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Quadrature Decoder/Counter Interface ICs Technical Data
HCTL-2000 HCTL-2016 HCTL-2020
Features
* Interfaces Encoder to Microprocessor * 14 MHz Clock Operation * Full 4X Decode * High Noise Immunity: Schmitt Trigger Inputs Digital Noise Filter * 12 or 16-Bit Binary Up/ Down Counter * Latched Outputs * 8-Bit Tristate Interface * 8, 12, or 16-Bit Operating Modes * Quadrature Decoder Output Signals, Up/Down and Count * Cascade Output Signals, Up/ Down and Count * Substantially Reduced System Software
Applications
* Interface Quadrature Incremental Encoders to Microprocessors * Interface Digital Potentiometers to Digital Data Input Buses
Description
The HCTL-2000, 2016, 2020 are CMOS ICs that perform the quadrature decoder, counter, and bus interface function. The HCTL-20XX family is designed to improve system performance
Devices
Part Number HCTL-2000 HCTL-2016 HCTL-2020 Description 12-bit counter. 14 MHz clock operation. All features of the HCTL-2000. 16-bit counter. All features of the HCTL-2016. Quadrature decoder output signals. Cascade output signals. Package Drawing A A B
ESD WARNING: Standard CMOS handling precautions should be observed with the HCTL-20XX family ICs.
5965-5894E
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in digital closed loop motion control systems and digital data input systems. It does this by shifting time intensive quadrature decoder functions to a cost effective hardware solution. The entire HCTL-20XX family consists of a 4x quadrature decoder, a binary up/down state counter,
and an 8-bit bus interface. The use of Schmitt-triggered CMOS inputs and input noise filters allows reliable operation in noisy environments. The HCTL-2000 contains a 12-bit counter. The HCTL-2016 and 2020 contain a 16-bit counter. The HCTL-2020 also contains quadrature decoder
output signals and cascade signals for use with many standard counter ICs. The HCTL20XX family provides LSTTL compatible tri-state output buffers. Operation is specified for a temperature range from -40 to +85C at clock frequencies up to 14 MHz.
MOTION SENSING AND CONTROL
25.91 0.25 (1.02 0.010)
Package Dimensions
19.05 0.25 (0.750 0.010)
15 1.52 0.13 (0.060 0.005) 9.40 (0.370) 15
Operating Characteristics
Table 1. Absolute Maximum Ratings (All voltages below are referenced to VSS) Parameter DC Supply Voltage Input Voltage Storage Temperature Operating Temperature Symbol VDD VIN TS TA[1] Limits -0.3 to +5.5 -0.3 to VDD +0.3 -40 to +125 -40 to +85 Units V V C C
Table 2. Recommended Operating Conditions Parameter DC Supply Voltage Ambient Temperature Symbol VDD TA[1] Limits +4.5 to +5.5 -40 to +85 Units V C
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Table 3. DC Characteristics VDD = 5 V 5%; TA = -40 to 85C Symbol VIL[2] VIH[2] VT+ VTVH IIN VOH[2] VOL[2] IOZ IDD CIN COUT Parameter Low-Level Input Voltage High-Level Input Voltage Schmitt-Trigger PositiveGoing Threshold Schmitt-Trigger NegativeGoing Threshold Schmitt-Trigger Hysteresis Input Current High-Level Output Voltage Low-Level Output Voltage High-Z Output Leakage Current Quiescent Supply Current Input Capacitance Output Capacitance VIN = VSS or VDD IOH -1.6 mA IOL = +4.8 mA VO = VSS or VDD VIN = VSS or VDD, VO = HiZ Any Input[3] Any Output[3] -10 1.0 1.0 -10 2.4 3.5 3.5 1.5 2.0 1 4.5 0.2 1 1 5 6 0.4 +10 5 +10 4.0 Condition Min. Typ. Max. 1.5 Unit V V V V V A V V A A pF pF
Notes: 1. Free air. 2. In general, for any VDD between the allowable limits (+4.5 V to +5.5 V), VIL = 0.3 VDD and VIH = 0.7 VDD; typical values are VOH = VDD - 0.5 V @ IOH = -40 A and VOL = VSS + 0.2 V @ IOL = 1.6 mA. 3. Including package capacitance.
Figure 1. Reset Waveform.
Figure 2. Waveform for Positive Clock Related Delays.
