel scan distributors and collectors and process of testing integrated circuits.” 
     The disclosure of this application also relates to U.S. Pat. No. 6,543,020, “Test pattern compression for an integrated circuit test environment”. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates generally to testing die on wafer and, more particularly, to testing of die on wafer using a reduced pin count JTAG interface. The reduced JTAG interface enables a larger number of die on wafer to be contacted by low cost testers and tested, which results in lowering the cost of manufacturing integrated circuits. 
     Description of Related Art 
     Today more and more wafer testing is being performed by low cost wafer testers. Low cost wafer testers are achieved primarily in two ways. (1) Decreasing the number of test contacts, and therefore expensive tester interfacing circuitry and probing mechanisms, between the tester and die on wafer, and (2) including more design for test circuitry (i.e. built in self test circuitry, scan design circuitry, test data compression/decompression circuitry) in the die to reduce the amount of test circuitry required in the tester. Furthermore wafer test cost can be reduced by using standardized design for test interfaces and circuitry in the die, such as the IEEE 1149.1 (JTAG) interface. The IEEE 1149.1 serial interface allows die on wafer to be tested via a standard 5 wire interface consisting of a test data input (TDI), a test data output (TDO), a test clock (TCK), a test mode select (TMS), and a test reset (TRST) signal. Using this interface, the tester only needs to contact each die using these 5 test signals to input and output test data and instructions. While the IEEE 1149.1 interface can be used for wafer testing, it still requires 5 contacts per die on wafer. For example, if 500 die on wafer are to be tested in parallel, the tester would have to have 2500 contacts to the wafer (500 die×5 signals each). Additionally, the IEEE 1149.1 serial interface bandwidth is relatively low since the TCK signal that times the interface typically operates at a frequency of only 20 to 50 MHz. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure provides a high speed, two signal test interface between a tester and a selected group or all of the die on wafer. The test interface operates to communicate IEEE 1149.1 signals between the tester and die at high speeds through the use of simultaneously bidirectional transceiver (SBT) technology. 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DISCLOSURE 
         FIG.  1 A  illustrates a first example of a tester contacting and communicating test data to die on wafer using the test interface of the present disclosure. 
         FIG.  1 B  illustrates a second example of a tester contacting and communicating test data to die on wafer using the test interface of the present disclosure. 
         FIG.  2 A  illustrates an example of the tester&#39;s test interface circuitry of  FIG.  1 A  whereby one controller is used to control all the tester interface channels. 
         FIG.  2 B  illustrates an example of the tester&#39;s test interface circuitry of  FIG.  1 B  whereby each tester interface channel has its own controller. 
         FIG.  3    illustrates an example of a die&#39;s test interface channel interfaced to IEEE 1149.1 TAP domains via a Tap Linking Module (TLM) according to the present disclosure. 
         FIGS.  4 A and  4 B , respectively, depict an example of the tester&#39;s test channel circuitry and a diagram of the channel circuitry signals. 
         FIGS.  5 A and  5 B , respectively, depict an example of the tester&#39;s test channel controller circuitry and a timing diagram of the controller signals. 
         FIGS.  6 A and  6 B , respectively, depict an example of a die&#39;s test channel circuitry and a timing diagram of the die&#39;s test channel signals. 
         FIGS.  7 A and  7 B , respectively, depict an example of the die&#39;s test channel controller circuitry and a timing diagram of the controller circuitry signals. 
         FIG.  8    illustrates an example of the interface between the SBT circuitry of the tester&#39;s test channel and the SBT circuitry of a die&#39;s test channel. 
         FIGS.  9 A-D  depict case statements of the operation of the SBT circuitry of  FIG.  8   . 
         FIGS.  10 A and  10 B , respectively, depict an example of the 2-signal interface between the tester&#39;s test channel and a die&#39;s test channel and a timing diagram of the interface signals. 
         FIG.  11    illustrates an example of performing scan testing using the die test channel. 
         FIG.  12    illustrates an example of performing parallel scan testing using the die test channel. 
