SRAM-Read-Write

A sketch for the Arduino Mega that allows it to read and write to some older generation SRAM chips

Background Information

The SRAM Read/Write Sketch sets up a set of control lines (WE/Write Enable, OE/Output Enable, and CE/Chip Enable) followed by
an address bus and a data bus and sends the proper signals to interface the Arduino Mega to an older generation
28-pin SRAM chip. The chip in this case is the UM61256AK-15 and the datasheet can be found here:
UM6126AK-15 Datasheet

Other 28-pin DIP SRAM chips should be compatible as well such as the N341256P-20:
N341256P-20 Datasheet

How it Works

There are three parts to the actual program itself, the initializing code that sets up the necessary control lines and busses,
the functions necessary for reading and writing, and the memory testing algorithm which doesn’t actually thoroughly test the
chip itself but simply writes a byte and attempts to read it back, as a way of testing if the read/write functions work.
Their method of operation is described below:

Setup

1. In order to save keystrokes and improve readability, the digitalWrite() functions needed to
   send the signals out of the control line are defined in the very beginning as OE_LOW, CE_LOW, WE_LOW, and WE_HIGH
2. The Serial Port is open at 9600 Baud to transmit Read/Write status to the Serial Monitor in the Arduino IDE
3. Pins 53, 52, and 51 of the Arduino Mega are set as output through the Port B Register for the control lines
4. Pins 22-29 are set as output through the Port A register and are part of the lower byte for addressing
5. Pins 37-31 are set as output through the Port C register and are part of the uppper byte for addressing
   • It should be noted that the entire byte of the register isn’t used as there are only 15 address lines

Due to the fact that the data bus will have to flip between Input and Output there isn’t much to set up for it and its status
will be set up and changed through the Read Write functions

Read/Write Functions

There are two main functions, write_data() and read_data(). write_data() relies on another function known as set_addr
which sets the proper address and data_op() which, given the proper arguments, will send data through the data bus.
read_data also relies on the same functions but with different arguments.

set_addr()

Argument:

• uint16_t address - 16 bit unsigned integer that holds address to be outputted

1. The first half of the address is set to PORTA (Lower Address Byte) and is outputted
2. The address is shifted 8 bits so that the second half of the address is set to PORTC(Upper Address Byte) and outputted

data_op()

Arguments:

• char rw - Takes the character ‘r’ for “read” or ‘w’ for “write”
• uint8_t data - 8 bit unsigned integer that holds data to be written

1. If the char rw argument is set to ‘w’ a write operation is performed
   1. The pins of the L Port Register are set to output
   2. the contents of uint8_t data are set to the L Port register, effectively setting them on the data bus
   3. the contents of the L Port register are returned (just as verification)

2. If the char rw argument is set to ‘r’ a read operation is performed

   1. The L Port Register is entirely cleared
   2. The pins of the L Port register are set to input
   3. The contents of the L Port register are returned

   write_data()

   Arguments:

   • uint16_t address - 16 bit unsigned integer that takes in the address of the memory location to be written
   • uint8_t data - 8 bit unsigned integer that holds the data to be written
   1. OE (Output Enable) is set low for the duration of the function
   2. CE (Chip Enable) is set to low for the duration of the function
   3. WE (Write Enable) starts of high
   4. A 1 ms delay is put in place to ensure all signals are LOW/HIGH enough
      • this can be removed without any problems according to my tests
   5. The address is sent using the set_addr() function with the uint16_t address argument of this function inputted as
      its argument
   6. WE is set low
   7. The data_op() function is called with the arguments ‘w’ to signal a write operation and the data from this functions
      argument
   8. WE is set high

   The timing diagram better illustrates the reasons for setting all these signals high or low and can be seen below
   

   read_data()

   Arguments:

   • uint16_t address - 16 bit unsigned integer that takes in the address of the memory location to be read
   1. WE (Write Enable) is set to high for duration of the function
   2. OE (Output Enable) is set to low for duration of the function
   3. CE (Chip Enable) is set to low for duration of the function
   4. the set_addr() function is called with its argument the address being the argument of this function
   5. The data read is returned by using return() in conjunction with the data_op() function and the arguments ‘r’ indicating
      a read operation and ‘NULL’ for the data to be written as no data will be written

   The timing diagram better illustrates the reasons for setting all these signals high or low and can be seen below
   

   Memory Testing Algorithm

   As mentioned before, this is by no means a thorough testing algorithm and is just something to show how all the functions
   presented would work in a real program

   1. Variable uint16_t address is declared and unitialized
   2. Variable uint8_t random_data is declared and unitialized
   3. The random number generator seed is chosen by a random read of the Analog I/O pin A0
   4. A “for” loop is set up that increments the uint16_t address variable up to 0x7FF
      • For each cycle, the random_data variable is set to some random number between 0x00 and 0xFF.
      • The write_data() function is called with the arguments address and random_data
      • an “if” statement is put in place that calls read_data() with address as an argument and tests for equality with random_data
        • If the data read and the data randomly generated previously are the same, the Address and Data written are displayed along with the string “STATUS: PASS”
        • If the data read and the data randomly generated previously are NOT the same, the information above is printed but with the string “STATUS: FAIL”
   5. A 500 millisecond delay is put in place to ensure that all the data from the Arduino is properly received and displayed on the Serial Monitor
   6. The program is terminated using the exit() function

   To-Do

   These are a couple of improvements I’d like to make to the program itself or just some things that I’ve been running
   through my head.

   • Adapt code for the Arduino UNO using shift registers to save on pin space
   • Add a blinking status light during I/O Operations
   • Possibly implement a more robust memory testing algorithm