Single-chip GPS receiver digital signal processing and microcomputer

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to integrated circuit semiconductor devices and more specifically to highly integrated implementations of global positioning system receivers.

2. Description of the Prior Art

The retail price of complete global positioning system (GPS) receivers including hand-held, battery-operated portable systems, continues to decrease. Competitive pressures drive manufacturers to reduce manufacturing costs while maintaining or actually improving functionality and reliability. Semiconductor fabrication advances have provided a vehicle to meet such goals, and to offer still smaller devices.

With a highly-integrated GPS receiver implementation, it is desirable to incorporate a GPS receiver's digital signal processing circuitry with a microcomputer with its associated peripherals, e.g., real time clock, serial input/output controllers, analog-to-digital converters, et cetera. In conventional GPS receivers, circuitry for digital signal processing and an associated microprocessor are separate, discrete devices mounted to one or more printed circuit boards.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide an integrated circuit for reducing the size, cost and complexity of a GPS receiver and thus improve reliability and performance.

Another object of the present invention is to provide a single integrated circuit device that integrates much of the digital functions associated with navigation and GPS satellite signal processing.

Briefly, an embodiment of the present invention combines on a single integrated circuit an eight channel GPS receiver, a 68330-type microprocessor, a 68681-type DUART serial communications controller, an analog-to-digital converter, a real-time clock, a random access memory and a boot read-only memory. A system integration module and inter-module bus allow tri-state control of the microprocessor such that a commercially available 68332-type emulator may be used for software development.

An advantage of the present invention is that a single-chip GPS digital integrated circuit is provided that has substantially lower system power consumption, compared to the prior art, because the data bus lines between the DSP and micro-controller functional units are internal and therefore very lightly loaded with parasitic capacitances. Thus, much smaller buffers can be used to interface between the various functional blocks.

Another advantage of the present invention is that a single-chip GPS digital integrated circuit is provided that has a much lower part count, compared to the prior art. Peripheral circuitry such as a real-time clock, a universal asynchronous receiver/transmitter, analog-to-digital converter, etc., which usually comprise discrete packages, are all combined into a single device. Therefore, packaging cost is lowered and smaller PC board space requirements are the result.

A further advantage of the present invention is that a single-chip GPS digital integrated circuit is provided that has improved reliability, where fewer packages and fewer interface circuitry and lower power dissipation translate directly into improved reliability.

Another advantage of the present invention is that a GPS digital integrated circuit is provided that has significant system cost savings. The resulting single-chip costs much less than the sum of the parts it replaces.

A still further advantage of the present invention is that a single-chip GPS digital integrated circuit is provided that is susceptible to integrated circuit process improvements.

A further advantage of the present invention is that a single-chip GPS digital integrated circuit is provided in which all the functions on the integrated circuit are fabricated using an advanced integrated circuit process that yields an increase in performance. Such performance is not otherwise attainable with separate commercially-available discrete parts, because such prior art devices are typically more mature products and fabricated with older processes.

Another advantage of the present invention is that a GPS digital integrated circuit is provided that can be re-scaled to take full advantage of the benefits of continuing process improvements in the art of semiconductor fabrication. Examples of such advantages are higher operating clock speed, lower power consumption, low supply voltage operation, lower price due to smaller die size, etc. Every function on the integrated circuit can be expected to participate in these benefits.

Another advantage of the present invention is that a single-chip GPS device is provided that is small in size.

Another advantage of the present invention is that a single-chip GPS digital integrated circuit is provided that can operate on a single three volt power supply.

These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment which is illustrated in the drawing figures.

IN THE DRAWINGS

FIG. 1 is a block diagram of a GPS digital integrated circuit embodiment of the present invention;

FIG. 2 is a general layout in plan view of the semiconductor chip of FIG. 1; and

FIGS. 3A-3C are block diagrams of the CPU, global controller and a representative one of the eight channels included in the GPS digital integrated circuit of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a GPS digital integrated circuit (IC) embodiment of the present invention, referred to herein by the general reference numeral 10. The IC 10 carries out the digital processing aspects of a GPS navigation system, including GPS code correlation and position and velocity calculation, local oscillator frequency synthesis, user keyboard and display interfacing, and other digital signal processing (DSP) and input/output (I/O) functions. IC 10 is implemented on a single semiconductor chip which may be physically laid out and fabricated on the surface of a silicon substrate, as shown in FIG. 2. Commercial standard-cell high-performance computer-core CMOS semiconductor fabrication processes and packaging facilities available in the United States, such as provided by Motorola Semiconductor (Phoenix, Ariz.), may be successfully used to build IC 10. IC 10 is preferably implemented using a commercially available semiconductor fabrication technology such as advanced, triple-metal, 0.8 micron CMOS processes directed at high performance and low power consumption. A 144-pin thin plastic quad flat-pack (TQFP) is preferably used to package IC 10.

