| 摘要 |
<p>PURPOSE:To eliminate the mask raising load in terms of both time and economy by setting a relation between the address of a PROM and that of a mask ROM and reading out the stored content of the PROM when necessary in place of the stored content of the mask ROM of an optional address. CONSTITUTION:The explanation is given in case of 7-address is accessed in assuming that (A0, A1, A2, A3)=(1,1,1,0) for the working of this circuit. A transistor TR21 is turned on since a clock phip is ''1'' in case phip=''1''. Only a word line a3 goes to ''0'' because of (A0, A1)=(1,1), then word lines a0, a1 and a2 go to ''1'' respectively. The TR21 is turned off in case phip=''0'', since the clock phip is set at ''0''. Therefore, a data line 17 is set at a high impedance in terms of a mask ROM50 regardless of the selected address of the ROM50. While a complementary signal -phip=''1'' of the clock phip is supplied to an FET62 from an inverter 63. Thus the storage contents of an EPROM60 can be read out. Then the output of the EPROM60 goes to ''0'' by inverters 600-603, and an FET613 is turned on. Then the data on a memory cell TR633 is read out to the line 17.</p> |