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Functional Pin Description
Table 4. Functional Pin Descriptions Symbol VDD VSS CLK CHA CHB Pin Pin 2000/2016 2020 16 8 2 7 6 20 10 2 9 8 Power Supply Ground CLK is a Schmitt-trigger input for the external clock signal. CHA and CHB are Schmitt-trigger inputs which accept the outputs from a quadrature encoded source, such as incremental optical shaft encoder. Two channels, A and B, nominally 90 degrees out of phase, are required. This active low Schmitt-trigger input clears the internal position counter and the position latch. It also resets the inhibit logic. RST is asynchronous with respect to any other input signals. This CMOS active low input enables the tri-state output buffers. The OE and SEL inputs are sampled by the internal inhibit logic on the falling edge of the clock to control the loading of the internal position data latch. This CMOS input directly controls which data byte from the position latch is enabled into the 8-bit tri-state output buffer. As in OE above, SEL also controls the internal inhibit logic. SEL 0 1 CNTDCDR U/D 16 5 BYTE SELECTED High Low
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Description
RST
5
7
OE
4
4
SEL
3
3
A pulse is presented on this LSTTL-compatible output when the quadrature decoder has detected a state transition. This LSTTL-compatible output allows the user to determine whether the IC is counting up or down and is intended to be used with the CNTDCDR and CNTCAS outputs. The proper signal U (high level) or D (low level) will be present before the rising edge of the CNTDCDR and CNTCAS outputs. A pulse is presented on this LSTTL-compatible output when the HCTL-2020 internal counter overflows or underflows. The rising edge on this waveform may be used to trigger an external counter. These LSTTL-compatible tri-state outputs form an 8-bit output port through which the contents of the 12/16-bit position latch may be read in 2 sequential bytes. The high byte, containing bits 8-15, is read first (on the HCTL-2000, the most significant 4 bits of this byte are set to 0 internally). The lower byte, bits 0-7, is read second.
CNTCAS
15
D0 D1 D2 D3 D4 D5 D6 D7 NC
1 15 14 13 12 11 10 9
1 19 18 17 14 13 12 11 6
Not connected - this pin should be left floating.
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Switching Characteristics
Table 5. Switching Characteristics Min/Max specifications at VDD = 5.0 5%, TA = -40 to + 85C. Symbol Description 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 tCLK tCHH tCD[1] tODE tODZ tSDV tCLH tSS
[2]
Min. 70 28
Max.
Units ns ns
Clock period Pulse width, clock high Delay time, rising edge of clock to valid, updated count information on D0-7 Delay time, OE fall to valid data Delay time, OE rise to Hi-Z state on D0-7 Delay time, SEL valid to stable, selected data byte (delay to High Byte = delay to Low Byte) Pulse width, clock low Setup time, SEL before clock fall Setup time, OE before clock fall Hold time, SEL after clock fall Hold time, OE after clock fall Pulse width, RST low Hold time, last position count stable on D0-7 after clock rise Hold time, last data byte stable after next SEL state change Hold time, data byte stable after OE rise Delay time, U/D valid after clock rise Delay time, CNTDCDR or CNTCAS high after clock rise Delay time, CNTDCDR or CNTCAS low after clock fall Hold time, U/D stable after clock rise Setup time, U/D valid before CNTDCDR or CNTCAS rise Hold time, U/D stable after CNTDCDR or CNTCAS rise
65 65 40 65 28 20 20 0 0 28 10 5 5 45 45 45 10 tCLK-45 tCLK-45
ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
tOS[2] tSH[2] tOH[2] tRST tDCD tDSD tDOD tUDD tCHD tCLD tUDH tUDCS tUDCH
Notes: 1. tCD specification and waveform assume latch not inhibited. 2. tSS, tOS, tSH, tOH only pertain to proper operation of the inhibit logic. In other cases, such as 8 bit read operations, these setup and hold times do not need to be observed.
Figure 3. Tri-State Output Timing.
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Figure 4. Bus Control Timing.
Figure 5. Decoder, Cascade Output Timing (HCTL-2020 only).
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Operation
A block diagram of the HCTL20XX family is shown in Figure 6. The operation of each major function is described in the following sections.
Figure 6. Simplified Logic Diagram.