         FIG.  13    illustrates an example of performing test data decompression/compression testing using the die test channel. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
       FIG.  1 A  illustrates an example of a tester  100  coupled to die  104 - 110  on a wafer  102  via a connectivity mechanism  124 . The die  104 - 110  could be a selected group of die on the wafer or all of the die on wafer. The connectivity mechanism can be any type of connection scheme that allows signals to flow between the tester and wafer. While not shown, the tester also supplies power and ground signals to the die on wafer to power up the die. The tester consists of a JTAG Pattern Engine  112  and test channels  114 - 120 . The JTAG Pattern engine consists of circuitry designed to communicate JTAG test patterns between the tester and wafer. The test channels receive the JTAG output test patterns (TMS and TDO) from the JTAG Pattern Engine and output them as serialized 2-bit packets on the Data I/O (DIO) paths  126 - 132  to each die on the wafer. The test channels are simultaneously bidirectional which allows them to simultaneously input JTAG test data inputs (TDI) from each die and input the test data input to the JTAG Pattern Engine, while, at the same time, the JTAG Pattern Engine outputs JTAG test patterns to the die. The test data transferred between the tester and wafer via the DIOs  126 - 132  is timed by a clock signal (CLK)  134  which is provided by the tester. 
     Each die  104 - 110  of wafer  102  includes a test channel  136 , a Tap Linking Module (TLM)  1138 , and TAP domains  140 . The test channels  136  serve to receive the serialized JTAG test data pattern (TMS and TDO packets) from the tester, via the DIOs, and convert the serialized JTAG test data pattern into a parallel JTAG test data pattern (TMS and TDI) which is input to the TLM. The TLM is a circuit used to select one or more of the TAP domains  140  to be coupled for access by the tester. The TLM circuitry can be implemented in a number of ways, including the implementation described in U.S. Pat. No. 6,073,254 which is incorporated herein by reference. The test channels  136  are simultaneously bidirectional which allows them to output JTAG test data output (TDO) from a selected TAP domain(s) to the tester simultaneously with the input of the JTAG test data pattern (TMS and TDO) from the tester via the DIOs  126 - 132 . 
       FIG.  1 B  illustrates another example of a tester  101  coupled to die  104 - 110  of wafer  102  via a connectivity mechanism  125 . The tester  101  differs from the tester  100  of  FIG.  1 A  in that each test channel  114 - 120  is driven by a separate clock (CLK) from the JTAG Pattern Engine  113  instead of a common clock (CLK) as shown in  FIG.  1 A . The advantages of using separate CLKs to drive the test channels include but are not limited to; (1) the transfer of test data between the tester channels and die channels can occur at the same or different frequencies, (2) different tests can be performed on each die, (3) each die test can be scheduled to occur at different times, and (4) each die test can be started, paused, or stopped at different times. All these advantages can effectively be used to reduce the heat generated on the wafer during the testing of a large number of die, which can be a major concern during wafer testing. 
       FIG.  2 A  illustrates a more detail example of the tester  100  of  FIG.  1 A . As seen the JTAG Pattern Engine outputs JTAG TMS and TDO signals to each test channel  1 -N, and inputs TDI data from each test channel  1 -N. A controller  200  receives the CLK and TRST outputs from the JTAG Pattern Engine, and outputs a load (LD) signal to the test channels and a CKIN signal to the JTAG Pattern Engine. The LD signal is used to cause the test channels to load the parallel TMS and TDO data output from the JTAG Pattern Engine which is shifted out of the test channel onto the DIO 1 -N signals. The CKIN signal output from the controller times the operation of the JTAG test pattern control circuitry within the JTAG Pattern Engine. The TRST signal is used to initialize the controller  200 . The TRST signal is also input to the test channels for initializing them as well. 
       FIG.  2 B  illustrates a more detail example of the tester  101  of  FIG.  2 B . As seen, each test channel  1 -N has its own controller circuit  200  which operates in response to a unique CLK output from the JTAG Pattern Engine  113  to generate the previously mentioned LD and CKIN signals. The JTAG Pattern Engine of  FIG.  2 B  has separate JTAG test pattern control circuitry which is interfaced to each of the test channels and controller combinations via separate TMS, TDO, CKIN, TRST, and CLK signals. Each JTAG test pattern control circuitry in the JTAG Pattern Engine is timed by the CKIN signal output from a channel controller  200 . In response to the CKIN signal, each the JTAG test control circuitry outputs TMS and TDO to the associated test channel and receives, during JTAG DR or IR Shift operations, TDI input from the associated test channel. The controller  200  of each test channel is timed by a separate CLK output  1 -N from clock generation circuitry within the JTAG Pattern Engine  113 . As mentioned, the CLK  1 -N outputs from the JTAG Pattern Engine can be controlled to operate at different frequencies and/or times to adjust how each die coupled to a tester via the test channels is tested. 
