High speed parallel/serial interface

Abstract: A circuit for serializing n parallel data bits (D1-D4) requires that the data clock (CLK), having a clock period T, be used to generate n phased clocks of the same frequency as the data clock, but varying in phase such that each phased clock is delayed T/n with respect to the previous one. This can be done using a digital phase locked loop device (17). These n phased clocks and n parallel data bits are then input to a logic circuit which uses an n input Register (16) and an n input multiplexer (22) to output one data bit (d1-d4) for each phased clock. The result is a serializer that converts parallel data to serial data without the need for generating a higher frequency clock.

Description: The present invention relates to a parallel/serial interface circuit according to the preamble of claim 1. In the art of digital image processing, there is a common need for the conversion of parallel data to serial data streams. For example, it is common for an image generating circuit to process image data in the form of a number of bits per word in parallel, but to convert this image data into serial bits for printing on a raster output scanner (ROS). To use a numerical example, if the image data words are eight bits, then these words could be loaded in parallel into an eight bit register by using a system clock, and clocked out serially to the ROS using a clock with a frequency eight times higher than the system clock. The higher frequency clock can be produced using a phase lock loop. The main problem with this system is that the higher frequency components may be required in the circuit. Current methods of meeting this requirement usually rely on analog techniques and multiplication of the original clock to higher frequencies which require very selective discrete components and very careful layout and electromagnetic emissions protections. One particular device (Model AD9560, Pulse Width Modulator from Analog Devices) has the limitation that only certain high addressability codes are allowed. Also, EP-A-477,582 discloses a frequency multiplication and data serialisation circuit in which a "low frequency" parallel clock is fed to a PLL circuit which multiplies the parallel clock frequency and outputs a "high frequency" serial data clock that is synchronised with the parallel data clock. In response to the high frequency clock, a ring counter generates a pulse at successive one of the output lines . The output clocks are sometimes insufficiently synchronised and the design of that prior art circuit is complex. It is the object of the invention to overcome the drawbacks of the prior art. This object is solved by the subject matter of claim 1. The present invention according to Claim 1 provides a parallel/serial interface circuit for receiving input clock pulses, the time between input clock pulses being the time T, and n data bits (D1-D4) per input clock pulse in parallel on n data lines, and for outputting said n data bits serially on a single output data line at the rate of n bits per input clock pulse, comprising: means, comprising a phased lock loop responsive to said input clock pulse for generating one of the output clocks, and delay means for delaying an output clock T/n from the previous output clock, and means responsive to said n data bits and said n output clocks for outputting one data bit for each output clock. This invention accomplishes the conversion by using a digital phase locked loop to generate new clocks which have the same frequency as the original system clock, but that differ in phase. These new clocks can be used to gate (shift) out the data serially, without the need for higher frequency clocks. A numerical example, using four clocks can be used to explain the conversion. A digital phase locked loop is set up to generate a stabilized clock from the system clock which is locked to the system clock with some jitter accuracy; this locked clock becomes phase 1, and to determine the time period T for this clock frequency . The phase 1 clock is delayed by T/4 to create phase 2. Phase 1 is inverted to create phase 3 and phase 2 is inverted to create phase 4. The four phases are now used to gate the four bits to the four data inputs of a multiplexer, and phase 1 and 2 are used as inputs to the two multiplexer control lines. The result is that the output of the multiplexer during one clock period (T) will be the four input bits in series. The circuit according to the invention is useful in image enhancement applications where each image generator output bit (each pixel of the bitmap) is presented to an image enhancement circuit, which generates a number of parallel output bits for each input bit. These must then be serialized prior to being sent to the ROS (raster output scanner). European patent application No. 94 120 610.4, filed 23.12.94, describes such a system. This invention offers a solution to the high speed data serialization problem associated with the hardware implementation of image enhancement algorithms (dilation, appearance tuning, edge enhancement, etc). The problem arises because through image enhancement, the single-bit pixel becomes algorithmically enhanced into a multi-bit pixel (usually 4 or 5 bits per pixel) and therefore 4 or 5 times as much information needs to be serially sent to the ROS in the same pixel clock cycle. For example, for a 50 MHz base pixel clock and 5 bits of high addressability, the effective data transfer rate from image enhancement logic to ROS will be 250 mega-bits per second (Mb/S) on a serial interface. The technique presented here relies on a digital phase locked loop and multiple phase generation of the base clock and thus avoids the generation of higher frequency clocks. Also, all possible combinations of high addressability codes (e.g. all 2 4 or 2 5 combinations of bits) can be processed by this design. Further, this technique can be implemented as a stand alone ASIC (application specific integrated circuit) device, or can