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CS5535-UDC Datasheet(PDF) 102 Page - National Semiconductor (TI) |
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CS5535-UDC Datasheet(HTML) 102 Page - National Semiconductor (TI) |
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102 / 555 page  www.national.com 102 Revision 0.8 PIT Functional Description (Continued) The read-back command present in the 8254 PIT is used to determine the current counter value and its status like counting format, the counting mode, the low-order or high- order byte or both being read or written, and the status of its output. Figure 4-16 shows the format of the read-back command. The two most significant bits define the read- back command with their value 11b. CT and ST indicate that the value and the status of the counter are to be deter- mined respectively. The C0-C2 bits define the counter whose value or status is to be determined. With the read- back command, several counter latch commands can be issued in parallel by indicating several counters simulta- neously with the C0-C2 bits. The 8254 then behaves as if several counter latch commands have been issued individ- ually, and transfers the individual count values into the out- put latches of each counter. All successive counter latch commands, whether issued by its own counter or a next read-back command, are ignored if the counter concerned has not been read by counter read instructions. To deter- mine the programmed mode of a particular counter, set CT = 1 and ST = 0. The read-back command latches the current mode and provides a status byte (see Figure 4-17) at the port address of the counter concerned. This status byte is fetched by a counter read instruction. The PIN bit indicates the current status of the concerned counter’s output pin. If PIN = 1, then the counter output is at logic 1, else at logic 0. Bit 0 shows whether the last written counter value has already been transferred to the counter. Not before zero = 0 is it meaningful to read back the counter value. Figure 4-16. PIT Read-Back Command Format Figure 4-17. PIT Status Byte Format CT: Determine count value of selected counter. 0 = Determine count value. 1 = Do not determine count value. ST: Determine status of selected counter. 0 = Determine count status. 1 = Do not determine count status. C2, C1, C0: Counter selection. 0 = Counter not selected. 1 = Counter selected. 7654321 0 CT ST C2 C1 C0 x 1 1 PIN: Status of counter output pin: 0 = Output pin low. 1 = Output pin high. LOAD: Is counter loaded with a start value? 0 = Counter loaded, count value can be read. 1 = Counter not yet loaded, count value cannot be read. LH: Corresponds to bits [5:4] of the Control Word register. MODE: Corresponds to bits [3:1] of the Control Word register. BCD: Corresponds to bit 0 of the Control Word register. 76543210 LH MODE BCD LOAD PIN |
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