       FIG.  3    illustrates a more detail example of the die  104 - 110  of  FIGS.  1 A and  1 B . As seen, the die&#39;s test channel is coupled to the DIO and CLK signals from a tester&#39;s test channel. The die test channel serves to convert the serialized JTAG TMS and TDO patterns on the DIO signal back into conventional parallel JTAG test TMS and TDO patterns and to apply the parallel JTAG TMS and TDI patterns to the TLM. The die test channel also outputs the TCK and TRST signals to the TLM and receives the TDI input from the TLM. The TLM serves to couple one or more of the die TAP domains to the die test channel, via a conventional 5-signal JTAG bus, and then allows the tester to communicate with the coupled TAP domain(s) using the conventional 5-signal JTAG bus. As seen, the TAP domains may be any type of TAP domain including but not limited to; Scan TAP Domains, BIST TAP Domains, Diagnostic TAP Domains, Debug TAP Domains, Fuse Programming TAP Domains, IEEE standard 1149.1, 1149.4, 1149.6, or 1149.7 TAP Domains, IEEE standard 1500 TAP Domains, IEEE standard 1532 TAP Domains, or any other type of domains using a TAP interface. 
       FIG.  4 A  illustrates a more detail example of the tester&#39;s test channel  114  coupled to a controller  200 . The test channel consists of a 2-bit parallel input serial output (PISO) register  402  and a simultaneously bidirectional transceiver (SBT)  404 . The TRST signal from the JTAG Pattern Engine is used to initialize the controller  200  and test channel&#39;s PISO  402 . Following initialization, the PISO repeats the steps of loading the TMS and TDO outputs from the JTAG Pattern Engine in response to the LD and CLK signals, then shifting out the TMS and TDO signals serially on DIO in response to the CLK signal. The SBT allows TDI input data from a die test channel, via the DIO signal, to be input to the JTAG Pattern Engine while simultaneously the TMS and TDO data from the JTAG Pattern Engine is being output to the die test channel on DIO. The timing diagram,  FIG.  4 B , illustrates this simultaneous input and output operation. The SBT is a known interface circuit that allows data to flow simultaneously in both directions over a single connection. A detail description of the operation of the circuit of  FIG.  4    is provided in regard to FIGS. 5A and 5B of patent application Pub. No. US 2006/0,236,174. In FIGS. 5A and 5B of patent application Pub. No. US 2006/0,236,174, the SBT circuit is referred to as input/output (I/O) circuit instead of SBT. The operation of the I/O circuit of Pub. No. US 2006/0,236,174 and the SBT circuit of this disclosure is the same. 
       FIG.  5 A  illustrates an example design of the controller  200  which consists of two flip flops, three AND gates, and a Delay Circuit connected as shown. The TRST signal resets the flip flops, the CLK signal times the flip flops, and the gating and delay circuits decode the states of the flip flops and the CLK input to produce the LD and CKIN output signals. As seen in the timing diagram,  FIG.  5 B , the controller responds to the CLK input to repetitively output the LD and CKIN signals. A detail description of the controller  200  is provided in regard to FIGS. 6A and 6B of referenced patent application Pub. No. US 2006/0,236,174. 
       FIG.  6 A  illustrates a more detailed example of the die&#39;s test channel  136  coupled to a TLM  138 . The die test channel  136  consists of an SBT circuit  602 , a master reset and synchronization circuit (MRS)  604 , a 2-bit serial input parallel output (SIPO) circuit  606 , a 2-bit register (REG) circuit  608 , a TAP state monitor (TSM) circuit  610 , a controller circuit  612 , and a power on reset (POR) circuit  614 . The SBT circuit is used to input (IN) data from the DIO signal to the MRS and SIPO circuits. The MRS circuit is used to reset the die channel circuitry, TLM, and TAP Domains, and to synchronize the operation of the die channel circuitry with the tester&#39;s channel circuitry. The MRS outputs a master reset (MRST) signal, which resets the channel circuitry, TLM, and TAP Domains, whenever the 5 TMS logic ones have been input on DIO and consequently output on the TMS output of the REG. 