be easily integrated into an image enhancement ASIC whose main function is template matching and selection (see the abovementioned application 94 120 610.4). Embodiments of the invention will now be described, by way of example, wih reference to the accompanying drawings, in which: Fig 1 is a block diagram of an image enhancement circuit using this invention; Fig 2 is a circuit diagram showing the multiplexer which serializes the data; and Fig 3 is a timing diagram for the circuit of Fig. 1. Fig 1 illustrates the basic block diagram and data flow for an ASIC implementation of an image enhancement algorithm using the parallel to serial interface according to the invention. In all cases the examples are shown using 4 data bits per clock period and 4 clock phases, but any other number could be used. Using current processes, this circuit can be fabricated using CMOS (complementary metal-oxide semiconductor) technology for output data rates of up to 200 Mb/s (for 0.8 µm CMOS technology) to the ROS, or BiCMOS technology for higher data rates The image data from the image generation circuits (e.g. the bitmap) are are fed to the FIFO 12 which stores them and then supplies them 5 (or 7 or 9) bits at a time, in this numerical example, to the image enhancement application specific integrated circuit (IE ASIC), which is described in detail in the above-mentioned application 94 120 610.4. For the purposes of this description, let us assume that the template logic circuit 13 creates a 5 x 5 bit window from the input bits, compares the result to a number of templates, and generates a 4-bit output for each system clock received from the clock generator 15. The other part of the image enhancement ASIC is the FIFO control 14 which controls the FIFO 12 to output data to the template logic circuit 13. The output from system clock 15 and the template logic 13 output data, D1-D4, are supplied to the conversion logic 1 1 which serialize the 4 bits of output data. For timing purposes, the original data D1 - D4 is latched through register 16 to become data d1-d4, prior to being applied to the serializer 22. The phase locked loop 17 receives the system clock and outputs an identical clock to the clock buffer 20 which generates the inverse and applies both to serializer 22 as phases 1 and 3. The delay circuit 19 receives the phase 1 clock output from the phase lock loop 17 and delays it by a quarter of a period (T/4) to create, through buffer 21, phases 2 and 4. To the extent that the delay may be slightly more or less than a quarter of a clock period, the delay is compared in comparator 18 to a quarter of a clock period timing signal from the phase locked loop 17 in a feedback loop, and a correction is applied to the delay if necessary. The relative timing of the various signals can be explained with reference to the circuit of Fig.2 and the timing diagram of Fig.3. The master clock, MCLK, received from the system clock generator 15, is applied to the digital phase locked loop 17, which generates an identical clock (which is locked to the input clock MCLK) at its output, CLK1, which is also used as the phase 1 clock. The clock buffer 20 also inverts this and produces the phase 3 clock. Delay 19 produces a clock delayed by a quarter of the time period of the clock to produce phase 2 and clock buffer 21 inverts that to produce phase 4. Assuming that the first set of enhanced image data bits is at lines D1-D4 as shown, then at T1, at the time of the rising edge of the phase 1 clock, that data will be clocked onto the d1-d4 lines. At T2, the phase 2 clock goes high, clocking the d1 bit into flip flop 22 of Fig 2. However, although this data is now available at the output of flip flop 25, the control lines of the multiplexer, phase 1 and phase 2, at this point are still 11. It is only at T3, when phase 1 goes to zero, and that the data will propagate through the multiplexer as the output, as shown on the timing diagram, which shows I1-d1 being output starting at T3. By the same reasoning, I1-d2 is output beginning at T4, I1-d3 is output at T5, etc. These timing relationships can be described more compactly as follows. d1: sample with phase 2, send out with phase 3. d2: sample with phase 3, send out with phase 4. d3: sample with phase 4, send out with phase 1. d4: sample with phase 1, send out with phase 2. The result is a circuit that will serialize data without the requirement of a higher frequency clock pulse generator.

Claims: A parallel/serial interface circuit for receiving input clock pulses having a period T and n data bits (D1-D4) per input clock pulse in parallel on n data lines, and for outputting said n data bits serially on a single output data line during each clock period of said input clock pulses, comprising means for generating n output clocks, each output clock being delayed T/n from the previous output clock, including: a delay means (19) responsive to a first output clock for generating a second output clock which is delayed relative to the first output clock, characterized in that the means for generating n output clocks further includes: a phase locked loop (17) being locked to said input clock pulses, for generating the first output clock and a clock period timing signal, both having the same frequency as said input clock pulses, a comparator (18) for comparing the delay of the second output clock relative to the first output clock and a fraction of the clock period timing signal and outputting a correction signal which is coupled to the delay means for adjusting the delay, and an inverter (21) for receiving and inverting at least one of the generated output clocks to generate a third output clock; and in that the interface circuit comprises means (12 -16, 22) responsive to said n data bits and said n output clocks for outputting one data bit (D1-D4) for each output clock.

Inventors: ROSTAMIAN FARHAD, ROSTAMIAN, FARHAD
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1995
Main classification: H03K5
IPC codes: G06F5, H03M9, H03K5