Digital Noise Filter
The digital noise filter section is responsible for rejecting noise on the incoming quadrature signals. The input section uses two techniques to implement improved noise rejection. Schmitt-trigger inputs and a three-clock-cycle delay filter combine to reject low level noise and large, short duration noise spikes that typically occur in motor system applications. Both common mode and differential mode noise are rejected. The user benefits from these techniques by improved integrity of the data in
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the counter. False counts triggered by noise are avoided. Figure 7 shows the simplified schematic of the input section. The signals are first passed through a Schmitt trigger buffer to address the problem of input signals with slow rise times and low level noise (approximately < 1 V). The cleaned up signals are then passed to a four-bit delay filter. The signals on each channel are sampled on rising clock edges. A time history of the signals is stored in the four-bit shift register. Any change on the
input is tested for a stable level being present for three consecutive rising clock edges. Therefore, the filtered output waveforms can change only after an input level has the same value for three consecutive rising clock edges. Refer to Figure 8 which shows the timing diagram. The result of this circuitry is that short noise spikes between rising clock edges are ignored and pulses shorter than two clock periods are rejected.
Figure 7. Simplified Digital Noise Filter Logic.
Figure 8. Signal Propagation through Digital Noise Filter.
Quadrature Decoder
The quadrature decoder decodes the incoming filtered signals into count information. This circuitry multiplies the resolution of the input signals by a factor of four (4X decoding). When using an encoder for motion sensing, the user benefits from the increased resolution by being able to provide better system control. The quadrature decoder samples the outputs of the CHA and CHB filters. Based on the past binary state of the two signals and the present state, it outputs a count signal and a direction signal to
the internal position counter. In the case of the HCTL-2020, the signals also go to external pins 5 and 16 respectively. Figure 9 shows the quadrature states and the valid state transitions. Channel A leading channel B results in counting up. Channel B leading channel A results in counting down. Illegal state transitions, caused by faulty encoders or noise severe enough to pass through the filter, will produce an erroneous count.
Design Considerations
The designer should be aware that the operation of the digital filter places a timing constraint on the relationship between incoming quadrature signals and the external clock. Figure 8 shows the timing waveform with an incremental encoder input. Since an input has to be stable for three rising clock edges, the encoder pulse width (tE - low or high) has to be greater than three clock periods (3tCLK). This guarantees that the asynchronous input will be stable during three consecutive rising clock edges. A realistic design also has to take
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into account finite rise times of the waveforms, asymmetry of the waveforms, and noise. In the presence of large amounts of noise, tE should be much greater than 3tCLK to allow for the interruption of the consecutive level sampling by the three-bit delay filter. It should be noted that a change on the inputs that is qualified by the filter will internally propagate in a maximum of seven clock periods. The quadrature decoder circuitry imposes a second timing constraint between the external clock and the input signals. There must be at least one clock period between consecutive quadrature states. As shown in Figure 9, a quadrature state is defined by consecutive edges on both
channels. Therefore, tES (encoder state period) > tCLK. The designer must account for deviations from the nominal 90 degree phasing of input signals to guarantee that tES > tCLK.
Position Counter
This section consists of a 12-bit (HCTL-2000) or 16-bit (HCTL2016/2020) binary up/down counter which counts on rising clock edges as explained in the Quadrature Decoder Section. All 12 or 16 bits of data are passed to the position data latch. The system can use this count data in several ways: A. System total range is 12 or 16 bits, so the count represents "absolute" position. B. The system is cyclic with 12 or 16 bits of count per cycle. RST is used to reset the counter every cycle and the system uses the data to interpolate within the cycle. C. System count is > 8, 12, or 16 bits, so the count data is used as a relative or incremental position input for a system software computation of absolute position. In this case counter rollover occurs. In order to prevent loss of position information, the processor must read the outputs of the IC before the count increments one-half of the maximum count capabil-
ity (i.e. 127. 2047, or 32,767 quadrature counts). Two'scomplement arithmetic is normally used to compute position from these periodic position updates. Three modes can be used: 1. The IC can be put in 8-bit mode by tying the SEL line high, thus simplifying IC interface. The outputs must then be read at least once every 127 quadrature counts. 2. The HCTL-2000 can be used in 12-bit mode and sampled at least once every 2047 quadrature counts. 3. The HCTL-2016 or 2020 can be used in 16-bit mode and sampled at least once every 32,767 quadrature counts. D. The system count is > 16 bits so the HCTL-2020 can be cascaded with other standard counter ICs to give absolute position.
Position Data Latch
The position data latch is a 12/ 16-bit latch which captures the position counter output data on each rising clock edge, except when its inputs are disabled by the inhibit logic section during two-byte read operations. The output data is passed to the bus interface section. When active, a signal from the inhibit logic section prevents new data from being captured by the latch, keeping the data stable while successive reads are made through the bus section. The latch is automatically reenabled at the end of these reads. The latch is cleared to 0 asynchronously by the RST signal.
