UM10462
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© NXP B.V. 2016. All rights reserved.
User manual
Rev. 5.5 — 21 December 2016
272 of 523
NXP Semiconductors
UM10462
Chapter 12: LPC11U3x/2x/1x USART
When the SCIEN bit in the SCICTRL register (
) is set as described, the USART
provides bidirectional serial data on the open-drain TXD pin. No RXD pin is used when
SCIEN is 1. The USART SCLK pin will output synchronously with the data at the data bit
rate. Software must use timers to implement character and block waiting times (no
hardware support via trigger signals is provided on the LPC11U3x/2x/1x). GPIO pins can
be used to control the smart card reset and power pins. Any power supplied to the card
must be externally switched as card power supply requirements often exceed source
currents possible on the LPC11U3x/2x/1x. As the specific application may accommodate
any of the available ISO 7816 class A, B, or C power requirements, be aware of the logic
level tolerances and requirements when communicating or powering cards that use
different power rails than the LPC11U3x/2x/1x.
12.6.2.1 Smart card set-up procedure
A T = 0 protocol transfer consists of 8-bits of data, an even parity bit, and two guard bits
that allow for the receiver of the particular transfer to flag parity errors through the NACK
response (see
). Extra guard bits may be added according to card requirements.
If no NACK is sent (provided the interface accepts them in SCICTRL), the next byte may
be transmitted immediately after the last guard bit. If the NACK is sent, the transmitter will
retry sending the byte until successfully received or until the SCICTRL retry limit has been
met.
The smart card must be set up with the following considerations:
Fig 29. Typical smart card application
LPC11Uxx
ISO 7816
Smart Card
pull-up
resistor
selectable
power rail
VCC
CLK
I/O
RST
Insertion Switch
Optional
Logic Level
Translation
pull-up
resistor
pull-up
resistor
GPIO
SCLK
GPIO
GPIO
TXD
Fig 30. Smart card T = 0 waveform
start
bit0
bit1
bit2
bit3
bit4
bit5
bit6
bit7
parity
NACK
guard1 guard2
extra
guard1
extra
guard2
extra
guard
n
start
bit0
Asynchronous transfer
Next transfer or
First retry
TXD
Clock