     Clocking, via TCK, the 5 TMS logic ones from the REG into the TSM, which is a TAP state machine, causes the TSM to enter the test logic reset (TRST) state which sets the RST output of the TSM low, resetting the MRS and controller circuit. The TAP of the selected TAP domain(s) also enters the TRST state whenever the TSM enters the TRST state, which resets the TAP domain(s). The reset state on the channel circuitry, TSM, and TAP Domains remains as long as DIO input logic ones on the IN input of the MRS circuit. To transition from the reset state, a synchronization code from the tester, consisting of say 2 logic zeros followed by 2 logic ones, is input to the MRS circuit via DIO. In response to the synchronization code, the MRS sets the CENA signal high for a period of time to enable the operation of the controller to input serialized TMS and TDO patterns into the SIPO to be output as TMS and TDI patterns to the TSM and TLM via the REG. With the controller and SIPO enabled, serialized JTAG TMS and TDI signal from the tester are again output from the REG to cause the TSM to transition from the TRST state, which removes the logic low on the RST output of the TSM and enables the operation of the TLM and selected TAP Domain(s). 
     The SIPO circuit is used to input, via the IN signal, the serialized TMS and TDO data packets from the SBT circuit and output them in parallel to the REG circuit. The REG circuit is used to hold the parallel TMS and TDI input signals to the TLM circuit stable as the next serialized TMS and TDO input packet is shifted into the SIPO circuit. The controller circuit is used to update TMS and TDI data from the SIPO into the REG, via the update clock (UCK) signal, and to produce a test clock (TCK) signal which times the selected TAP domain(s) via the TLM circuit. The TSM is used to allow the test channel circuitry to track the sixteen TAP states of the connected TAP domain circuit. The POR circuit is used to force the test channel, TLM, and TAP Domains into a reset state when power is first applied to the die. As seen in the timing diagram,  FIG.  6 B , the die test channel inputs serialized TMS and TDO signals from the DIO signal and converts them into parallel TMS and TDI patterns, via the SIPO and REG, which are input to the TLM. Simultaneous with the inputting of the serialized TMS and TDO signals on DIO, the test channel can output, during TAP IR or DR Shift operations, TDO data on DIO from the TAP Domain(s) selected by the TLM. 
     A detailed description of circuitry very similar to the test channel circuitry of  FIG.  6 A  is given in regard to FIGS. 7A, 7B, 8A, 8B, 9A, 9B, and 9C of referenced patent application Pub. No. US 2006/0,236,174. In FIGS. 7A and 7B of patent application Pub. No. US 2006/0,236,174, the SBT circuit is referred to as input/output (I/O) circuit instead of SBT. The operation of the I/O circuit of Pub. No. US 2006/0,236,174 and the SBT circuit of this disclosure is the same. In FIG. 6 of this application the TLM is shown outside the TAP domain block, whereas in FIG. 7A of reference application Pub. No. US 2006/0,236,174 the TLM, if used, is embedded within the TAP domain block and thus not shown. 
       FIG.  7 A  illustrates an example design of the controller  612  which consists of two flip flops and two AND gates connected as shown. The flop flops can be reset by either the controller enable (CENA) or reset (RST) signals. The CLK signal times the flip flops and the gating circuits decode the states of the flip flops and the CLK input to produce the update clock (UCK) and TCK output signals. As seen in the timing diagram,  FIG.  7 B , the controller responds to the CLK input to repetitively output an update clock (UCK) to the REG and a TCK to the TLM and coupled TAP Domain(s). A detail description of the controller  612  is provided in regard to FIGS. 8A and 8B of referenced patent application Pub. No. US 2006/0,236,174. 
       FIG.  8    illustrates the DIO connection between the SBT circuit of the tester&#39;s channel and SBT circuit of the die&#39;s channel. For simplification, the accompanying CLK signal connection is not shown. The tester channel&#39;s SBT circuit consists of an output buffer  802 , a resistor  803 , and an input (I) circuit  804 . The die channel&#39;s SBT circuit includes an output buffer  806 , a resistor  805 , an input (I) circuit  808 , and a pull up (PU) element  810 . PU element  810  is used to hold the DIO input to the die channel high whenever the DIO input is not being externally driven, which forces the die&#39;s test channel, TLM, and TAP Domain circuits into a reset state, as previously described in regard to  FIG.  6   . 
