SPRUHM9H October 2014 – May 2024 TMS320F28075 , TMS320F28075-Q1 , TMS320F28076
When the EMIF receives a read request to the SDRAM from one of the requesters listed in Section 23.2.2, the EMIF performs one or more read access cycles. A read access cycle begins with the issuing of the ACTV command to select the desired bank and row of the SDRAM device. After the row has been opened, the EMIF proceeds to issue a READ command while specifying the desired bank and column address. EM1A[10] is held low during the READ command to avoid auto-precharging. The READ command signals the SDRAM device to output data from the specified address while EMIF issues NOP commands. Following a READ command, the CL field of the SDRAM configuration register (SDRAM_CR) defines how many delay cycles are present before the read data appears on the data bus. This is referred to as the CAS latency.
Figure 23-6 shows the signal waveforms for a basic SDRAM read operation in which multiple data is read from a single page. On this device, burst accesses are not supported; hence, the EMIF issues a READ command for each data access. Only when the EMIF SDRAM interface is configured to 16-bit by setting the NM bit of the SDRAM configuration register (SDRAM_CR) to 1 and CPU (or any other master) does a 32-bit READ access, a burst access is issued with a size of two.
Several other pins are also active during a read access. The EM1DQM[x:0] pins are driven low during the READ commands and are kept low during the NOP commands that correspond to the burst request. The state of the other EMIF pins during each command can be found in Table 23-6.
The EMIF schedules the commands based on the timing information that is provided to the EMIF in the SDRAM timing register (SDRAM_TR). The values for the timing parameters in this register must be chosen to satisfy the timing requirements listed in the SDRAM data manual. The EMIF uses this timing information to avoid violating any timing constraints related to issuing commands. This is commonly accomplished by inserting NOP commands between various commands during an access. Refer to the register description of SDRAM_TR in the SDTIMER register for more details on the various timing parameters.