FIG. 1 shows that IC 10 comprises a pair of IC pin multiplexers 12 and 14, a dual universal asynchronous receiver transmitter (DUART) 16, a read only memory (ROM) 18, a random access memory (RAM) 20, a microcomputer (CPU) 22, a global channel controller 24 and a set of eight GPS receiver channels 26-33 for demodulating carrier signals simultaneously received from a plurality of orbiting GPS satellites.

IC pin multiplexers 12 and 14 make it possible for a 144-pin TQFP package to contain IC 10. Since not all input/output signals associated with IC 10 need to be simultaneously available to the external interface, some pins are configurable to serve more than one function.

The use of standard cell devices can substantially simplify the task of implementing IC 10. DUART 16 is preferably a standard cell device, for example the Motorola 68681 DUART. CPU 22 is also preferably a standard cell device, for example the Motorola 68330 microcomputer, which is integrated into the substrate of IC 10. DUART 16 is also integrated into the substrate of IC 10. As a consequence, these and all the other functional elements of IC 10 share a common semiconductor fabrication process and are susceptible to process improvements and scaling on a substantially equal basis. Since such elements within IC 10 constitute a major part of the whole function of a complete GPS navigation system, such semiconductor process improvements will operate on a system-wide basis. Therefore piecemeal improvements are avoidable over a product life and little inertia exists to retain older, less advanced components in a system. Due to the common basis of fabrication of the elements of IC 10 on a single-chip, operation from a single power supply voltage is obtainable. A single three volt supply is therefore preferably used for all the functions, including analog to digital conversion. (Prior art ADC's seldom use three volt supplies.)

GPS receiver channels 26-33 are independent coarse acquisition (C/A) code GPS receiver channels. For example, a conventional GPS receiver that has been adapted to standard-cell integrated circuit fabrication techniques can provide good results when used to implement channels 26-33.

During acquisition and tracking of a particular GPS satellite vehicle (SV), IC 10 is operated to find a Doppler influenced GPS carrier frequency and C/A code phase from the GPS SV. As is conventional, a GPS receiver will search over a range of frequencies, a range that can be widened by local oscillator uncertainties when first starting up. A range of 1023 code phases will also be searched (chip-by-chip) until a signal from the expected GPS SV is found. If the expected GPS SV is not found, C/A codes for other likely candidate satellite vehicles are tried. This search is carried out by mixing a received signal with various local oscillator frequencies in a super-heterodyne configuration and correlating a detected signal with a trial C/A code that is predicted to be valid for the locale. The correlation interval is typically one millisecond, which is the periodicity of the C/A code.

After an initial detection of a GPS signal, a phase locked loop is used to help bring it in to a full track condition. The success of the phase locked loop and the phase locked loop delay are dependent upon bringing the signal close enough to a pseudo-baseband via a carrier numerically controlled oscillator (NCO), and being close enough to an actual peak in C/A code phase.

After a GPS receiver acquires a satellite signal, it may temporarily lose lock due to some physical obstruction between the receiver and the satellite, e.g. the user drives by a very tall building. Rather than resuming a search over a range of code phases, it is possible to use information from the last lock to predict the new code phase. This makes for fast re-acquisition once the obstruction is out of the way. However, such concerns and their solutions are conventional and well documented in the background art which a person skilled in the art will be conversant.

Intermediate frequency signals from an external radio frequency (RF) and mixer stage are received at a pair of differential inputs IF1 and IF2, each of which is a two-bit quantized quadrature input. A multiplexer function within IC 10 allows several different types of IF signal inputs to be accommodated by IC 10. In a normal mode, the IF1 and IF2 inputs are directly available. In a sampler mode, IF1 and IF2 are directed to a sampler circuit. In a gyro mode, such as used in inertial instrumentation, two single-bit inputs are steered to the sampler circuit and two additional inputs connect to a pair of gyro sampler flip-flops. In a fourth mode, a sign input and two magnitude inputs to the sampler are accepted, together with a sampler clock, which can either be driven internally, or-externally. A set of four channel test outputs for IC 10 are provided: CHT1A, CHT1B, CHT2A and CHT2B.