Figure 9. 4x Quadrature Decoding.
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Inhibit Logic
The Inhibit Logic Section samples the OE and SEL signals on the falling edge of the clock and, in response to certain conditions (see Figure 10 below), inhibits the position data latch. The RST signal asynchronously clears the inhibit logic, enabling the latch. A simplified logic diagram of the inhibit circuitry is illustrated in Figure 11.
output and whether or not the output bus is in the high-Z state. In the case of the HCTL-2000 the data latch is only 12 bits wide and the upper four bits of the high byte are internally set to zero.
Bus Interface
The bus interface section consists of a 16 to 8 line multiplexer and an 8-bit, three-state output buffer. The multiplexer allows independent access to the low and high bytes of the position data latch. The SEL and OE signals determine which byte is Inhibit Signal 1 1 0
The quadrature decoder output section consists of count and up/ down outputs derived from the 4X decode logic of the HCTL2020. When the decoder has detected a count, a pulse, onehalf clock cycle long, will be output on the CNTDCDR pin. This output will occur during the clock cycle in which the internal counter is updated. The U/D pin
Cascade Output (HCTL2020 Only)
The cascade output also consists of count and up/down outputs. When the HCTL-2020 internal counter overflows or underflows, a pulse, one-half clock cycle long, will be output on the CNTCAS pin. This output will occur during the clock cycle in which the internal counter is updated. The U/D pin will be set to the proper voltage level one clock cycle before the rising edge of the CNTCAS pulse, and held one clock cycle after the rising edge of the CNTCAS pulse. These outputs are not affected by the inhibit logic. See Figures 5 and 12 for detailed timing.
Step SEL 1 2 3 L H X
OE CLK L L H
Action Set inhibit; read high byte Read low byte; starts reset Completes inhibit logic reset
Figure 10. Two Byte Read Sequence.
Figure 11. Simplified Inhibit Logic.
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Quadrature Decoder Output (HCTL-2020 Only)
will be set to the proper voltage level one clock cycle before the rising edge of the CNTDCDR pulse, and held one clock cycle after the rising edge of the CNTDCDR pulse. These outputs are not affected by the inhibit logic. See Figures 5 and 12 for detailed timing.
INTERNAL COUNTER ROLL OVER CLK
FF 00 CHAFILT* START INHIBIT
CHBFILT*
U/D
CNTDCDR*
CNTCAS
COUNT
FFFDH
FFFEH
FFFFH
0000H
FFFFH
FFFDH
*CHAFILT AND CHBFILT ARE THE OUTPUTS OF THE DIGITAL NOISE FILTER (SEE FIGURES 7 AND 8).
Figure 12. Decode and Cascade Output Diagram.
Cascade Considerations (HCTL-2020 Only)
The HCTL-2020's cascading system allows for position reads of more than two bytes. These reads can be accomplished by latching all of the bytes and then reading the bytes sequentially over the 8-bit bus. It is assumed here that, externally, a counter followed by a latch is used to count any count that exceeds 16 bits. This configuration is compatible with the HCTL-2020 internal counter/latch combination. Consider the sequence of events for a read cycle that starts as the HCTL-2020's internal counter rolls over. On the rising clock edge, count data is updated in the internal counter, rolling it over. A count-cascade pulse (CNTCAS)
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will be generated with some delay after the rising clock edge (tCHD). There will be additional propagation delays through the external counters and registers. Meanwhile, with SEL and OE low to start the read, the internal latches are inhibited at the falling edge and do not update again till the inhibit is reset. If the CNTCAS pulse now toggles the external counter and this count gets latched a major count error will occur. The count error is because the external latches get updated when the internal latch is inhibited. Valid data can be ensured by latching the external counter data when the high byte read is started (SEL and OE low). This latched external byte corresponds to the
count in the inhibited internal latch. The cascade pulse that occurs during the clock cycle when the read begins gets counted by the external counter and is not lost. For example, suppose the HCTL2020 count is at FFFFH and an external counter is at F0H, with the count going up. A count occurring in the HCTL-2020 will cause the counter to roll over and a cascade pulse will be generated. A read starting on this clock cycle will show FFFFH from the HCTL2020. The external latch should read F0H, but if the host latches the count after the cascade signal propagates through, the external latch will read F1H.
