4-Mbit parallel NOR Flash in a PQCC-32 footprint
The Texas Instruments TMS29F040-90C5FML is a 4-Mbit parallel NOR Flash memory organized as 512K words of 8 bits each, with a 90 ns access time. It comes in a 32-lead plastic leaded chip carrier (PQCC-32), the same footprint as a standard PLCC-32 socket or solder pad. This is a legacy parallel-interface part suited for code storage or data logging in embedded systems that already use a 5 V tolerant bus and a byte-wide memory map.
90 ns access time — timing margin on a legacy bus
The 90 ns access time is the key timing parameter: it defines how quickly the memory outputs valid data after the address is latched. For a system with a 10 MHz to 12 MHz bus clock (80–100 ns cycle), this part leaves little margin for additional decode logic or long trace delays. It is a direct fit for designs originally specified for 90 ns or slower parallel Flash, but a 70 ns or 55 ns part would be needed if the bus runs faster or if you add address buffers.