A set of eleven chip select outputs, CS0-CS10, are provided for connecting to external memory and peripheral chips and for reducing the need for external "glue" logic to interface IC 10.

A sixteen-bit bi-directional data bus, DB0-DB15, is multiplexed into the thirty-two bits of CPU 22. The least significant nineteen bits of the address bus from CPU 22 are brought out as an address bus A0-A18. Address lines A19-A31 internal to IC 10 are distributed normally, but are multiplexed with various other functions to save package pins when brought out externally. The data transfer direction for a current cycle is indicated by a read/not-write control signal, R/W. RMC is an output that signals a read-modify-write cycle. An address strobe, AS, is used by external devices to latch address information from A0-A31. A data strobe, DS, signals when data is valid on DB0-DB15. A pair of active-LOW upper byte and lower byte write strobes, UWE and LWE, are used for byte-wide data transfers. A read strobe, RD, latches in read data. A dynamic bus sizing is implemented with a pair of signals, SIZ0 and SIZ1. Data strobe acknowledge signals, DACK0 and DACK1, are used to terminate an access cycle and are used for dynamic bus sizing. An active LOW boot chip select output, CSB, is derived from CPU 22. A boot code program running in ROM 14 typically causes this output to select an external memory that may contain a custom initialization code. A chip select, CSX, enables access to a set of internal registers and memories that are not part of the core of CPU 22.

The data bus lines between the functional units of IC 10, e.g., DB0-DB15, are internal and therefore very lightly loaded with parasitic capacitances. Thus, much smaller buffers can be used to interface between the various functional blocks. In a discretely implemented prior art system having nearly the same functions as IC 10, these same interfaces would unavoidably go off-chip and encounter as much as fifty picofarads of capacitance per line that would have to be overcome by suitable buffer/drivers.

An autovector input, AVEC, is provided to CPU 22 that will cause an interrupt vector address to be loaded in the CPU address register when a hardware signal is received. The AVEC function can also be generated automatically. A chip select output controlled by a register external to the core of CPU 22 is labeled CS0. When other masters wish to use the address and data buses, a bus request input, BR, is received from the requester. BG is a bus grant output. BGACK is a bus grant acknowledge input.

Interrupt acknowledges, corresponding to levels zero through seven are received on lines labeled IACK0-IACK7. A pair of ports, with bit lines labeled PORT A0-PORT A7 and PORT B0-PORT B7, are provided for general-purpose input and output. One or more lines of PORT B can be configured by software running on CPU 22 to be either push/pull, or simply open-drain for external pull-up. The PORT A and B input/output signals are able to support direct scanning of an external keyboard, thereby eliminating additional external buffers or diodes.

IC 10 can assert an interrupt request GPSIRQ. A response signal, GPSIACK, is an interrupt acknowledge signal that causes IC 10 to drive an interrupt vector onto DB0-DB15. DUART 16 receives an interrupt acknowledge from a signal output from CPU 22 which is labeled DRTIACK and is also made available externally. DRTIACK causes DUART 16 to drive an interrupt vector onto DB0-DB15.

Memory protector 18 provides for external battery-backed CMOS static RAMs. An external signal pin is pulled HIGH by the standby power supply (VSTBY) whenever a memory protect signal input, MPROT, is HIGH. Chip selects CS3 and CS4 are used for CMOS memory selection in such instances.

A combination power-up hardware reset output from IC 10 and reset input for all blocks is provided by a signal labeled RESET. CPU 22 may be halted by an input labeled HALT and produces a halt output in response.

Bus transfer errors are signaled to CPU 22 with an input labeled BERR. Eight interrupt request levels are provided with IRQ0-IRQ7. The source of a processor clock for CPU 22 can be specified by setting the logic state of an input signal, MODCK, at reset

An external 32.768 KHz crystal can be connected to signal pins labeled XTAL and EXTAL to control internal oscillator 28. Alternatively, an externally supplied processor clock can be input at EXTAL. Phase-locked loop 21 is used to synthesize a number of internal frequency references for IC 10 from the XTAL/EXTAL source and has a connection provided for an external filter capacitor, labeled XFC. The processor clock has an external output, labeled CLKOUT.

