Below is a rough schematic of the layout of the accelerometer PC board looking from the component side. The microcontroller is an Atmel AT89S8252, an 8051 clone. This microcontroller is in-circuit programmable using an SPI interface. The SPI pins are also used to drive the MMC. To permit the dual use there is a jumper block (located below the 74AHC244, marked "P" and "R") that allows the pins to be connected for programming (P) or running the code (R). Speaking of the 74AHC244 chip, it MUST be the 74AHC family version. Regular old 74LS or 74HC etc. will not work, at least not for long. The AHC family allows the chip to be powered by 3.3v yet will not be destroyed by 5v inputs. It is used in this circuit to allow the 5v microcontroller to talk to the 3.3v or so MMC. Without, hopefully, damaging anything. Pins 31 through 40 of the microcontroller are connected with a 4.7Kohm resistor pack. This effectively ties pins 31 through 40 to 5v. This was done so port 0 could be used as I/O at a later date if desired and, more importantly, to tie pin 31 high so the microcontroller would work at all. If this resistor pack is left out, pin 31 must still be held high some way.
The accelerometer itself is a surface mount chip and is thus shown from the bottom on this schematic as if you were looking through the board. The same holds true for the MMC socket. The accelerometer has an external resistor used to select the data rate and external capacitors to smooth the output. The values shown cause output at a theoretical 250 Hz with the bulk of the signal beyond 100 Hz filtered out. In reality this unit runs at 273 Hz. There is a jumper next to the accelerometer that can be used to perform a self test. I never used this feature but it's explained by the accelerometer datasheet which is available from the Analog Devices web site.
There are two linear power supplies on this board. A 7805 at the lower left powers everything except the MMC and 74AHC244. A 7833 shown above the 7805 drops the voltage further to power these two devices.
There are two switches on this unit. The main switch, which runs to the outside of the box is shown at the lower right. This is a two pole, three position switch with the added feature that one pole is closed in switch positions 2 and 3 while the other is closed only in position 3. The leg that is closed in positions 2 and 3 is the power switch for the device. It is the intermediate position so when the switch is moved from "off" to the first detent the unit powers up. It immediately reads the address of the first empty memory location in the MMC and then enters a loop and waits. When the switch is then moved to the third position, the second pole grounds an I/O pin on the microcontroller. This signals it to leave the loop and start taking data. When data collection is over, the switch is moved back to the middle position. The microcontroller again detects this, finish writing the current memory block, stores the next memory address in block 1 then goes into an endless loop until power is turned off.
The second switch is a momentary contact switch located on the circuit board along the top edge. This switch is used to "erase" the MMC. If it is held down while the unit is powered up it will write the address of "0000h" into the first block of the card. The next time the unit is powered up normally it will thus begin writing at the start of memory.
The only visual indication that data is being taken is given through two LED's. A green LED blinks about twice per second when the power switch is in the intermediate position. This blinking occurs only when the required reads/writes of block one are completed to let the user know it's safe to proceed to recording data or to turn the unit off. A red LED gives one quick flash when the memory is "erased". It also blinks if an error occurs. The number of flashes in each sequence yields the error number which is halfway documented in the assembly language routine.
The final component of note is the 24MHz CMOS clock oscillator at the bottom of the schematic. This speed was needed to keep up with the data rate selected. In reality, the MMC sometimes takes a hair too long to store a block of data causing a glitch in the data. These usually occur about once a second and affect one or two data points so I tend to ignore them rather than try to fix them.
Here's the 8051 assembly language routine that makes this all work: 20,077 Bytes.
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