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HCTL2000 Datasheet(PDF) 15 Page - Agilent(Hewlett-Packard) |
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HCTL2000 Datasheet(HTML) 15 Page - Agilent(Hewlett-Packard) |
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15 / 19 page  15 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 HCTL- 2020. This line selects the low or high byte of data from the HCTL- 2020. 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 HCTL- 2020. Figure 16 shows the interface timing for the circuit. Address Function CXXX Reset Counters 4XXX Enable High Byte on Data Lines 2XX0 Enable Mid Byte on Data Lines 2XX1 Enable Low Byte on Data Lines Read Example LDX 2000 Loads mid byte and then low byte into STX 0100 memory locations 0100 and 0101 LDAA 4000 Loads the high byte into memory STAA 0102 location 0102 Figure 15. Memory Addresses and Read Example. |
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