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IS82C54Z Datasheet(PDF) 10 Page - Intersil Corporation |
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IS82C54Z Datasheet(HTML) 10 Page - Intersil Corporation |
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10 / 22 page  10 82C54 the counters selected by setting their corresponding bits D3, D2, D1 = 1. The read-back command may be used to latch multiple counter output latches (OL) by setting the COUNT bit D5 = 0 and selecting the desired counter(s). This signal command is functionally equivalent to several counter latch commands, one for each counter latched. Each counter’s latched count is held until it is read (or the counter is reprogrammed). That counter is automatically unlatched when read, but other counters remain latched until they are read. If multiple count read-back commands are issued to the same counter without reading the count, all but the first are ignored; i.e., the count which will be read is the count at the time the first read-back command was issued. The read-back command may also be used to latch status information of selected counter(s) by setting STATUS bit D4 = 0. Status must be latched to be read; status of a counter is accessed by a read from that counter. The counter status format is shown in Figure 6. Bits D5 through D0 contain the counter’s programmed Mode exactly as written in the last Mode Control Word. OUTPUT bit D7 contains the current state of the OUT pin. This allows the user to monitor the counter’s output via software, possibly eliminating some hardware from a system. NULL COUNT bit D6 indicates when the last count written to the counter register (CR) has been loaded into the counting element (CE). The exact time this happens depends on the Mode of the counter and is described in the Mode Definitions, but until the counter is loaded into the counting element (CE), it can’t be read from the counter. If the count is latched or read before this time, the count value will not reflect the new count just written. The operation of Null Count is shown below. THIS ACTION: CAUSES: A. Write to the control word register:(1) . . . . Null Count = 1 B. Write to the count register (CR):(2) . . . . . Null Count = 1 C. New count is loaded into CE (CR - CE) . . Null Count = 0 1. Only the counter specified by the control word will have its null count set to 1. Null count bits of other counters are unaffected. 2. If the counter is programmed for two-byte counts (least significant byte then most significant byte) null count goes to 1 when the second byte is written. If multiple status latch operations of the counter(s) are performed without reading the status, all but the first are ignored; i.e., the status that will be read is the status of the counter at the time the first status read-back command was issued. Both count and status of the selected counter(s) may be latched simultaneously by setting both COUNT and STATUS bits D5, D4 = 0. This is functionally the same as issuing two separate read-back commands at once, and the above discussions apply here also. Specifically, if multiple count and/or status read-back commands are issued to the same counter(s) without any intervening reads, all but the first are ignored. This is illustrated in Figure 7. If both count and status of a counter are latched, the first read operation of that counter will return latched status, regardless of which was latched first. The next one or two reads (depending on whether the counter is programmed for one or two type counts) return latched count. Subsequent reads return unlatched count. A0, A1 = 11; CS = 0; RD = 1; WR = 0 D7 D6 D5 D4 D3 D2 D1 D0 1 1 COUNT STATUS CNT 2CNT 1CNT 0 0 D5:0=Latch count of selected Counter (s) D4:0=Latch status of selected Counter(s) D3:1=Select Counter 2 D2:1=Select Counter 1 D1:1=Select Counter 0 D0:Reserved for future expansion; Must be 0 FIGURE 5. READ-BACK COMMAND FORMAT D7 D6 D5 D4 D3 D2 D1 D0 OUTPUT NULL COUNT RW1 RW0 M2 M1 M0 BCD D7:1=Out pin is 1 0=Out pin is 0 D6:1=Null count 0=Count available for reading D5-D0=Counter programmed mode (See Control Word Formats) FIGURE 6. STATUS BYTE |
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