     For simplification, the REG and the TLM circuits are not shown between the SIPO and TAP Domains. The output buffers  802  and  806  serve to output data onto the DIO path from the tester and die channels respectively. The resistors  803  and  805  of the SBT circuits serve to limit current flow when the tester and die channels are outputting opposite logic states. The input (I) circuit compares the voltage level on the DIO path to the logic voltage level being output from the tester and die channel. If the voltage level on DIO is the same as the logic voltage level being output from the tester and die channel, the input (I) circuit inputs that voltage level to the TDI input of the JTAG Pattern Engine of the tester channel and to the IN input of the SIPO of the die channel. If the voltage level on DIO is at a mid-point level, indicating the output buffers  802  and  806  are outputting opposite logic voltage levels, the JTAG Pattern Engine will input the opposite logic level being output from output buffer  802  and the SIPO will input the opposite logic level being output from output buffer  806 . 
     The simultaneous bidirectional data operation of the SBT circuits are described in case statements A-D of  FIG.  8    and illustrated in  FIG.  9     FIGS.  9 A- 9 D . A detail description of the operation of the SBT circuits is provided in regard to FIGS. 11A, 11B, 12, and 13 of referenced patent application Pub. No. US 2006/0,236,174. 
       FIG.  10 A  illustrates the DIO and CLK connections between a tester channel and a die channel. For simplification, the die channel circuit only shows the SBT  602 , SIPO  606 , REG  608 , and Controller  612  circuits and their associated signals, with the exception of the TRST signal. Also the TLM circuit  138  is not shown between the die channel and selected TAP Domain(s). It should be noted that in instances where there is only one TAP domain in the die, the TLM circuit will not be required at all since that TAP domain will be permanently coupled to the die channel as shown in  FIG.  10   . 
     The operation of the channels is shown in the timing diagram,  FIG.  10 B . As seen in the timing diagram, the operation includes repetitively serializing TMS and TDO output patterns from the JTAG Pattern Engine over the DIO path using the PISO  402 , then de-serializing the TMS and TDO signals into parallel TMS and TDI inputs to the selected TAP Domain(s) using the SIPO  606  and REG  608 . During the serialize and de-serialize operations, controller  200  outputs the LD signal to the PISO to load the TMS and TDO pattern from the JTAG Pattern Engine and outputs the CKIN signal to time the operation of JTAG control circuitry within the JTAG Pattern Engine while, concurrently, controller  612  outputs the UCK signal to update the REG with the parallel TMS and TDI signals from the SIPO and outputs the TCK signal to time the operation of the TAP Domain. A more detail description of this serialize and de-serialize operation is provided in regard to FIGS. 14A, 14B, and 14C of referenced patent application Pub. No. US 2006/0,236,174. 
       FIG.  11    illustrates a first example use of the present disclosure to perform scan testing of combinational logic within a core of an IC. In this example, a scan path is accessed by the die channel to shift in stimulus test data, apply the stimulus data to combinational logic, capture the response of the combinational logic into the scan path, and shift the response data out of the scan path to the tester. The use of a scan path to test combinational logic is known. What is new is the use of the die channel to provide the stimulus test data input to the scan path and the response test data output from the scan path. 
       FIG.  12    illustrates a second example use of the present disclosure to perform scan testing of combinational logic within a core of an IC. In this example, a SIPO register is used to serially input and apply stimulus test data to inputs of parallel scan paths and a PISO register is used to capture and shift out response test data from the outputs of the parallel scan paths. The use of a SIPO and a PISO to input and output test data to parallel scan paths is known, see U.S. Pat. No. 6,242,269. What is new is the use of the die channel to provide the stimulus test data input to the SIPO and the response test data output from PISO. 
       FIG.  13    illustrates a third example use of the present disclosure to perform scan testing of combinational logic within a core of an IC using decompression and compression circuits. In this example, a decompresser circuit is loaded with compressed stimulus test data patterns from TDI which are decompressed by the decompresser and input to the parallel scan paths, while the compresser circuit compresses the output responses of the parallel scan paths and outputs the compressed response test data to the tester. The use of test data compression and decompression circuits is known, see U.S. Pat. No. 6,543,020. What is new is the use of the die channel to provide the compressed stimulus data input to the decompresser and the compressed response data output from the compresser. 
     Although exemplary embodiments of the present disclosure are described above, this does not limit the scope of the present disclosure, which can be practiced in a variety of embodiments. Further, references to detailed descriptions in referenced patent Pub. No. US 2006/0,236,174 are intended to provide more detailed teaching of the operation of circuits similar to those described in this application. It should be understood that subtle differences in circuit and signal names may be seen between the circuitry described in Pub. No. US 2006/0,236,174 and the circuitry contained in this application.