General Interfacing
The 12-bit (HCTL-2000) or 16-bit (HCTL-2016/2020) latch and inhibit logic allows access to 12 or 16 bits of count with an 8-bit bus. When only 8-bits of count are required, a simple 8-bit (1byte) mode is available by holding SEL high continuously. This disables the inhibit logic. OE provides control of the tri-state bus, and read timing is shown in Figures 2 and 3.
For proper operation of the inhibit logic during a two-byte read, OE and SEL must be synchronous with CLK due to the falling edge sampling of OE and SEL. The internal inhibit logic on the HCTL-20XX family inhibits the transfer of data from the counter to the position data latch during the time that the latch outputs are being read. The inhibit logic allows the microprocessor to first
read the high order 4 or 8 bits from the latch and then read the low order 8 bits from the latch. Meanwhile, the counter can continue to keep track of the quadrature states from the CHA and CHB input signals. Figure 11 shows the simplified inhibit logic circuit. The operation of the circuitry is illustrated in the read timing shown in Figure 13.
*OE can consist of two short low pulses, as well as one long pulse, and still satisfy the inhibit logic sequence. During the time that OE is high, the data lines are tri-seated.
Figure 13. Typical Interface Timing.
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Actions
1. On the rising edge of the clock, counter data is transferred to the position data latch, provided the inhibit signal is low. 2. When OE goes low, the outputs of the multiplexer are enabled onto the data lines. If SEL is low, then the high order data bytes are enabled onto the data lines. If SEL is high, then the low order data bytes are enabled onto the data lines.
3. When the IC detects a low on OE and SEL during a falling clock edge, the internal inhibit signal is activated. This blocks new data from being transferred from the counter to the position data latch. 4. When SEL goes high, the data outputs change from the high byte to the low byte. 5. The first of two reset conditions for the inhibit logic is met when the IC detects a logic high on SEL and a logic
low on OE during a falling clock edge. 6. When OE goes high, the data lines change to a high impedance state. 7. The IC detects a logic high on OE during a falling clock edge. This satisfies the second reset condition for the inhibit logic.
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Interfacing the HCTL-2020 to a Motorola 6802/8 and Cascading the Counter for 24 Bits
Figure 14. A Circuit to Interface to the 6802/8.
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In this circuit an interface to a Motorola 6802/8 and a cascading scheme for a 24-bit counter are shown. This circuit provides a minimum part count by: 1) using two 74LS697 Up/Down counters with output registers and tri-state outputs and 2) using a Motorola 6802/8 LDX instruction which stores 16 bits of data into the index registers in two consecutive clock cycles. The HCTL-2020 OE and the 74LS697 G lines are decoded from Address lines A15-A13. This results in counter data being enabled onto the bus whenever an external memory access is made to locations 4XXX or 2XXX. Address line A12 and processor clock E enables the 74LS138. The processor clock E is also
used to clock the HCTL-2020. Address AO is connected directly to the SEL pin on the HCTL2020. This line selects the low or high byte of data from the HCTL2020. Cascading is accomplished by connecting the CNTCAS output on the HCTL-2020 with the counter clock (CCK) input on both 74LS697s. The U/D pin on the HCTL-2020 and the U/D pin on both 74LS697s are also directly connected for easy expansion. The RCO of the first 4-bit 74LS697 is connected to the ENT pin of the second 74LS697. This enables the second counter only when there is a RCO signal on the first counter. This configuration allows the 6802 to read both data bytes with
a single double-byte fetch instruction (LDX 2XX0). This instruction is a five cycle instruction which reads external memory location 2XX0 and stores the high order byte into the high byte of the index register. Memory location 2XX1 is next read and stored in the low order byte of the index register. The high byte of counter data is clocked into the 74LS697 registers when SEL is low and OE goes low. This upper byte can be read at any time by pulling the 74LS697 G low when reading address 4XXX. Figure 15 shows memory addresses and gives an example of reading the HCTL2020. Figure 16 shows the interface timing for the circuit.
Address CXXX 4XXX 2XX0 2XX1
Function Reset Counters Enable High Byte on Data Lines Enable Mid Byte on Data Lines Enable Low Byte on Data Lines Read Example
LDX 2000 STX 0100 LDAA 4000 STAA 0102
Loads mid byte and then low byte into memory locations 0100 and 0101 Loads the high byte into memory location 0102
Figure 15. Memory Addresses and Read Example.
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HCTL-2020 INTERNAL CLOCK
FFFF
0000
CLK
SEL
OE
INTERNAL INHIBIT
CNTCAS
RCK
G
DATA BUS
HIGH Z
MID BYTE
LOW BYTE
HIGH BYTE
ACTIONS
1
1
1 2 3
4
5 6
7
8
9
10
Figure 16. Interface Timing for the 6802/8.
