A Dual-Mode Synchronous/Asynchronous CORDIC Processor Eckhard Grass, Bodhisatya Sarker and Koushik Maharatna IHP Im Technologiepark 25 15236 Frankfurt (Oder), Germany [email protected] Abstract a frequency correction on the incoming signals. A Coordinate Rotation Digital Computer (CORDIC) offers For application in a software defined radio a CORDIC an elegant way of implementing an NCO. processor has been developed that can operate both in To open a broad market for consumer products, low synchronous and asynchronous mode. Each mode of cost of the required hardware is essential. One way to operation has advantages and drawbacks. Depending on realise low-cost systems is to reduce system complexity the actual application, an optimal trade-off can be and implement all functions of a wireless modem in a achieved by selecting the mode of operation that fits best single chip. Potentially, a single-chip solution is also with system demands. We believe that for a system advantageous in terms of performance and power developer this additional degree of freedom significantly dissipation when compared with multi-chip increases the application space. µ implementations. Fewer signals have to be routed via slow The design has been implemented on a 0.25 m SiGe:C and power-hungry pad drivers. In addition, short BiCMOS process. Simulation results using post-layout interconnections allow faster operation of the system. extracted parameters indicate that the design is µ competitive both with purely synchronous and purely Our in-house 0.25 m SiGe:C BiCMOS technology asynchronous implementations. enables the integration of complex digital baseband functionality together with the analog RF front-end (AFE). This process technology forms the basis for the single-chip 1. Introduction implementation of a complete wireless modem. This single-chip wireless modem will hence comprise analog Fourth generation (4G) wireless and mobile systems circuitry, processor cores as well as dedicated hardware. are currently the focus of research and development. They For the single-chip modem, a number of critical design will allow new types of services to be universally available problems have to be solved. One major issue is the to consumers and for industrial applications. Broadband reduction of crosstalk from the digital part to the analog wireless networks will enable packet based high datarate circuitry. The high energy which is delivered by clock communication suitable for video transmission and mobile drivers can influence the AFE and deteriorate its signal to Internet applications. noise (S/N) ratio. The most predominant crosstalk This paper is based on a project which aims to develop mechanisms are substrate noise, supply voltage variations a single-chip wireless broadband communication system in as well as electro-magnetic effects (EMR) [3]. Secondly, the 5 GHz band, compliant with the Hiperlan/2 [1] and system power dissipation is a critical issue. Since many IEEE 802.11a [2] standards. Both standards specify functional blocks are active for a limited period only, there broadband communication systems using Orthogonal is potential to reduce power dissipation drastically. A Frequency Division Multiplexing (OFDM) with data rates token-flow approach [4] is adopted by adding signals ranging from 6-54 Mbit/s. The duration of one OFDM which indicate the validity of data at inputs and outputs of symbol, which can carry 24 to 216 bits of information is functional blocks. Distributing the control of data flow, 4 µs. For such a system a large portion of the digital this token-flow approach allows easy implementation of baseband processor has to operate on complex signals clock gating and lends itself to asynchronous which are sampled at a rate of 20 MS/s. The most difficult implementations of blocks. to implement functional blocks of the baseband processor Currently a number of extensions to the standards for are FFT/IFFT processor, Viterbi Decoder, and Numeric 5 GHz broadband communication systems are being Controlled Oscillator (NCO). The NCO is used to perform discussed by the standardisation committee. Therefore, it w ou l d b e b e n e f i c i a l f o r c u s t o m e r s a n d s upp li e r s o f t ho s e a nd t h e v ec t o r i n g m od e w h e r e , v a r i a b l e s z a n d y a r e f o r c e d s y s t e m s i f h a r d w a r e w ou l d b e a b l e t o acc o m m od a t e f u t u r e t o ze r o r e s p ec ti v e l y . E ac h o f t h e s e m o d e s ca n b e u tili s e d i n c o mm un i ca ti o n s t a nd a r d s . A s o f t w a r e d e f i n e d r a d i o , i n c i r c u l a r , li n ea r a nd h yp e r bo li c c o o r d i n a t e s y s t e m s . T h i s p r i n c i p a l , o ff e r s t h i s n ee d e d f l e x i b ilit y . ill u s t r a t e s t h e v e r s a tilit y o f t h e C O R D I C a nd e xp l a i n s w hy H e r e w e e x t e nd t h e i d ea o f s o f t w a r e d e f i n e d r a d i o s u c h iti s r e g a r d e d a s on e o f t h e m a j o r t oo l s i n d i g it a l c o m p u t e r t h a t t h e s o f t w a r e d e v e l op e r ca n c ho s e ce r t a i n h a r d w a r e a r it h m e ti c . acce l e r a t o r s t o op e r a t e e it h e r s yn c h r o n ou s l y o r H o w e v e r , f o r o u r p u r p o s e it i s s u ff i c i e n t t o u s e t h e a s yn c h r o n ou s l y . T h i s a dd iti on a l d e g r ee o f fr e e do m r o t a ti on a n d v ec t o r i ng o p e r a ti on s i n t h e c i r c u l a r c oo r d i n a t e i n c r ea s e s t h e a pp li ca ti on s p ace o f t h e c o m p l e t e s y s t e m . s y s t e m on l y . H e n ce w e c o n ce n t r a t e on t h e r ea li za ti on o f a F u r t h e r m o r e , a l s o w it h a g i v e n s t a nd a r d , d i ff e r e n t c i r c u l a r C O R D I C p r o ce ss o r . I t ’ s op e r a ti on r e li e s on t h e a pp li ca ti on s ce n a r i o s r e q u i r e d i f f e r e n t p r o p e r ti e s o f t h e p r i n c i p l e o f v ec t o r r o t a ti on . T h e b a s i c op e r a ti on i s h a r d w a r e . W e a r e c onv i n c e d t h a t h a v i ng t h e fr ee d o m f o r d e s c r i b e d b e l o w : ce r t a i n k e y b l o c k s o f c h o o s i ng b e t w ee n s yn c h r o nou s a n d T h e r o t a ti on o f a v ec t o r [ x0 y 0 ] T i n t h e C a r t e s i a n c o - a s yn c h r o n ou s o p e r a ti on a ll o w s f i n d i ng a s u it a b l e t r a d e - o ff o r d i n a t e s y s t e m ca bn e d e s c r i b e d a s , b e t w ee n po w e r d i ss i p a ti on , c r o ss t a l k , d a t a t h r ou g hpu t , é x 1 ù é cos q sin q ù é x 0 ù l a t e n c y a nd ( s e l f-) t e s t a b ilit y .