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I/O Model Reference 87

Table 33. Magcard Input Latency Values for Series 3100 Devices

Symbol Description Minimum Typical Maximum

fin

I/O call to first clock input — 45.0 μs —

hold

Data hold 0 μs — —

setup

Data setup 0 μs — —

low

Clock low width 60 μs — —

high

Clock high width 60 μs — —

wto

Width of timeout pulse 60 μs — —

clk

Clock period 120 μs — —

tret

Return from timeout 21.6 μs — 81.6 μs

ret

Return from function — — 301.8 μs

Programming Considerations

The magcard input model reads track 2 in the forward direction only (the

magcard_bitstream input model can read in either direction). The data is

presented as a data signal input on pin IO_9, and a clock, or data strobe, signal

input on pin IO_8. The data on pin IO_9 is clocked on or just following the falling

edge of the clock signal on IO_8, with the least significant bit first.

Data is recognized as a series of 4-bit characters plus an odd parity bit per

character. This process begins when the start sentinel (0x0B) is recognized, and

continues until the end sentinel (0x0F) is recognized. No more than 40

characters, including the two sentinels, will be read. The data is stored as

packed binary-coded decimal (BCD) digits in the buffer space pointed to by the

buffer pointer argument to the io_in( ) function with the parity bit stripped, and

includes the start and end sentinel characters. This buffer should be 20 bytes

long. The data is stored with the first character in the most significant nibble of

the first byte in the buffer.

For magcard input, the io_in( ) function requires a pointer to a data buffer, into

which the series of BCD pairs are stored. The io_in( ) function returns a signed

int that contains the actual number of characters stored.

The parity of each character is checked. The longitudinal redundancy check

(LRC) character, which appears just after the end sentinel, is also checked. If

either of these tests fail, if more than 40 characters are being clocked in, or if the

process aborts due to an input pin event, the io_in( ) function returns the value (-

1). The LRC character is not stored.

The magcard object optionally uses one of I/O pins IO_0 through IO_7 as a

timeout/abort pin. Use of this feature is suggested because the io_in( ) function

updates the watchdog timer during clock wait states, and could result in a lockup

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