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M27256-25F6 Datasheet(PDF) 3 Page - STMicroelectronics |
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M27256-25F6 Datasheet(HTML) 3 Page - STMicroelectronics |
3 / 11 page For the most efficient use of these two control lines, E should be decoded and used as the primary device selecting function, while GVPP should be made a common connection to all devices in the array and connected to the READ line from the system control bus. This ensures that all dese- lected memory devices are in their low power standby mode and that the output pins are only active when data is required from a particular mem- ory device. System Considerations The power switching characteristics of fast EPROMs require careful decoupling of the devices. The supply current, ICC, has three segments that are of interest to the system designer : the standby current level, the active current level, and transient current peaks that are produced by the falling and rising edges of E. The magnitude of the transient current peaks is dependent on the capacitive and inductive loading of the device at the output. The associated transient voltage peaks can be sup- pressed by complying with the two line output control and by properly selected decoupling ca- pacitors. It is recommenced that a 1 µF ceramic capacitor be used on every device between VCC and VSS. This should be a high frequency capacitor of low inherent inductance and should be placed as close to the device as possible. In addition, a 4.7 µF bulk electrolytic capacitor should be used between VCC and VSS for every eight devices. The bulk capacitor should be located near the power supply connection point. The purpose of the bulk capacitor is to overcome the voltage drop caused by the inductive effects of PCB traces. Programming When delivered, and after each erasure, all bits of the M27512 are in the “1" state. Data is introduced by selectively programming ”0s" into the desired bit locations. Although only “0s” will be programmed, both “1s” and “0s” can be present in the data word. The only way to change a “0" to a ”1" is by ultraviolet light erasure. The M27512 is in the programming mode when GVPP input is at 12.5V and E is at TTL-low. The data to be programmed is applied 8 bits in parallel to the data output pins. The levels required for the address and data inputs are TTL. The M27512 can use PRESTO Programming Algo- rithm that drastically reduces the programming time (typically less than 50 seconds). Nevertheless to achieve compatibility with all programming equipment, the standard Fast Programming Algo- rithm may also be used. Fast Programming Algorithm Fast Programming Algorithm rapidly programs M27512 EPROMs using an efficient and reliable method suited to the production programming en- vironment. Programming reliability is also ensured as the incremental program margin of each byte is continually monitored to determine when it has been successfully programmed. A flowchart of the M27512 Fast Programming Algorithm is shown in Figure 8. Mode E GVPP A9 Q0 - Q7 Read VIL VIL X Data Out Output Disable VIL VIH X Hi-Z Program VIL Pulse VPP X Data In Verify VIH VIL X Data Out Program Inhibit VIH VPP X Hi-Z Standby VIH X X Hi-Z Electronic Signature VIL VIL VID Codes Note: X = VIH or VIL, VID = 12V ± 0.5%. Table 3. Operating Modes Identifier A0 Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q0 Hex Data Manufacturer’s Code VIL 00100 00 0 20h Device Code VIH 00001 10 1 0Dh Table 4. Electronic Signature DEVICE OPERATION (cont’d) M27512 3/11 |
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