Actions
1. The microprocessor clock output is E. If the internal HCTL-2020 inhibit is not active, new data is transferred from the internal counter to the position data latch. 2. An even address output from the 6802 causes SEL to go low. When E goes high, the address decoder output for the HCTL-2020 OE signal goes low. This causes the HCTL-2020 to output the middle byte of the system counter (high byte of the HCTL-2020 counter). This middle byte, FFFFH is available at (2) through (4), the first time OE is low. In this example an overflow
has occurred and OE has been pulled low to start a read cycle. SEL and OE are gated to give RCK which latches the external high byte, equal to 00H. The falling edge, of the CNTCAS signal counts up the external counter to 0001H. 3. With the first negative edge of the clock after SEL and OE are low the internal latches are inhibited from counting and the 6802 reads the high byte in. 4. OE goes high and the data bus goes into a high impedance state. 5. OE is low and SEL is high and the low byte is enabled onto the data bus. The low byte is valid through (7).
6. With the first negative edge after OE and SEL go high, the first of the two HCTL2020 inhibit reset conditions is met and the 6802 reads the low byte in. 7. The data bus returns to the high impedance state, when OE goes high. 8. With the first negative edge of the clock after OE goes high, inhibit reset is complete. 9. With the positive going edge of the clock, G is asserted and the external high byte, 00H is available on the data bus from 9 through 10 and the 6802 reads the high byte in at (10).
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Interfacing the HCTL20XX to an Intel 8748
The circuit shown in Figure 17 shows the connections between an HCTL-20XX and an 8748. Data lines D0-D7 are connected to the 8748 bus port. Bits 0 and 1 of port 1 are used to control the OE and SEL inputs of the HCTL20XX respectively. T0 is used to provide a clock signal to the HCTL-20XX. The frequency of T0
is the crystal frequency divided by 3. T0 must be enabled by executing the ENT0 CLK instruction after each system reset, but prior to the first encoder position change. An 8748 program which interfaces to the circuit in Figure 17 is given in Figure 18. The resulting interface timing is shown in Figure 19.
* NOTE: PIN NUMBERS ARE DIFFERENT FOR THE HCTL-2020.
Figure 17. An HCTL-20XX-to-Intel 8748 Interface.
LOC 000 002 003 004 006 008 009 00B
Object Code 99 00 08 A8 89 02 08 A9 89 03 93
Source Statements ANL P1, 00H INS A, BUS MOVE R0, A ORL P1, 02H INS A, BUS MOV R1, A ORL P1, 03H RETR
Comments Enable output and higher order bits Load higher order bits into ACC Move data to register 0 Enable output and lower order bits Load order bits into AC Move data to register 1 Disable outputs Return
Figure 18. A Typical Program for Reading HCTL-20XX with an 8748.
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ANL P1, OOH
ORL P1, 02H
Figure 19. 8748 READ Cycle from Figure 18.
Actions
1. ANL P1, 00H has just been executed. The output of bits 0 and 1 of Port 1 cause SEL and OE to be logic low. The data lines output the higher order byte. 2. The HCTL-20XX detects that OE and SEL are low on the next falling edge of the CLK and asserts the internal inhibit signal. Data can be read without regard for the phase of the CLK. 3. INS A, BUS has just been executed. Data is read into the 8748. 4. ORL PORT 1, 02H has just been executed. The program sets SEL high and leaves OE low by writing the correct values to port 1. The HCTL-
20XX detects OE is low and SEL is high on the next falling edge of the CLK, and thus the first inhibit reset condition is met. 5. INS A, BUS has just been executed. Lower order data bits are read into the 8748. 6. ORL P1, 03H has just been executed. The HCTL-20XX detects OE high on the next falling edge of CLK. The program sets OE and SEL high by writing the correct values to port 1. This causes the data lines to be tristated. This satisfies the second inhibit and reset condition. On the next rising CLK edge new data is transferred from the counter to the position data latch.
Additional Information from Hewlett-Packard
Application briefs are available from the factory. Please contact your local HP sales representative for the following. M027 Interfacing the HCTL-20XX to the 8051 M019 Commonly Asked Questions about the HCTL2020 and Answers M020 A Simple Interface for the HCTL-2020 with a 16-bit DAC without Using a Processor M023 Interfacing the MC68HCII to the HCTL-2020
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