Standby power supply, VSTBY, powers crystal oscillator 28, RTC 26 and the memory protect circuit 18. Power for the processor clock generator phase-locked loop frequency synthesizer circuit 21 is labeled, VCCSYN. When an output of RTC 26 is activated, an output, labeled ALARM, is pulled HIGH by the standby power supply (VSTBY). A tri-state control signal, TSC, when held LOW, puts IC 10 in a master mode in which CPU 22 is active and in control of the internal functional blocks and the external processor bus, e.g., DB0-DB15. When TSC is held HIGH, IC 10 is placed in a slave mode, in which CPU 22 is held in reset, and is effectively removed from participation in IC 10 functions. All dedicated CPU 22 output signals are tri-stated, while all other blocks remain active. This allows the assets of IC 10 to be accessed externally via the processor bus signals.

The state of an instruction fetch pipeline is communicated via two signal outputs from CPU 22, IPIPE and IFETCH. They also furnish data in and out signals for a background debug monitor for CPU 22. A breakpoint to CPU 22 is signaled with a signal labeled BKPT. CPU 22 acknowledges a breakpoint with a signal labeled FREEZE.

An Institute of Electronic and Electrical Engineers (IEEE) industry-standard number 1149.1-JTAG test function is provided with a set of four signals: TCK, TMS, TDI and TDO. EVENT1 and EVENT2 are event timer input signals. A millisecond generator toggle signal, TOGGLE, is used to identify in which millisecond an event or channel interrupt occurred.

A master clock input signal, MCLK, is provided to IC 10. This signal can be driven internally by CLKDVR 29, or it can be driven externally. An external millisecond input to IC 10 is provided as MSI. A millisecond generator output signal is labeled MSO.

PPS1 and PPS2 are general purpose pulse generator outputs. DAO0-DA07 are outputs from a pulse-width modulator circuit. OP0-OP7 are labels for the individual bit lines of an output port signal from DUART 16. Output signal OP3 is capable of sourcing or sinking 48 mA, and can also be inverted or non-inverted. OP3-OP5 can be configured as push-pull outputs or an open-drain outputs.

The receive data input signals for the two DUART channels A and B are labeled RXD. The two transmit data output channels A and B are labeled TXD. Two DUART input port signals, IP0 and IP1, provide a clear-to-send (CTS) input function for the A and B channels in DUART 16. IP2 is a DUART input port signal that can also be used as an external clock input for the sixteen-bit counter/timer 26. A request-to-send (RTS) output function is also included for channels A and B of DUART 16. DUART 16 has a pair of crystal oscillator signals, X1 and X2. An external 3.6864 MHz crystal is connected for DUART 16 to operate a crystal oscillator. Alternatively, an external clock can be supplied to an input signal, CLK.

Table I lists a set of input/output interface pins preferably included with IC 10. A complete list of these input/output pins and their functions are summarized. Some of the signals shown individually in FIG. 1 are shown as sharing package pins with other signals. This sharing conserves the number of total pins required and does not unduly constrain the connection of IC 10 into a larger system.

