Information

depth of one. This is the default/reset behavior of the module and can be adjusted using
the PFIFO[RXFE] and PFIFO[TXFE] bits. Individual watermark levels are also provided
for transmit and receive.
There are multiple ways to ensure that a data block, which is a set of characters, has
completed transmission. These methods include:
1. Set TXFIFO[TXWATER] to 0. TDRE asserts when there is no further data in the
transmit buffer. Alternatively the S1[TC] flag can be used to indicate when the
transmit shift register is also empty.
2. Poll TCFIFO[TXCOUNT]. Assuming that only data for a data block has been put
into the data buffer, when TCFIFO[TXCOUNT] = 0, all data has been transmitted or
is in the process of transmission.
3. S1[TC] can be monitored. When S1[TC] asserts, it indicates that all data has been
transmitted and there is no data currently being transmitted in the shift register.
45.8.2 ISO-7816 initialization sequence
This section outlines how to program the UART for ISO-7816 operation. Elements such
as procedures to power up or power down the smartcard, and when to take those actions,
are beyond the scope of this description. To set up the UART for ISO-7816 operation:
1. Select a baud rate. Write this value to the UART baud registers (BDH/L) to begin the
baud rate generator. Remember that the baud rate generator is disabled when the
baud rate is zero. Writing to the BDH has no effect without also writing to BDL.
According to the 7816 specification the initial (default) baud rating setting should be
Fi = 372 and Di = 1 and a maximum frequency of 5 MHz. In other words, the BDH,
BDL, and C4 registers should be programmed such that the transmission frequency
provided to the smartcard device must be 1/372th of the clock and must not exceed 5
MHz.
2. Write to set BDH[LBKDIE] = 0.
3. Write to C1 to configure word length, parity, and other configuration fields (LOOPS,
RSRC) and set C1[M] = 1, C1[PE] = 1, and C1[PT] = 0.
4. Write to set S2[RWUID] = 0 and S2[LBKDE] = 0.
5. Write to set MODEM[RXRTSE] = 0, MODEM[TXRTSPOL] = 0,
MODEM[TXRTSE] = 0, and MODEM[TXCTSE] = 0.
Chapter 45 Universal Asynchronous Receiver/Transmitter (UART)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 1137