TABLE I ______________________________________ Pin Descriptions ______________________________________ DBO-DB15 The sixteen-bit bidirectional data bus of CPU 22. A0-A18 The least significant nineteen bits of the address bus of CPU 22. R/W Read/write signal. AS Active-LOW address strobe. DS Active-LOW data strobe. L7WE/ In master mode, these are active-LOW DSACK0S, upper byte and lower byte write UWE/ strobes. In slave mode, these two DSACK1 pins can be tied to DSACK0 and DSACK1 to enable active deassertion of these signals. To disable active deassertion of DSACK0 and DSACK1 in slave mode, DSACKOS and DSACK1S are tied HIGH. RD Active-LOW read strobe. SIZO-SIZ1 CPU 22 outputs used for dynamic bus sizing. DSACK0- Data strobe acknowledge signals used DSACK1 to terminate an access cycle and for dynamic bus sizing. CSB Active LOW boot chip select output. This signal is derived from the CPU 22 CS1 output. Boot code running in ROM 14 uses this output to select external memory that contains the initialization code. CSX/ This pin has three selectable FC2/ functions. CSX is an active-LOW chip AVEC select that enables access of all internal registers and memories that are not part of the core of CPU 22. In a master mode, this signal is derived from CS0 output of CPU 22 and can be used to signal external logic or to signal an emulator that internal registers are being accessed. In a slave mode, this pin is a CSX input that enables an external access to IC 10. FC2 is a CPU function code output. AVEC is an active-LOW CPU auto-vector input, and may not be needed, where an AVEC function can be generated automatically. CS0/ This pin has five selectable BR/ functions. CS0 is a chip select A31/ output that is controlled by IACK7/ registers external to CPU 22. BR is PORT A7 an active-LOW bus request input. A31 is a high-order address line. IACK7 is an active-LOW priority level seven interrupt acknowledge output. PORT A7 is a general purpose port pin. CS1/ In slave mode, this pin always has BG/ the GPSIACK function. This is an A25/ active-LOW interrupt acknowledge IACK1/ signal that causes IC 10 to drive an PORT Al/ interrupt vector onto DB0-DB15. In GPSIACK master mode, this pin has one of five selectable functions. CS1 is an active-LOW chip select output that is derived from the CS2 output of CPU 22. BG is an active-LOW bus grant output. A25 is a high-order address line. IACK1 is an active-LOW priority level one interrupt acknowledge output. PORT A1 is a general purpose port pin. CS2/ In slave mode, this pin always has BGACK/ DUART 16 IACK function. This is an A26/ active-LOW interrupt acknowledge IACK2/ signal that causes DUART 16 to drive PORT A2/ an interrupt vector onto DB0-DB15. In DRTIACK a master mode, this pin has one of four selectable functions. CS2 is an active-LOW chip select output that is derived from the CPU 22 CS3 output. BGACK is an active-LOW bus grant acknowledge input. A26 is a high- order address line. IACK2 is an active-LOW priority level 2 interrupt acknowledge output. PORT A2 is a general purpose port pin. CS3/ This pin has five selectable FC0/ functions. CS3 is a chip select A27/ output that is controlled by IACK3/ registers external to the core of CPU PORT A3 22. FC0 is one of the CPU 22 function code outputs. A27 is a high-order address line. IACK3 is an active-LOW priority level 3 interrupt acknowledge output. This pin is pulled HIGH by the standby power supply (VSTBY) whenever the MPROT input is HIGH. This provides a memory protect function when CS3 is used to select battery-backed CMOS static RAMs. PORT A3 is a general purpose port pin. CS4/ This pin has five selectable FC1/ functions. CS4 is a chip select A30/ output that is controlled by IACK6 registers external IACK6/ to the core PORT A6 of CPU 22. FC1 is one of the CPU 22 function code outputs. A30 is a high order address line. IACK6 is an active-LOW priority level 6 interrupt acknowledge output. This pin is pulled HIGH by the standby power supply (VSTBY) whenever the MPROT input is HIGH. This provides a memory protect function when CS4 is used to select battery-backed CMOS static RAMS. PORT A6 is a general purpose port pin. CS6/ This pin has two selectable A19 functions. CS6 is a chip select output that is controlled by registers external to the core of CPU 22. A19 is an address bus output. CS7/ This pin has two selectable A20 functions. CS7 is a chip select output that is controlled by registers external to the core of CPU 22. A20 is an address bus output. CS8/ This pin has two selectable A21 functions. CS8 is a chip select output that is controlled by registers external to the core of CPU 22. A21 is an address bus output. CS9/ This pin has two selectable A22 functions. GS9 is a chip select output that is controlled by registers external to the core of CPU 22. A22 is an address bus output. CS10/ This pin has two selectable A23 functions. CS10 is a chip select output that is controlled by registers external to the core of CPU 22. A23 is an address bus output. RESET Active-low power-up reset output and external rese input for all blocks. HALT/ This pin has three selectable IP2/ functions. HALT is the CPU 22 halt AD0 input and output. IP2 is a DUART input port pin that can also be used as the external clock input for the sixteen-bit counter/timer. This pin also serves as an AD0 synchronized analog-to-digital conversion input. BERR Active-LOW bus error input. IRQ1/ Active-LOW interrupt priority level PORT B1/ one input. This pin can also be AD1 configured as general purpose port pin B1. As an output port pin, it can be configured to be either a push- pull output or an open-drain output. This pin also serves as an AD1 synchronized analog-to-digital conversion input. IRQ2/ Active-LOW interrupt priority level 2 PORT B2/ input. This pin can also be AD2 configured as general purpose port pin B2. As an output port pin, it can be configured to be either a push- pull output or an open-drain output. This pin also serves as an AD2 synchronized analog-to-digital conversion input. IRQ3/ Active-LOW interrupt priority level 3 PORT B3/ input. This pin is also pulled LOW by AD3 an active DUART interrupt if DUART 16 is set to interrupt on priority level 3. This pin can also be configured as general purpose port pin B3. As an output port pin, it can be configured to be either a push-pull output or an open-drain output. This pin also serves as an AD3 synchronized analog- to-digital conversion input. IRQ4/ Active-LOW interrupt priority level 4 GPSIRQ/ input. This pin GPSIRQ/ is also PORT B4/ pulled LOW by an active IC 10 AD4 interrupt. This pin can also be configured as general purpose port pin B4. As an output port pin, it can

be configured to be either a push- pull output or an open-drain output. This pin also serves as an AD4 synchronized analog-to-digital conversion input. IRQ5/ Active-LOW interrupt priority level 5 PORT B5/ input. This pin is also pulled LOW by AD5 an active DUART interrupt if DUART 16 is set to interrupt on priority level 5. This pin can also be configured as general purpose port pin BS. As an output port pin, it can be configured to be either a push-pull output or an open-drain output. This pin also serves as an AD5 synchronized analog- to-digital conversion input. IRQ6/ Active-LOW interrupt priority level 6 TIRQ/ input. This pin can be pulled LOW by PORT B6/ an activated timer interrupt within AD6 DUART 16 if this function is enabled. This pin can also be configured as general purpose port pin B6. As an output port pin, it can be configured to be either a push-pull output or an open-drain output. This pin also serves as an AD6 synchronized analog- to-digital conversion input. IRQ7/ Active-LOW interrupt priority level PORT B7/ seven input. This pin can also be AD7 configured as general purpose port pin B7. As an output port pin, it can be configured to be either a push- pull output or an open-drain output. This pin also serves as an AD7 synchronized analog-to-digital conversion input. MODCK/ The logic state of this input pin at PORT BO/ reset determines the source of the ECLKO processor clock. This pin can also be configured as general purpose port pin B0. As an output port pin, it can be configured to be either a push- pull output or an open-drain output. This pin can also be configured as the "E-clock" output for 68000-style peripherals. EXTAL, These two pins can be connected to an XTAL external 32:768 MHz crystal. alternatively, EXTAL is the input pin for an externally supplied processor clock. XFC Connection pin for an external filter capacitor for the processor clock generator phase-locked loop. CLKOUT/ Output pin for the processor clock. PCLK In slave mode, this pin is the PCLK input for a chip select (CS) logic wait-state generator. ALARM This output is pulled HIGH by the standby power supply (VSTBY) when the alarm output of RTC 26 goes active. MPROT When this input is pulled HIGH, the CS3 and CS4 pins are pulled HIGH by the standby power supply (VSTBY). VSTBY The standby power supply (VSTBY) input pin. This pin powers the 32.768 KHz crystal oscillator, RTC 26, and the memory protect circuit 18. VCCSYN This power pin supplies power to a processor clock generator phase- locked loop circuit. TSC Tri-state control pin. When this pin is held LOW, IC 10 is in master mode. The CPU 22 is active and controls the internal blocks and the external processor bus. When this pin is held HIGH, IC 10 is in slave mode, CPU 22 core is held in reset and effectively removed from the system. All dedicated CPU 22 output pins are tri- stated. All other blocks are still active, and can be accessed externally via the processor bus pins. BKPT/SE This active-LOW input signals a breakpoint to the CPU 22. When IC 10 is in test mode, this is the scan enable input IPIPE, These two pins are outputs from the IFETCH CPU 22 that indicate the state of the instruction fetch pipeline. They also furnish data in and out pins for the CPU 22 background debug monitor. FREEZE This active-LOW output indicates that the CPU 22 has acknowledged a breakpoint. IS1/C8, These four pins are the two-bit QS1/C9, quadrature IF input pins for a first IM1/C10, IF input. These pins provide QM1/C11 different functions, depending on the mode setting of the IF input circuitry. In a first mode, these pins provide the IF1 inputs directly. In a second mode, they serve as inputs to the sampler. In a third mode, IS1 and IM1 are two single-bit inputs to the sampler and QS1 and QM1 are inputs to two gyro input sampler flip-flops. In a fourth mode, QS1 is the sign input